ESSL LAR 2008: Strategic Goal #1, Priority #2
Earth and Sun Systems Laboratory endeavors are central to NCAR's Strategic Goal #1, Improve understanding of the atmosphere, the Earth system, and the Sun. This Strategic Goal encompasses four Strategic Priorities, each of which is dependent on the efforts and accomplishments of ESSL staff.
ESSL developed an action plan with seven priority themes which involve direct partnerships with the university community and contribute directly to the ESSL Strategic Vision and to several priority items of the NCAR Strategic Plan.
ESSL's seven priority themes:
- Climate projection, with emphasis on short-term prediction.
- Biosphere-Atmosphere-Hydrosphere interactions and specifically development of BEACHON Project (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, Hydrology, Organics and Nitrogen).
- Water system research, specifically the development of the Society, Water, Atmosphere and Natural Systems Project (SWANS).
- An advanced Weather Research and Forecasting system, specifically the development of the Hurricane Intensity and Forecasts (HiFi) Project.
- Space Weather, specifically the development of the Coronal Solar Magnetism Observatory Project (COSMO).
- Chemical Weather, including interpretation of observed data gathered during the Megacity Impact on Regional and Global Environments (MIRAGE) campaign, and the development of a capability for chemical data monitoring and prediction.
- Prediction across scales, specifically the development of an advanced next-generation, climate-weather modeling system and an integrated Earth system model of intermediate complexity.
Goal #1, Priority #2: Investigating the Interactions of the Atmosphere, the Broader Earth System and Human Society, is described as a combination of "...meteorology and climatology," which "were (previously) considered separate fields, largely because of disparate time and length scales. Today, the two fields are strongly coupled, not only because climate provides boundaries for investigating the weather, but also because localized events can influence larger climatological scales. The activities that NCAR scientists focused on this year ranged from collecting in situ data to better understand climate, weather and related phenomena, to developing and analyzing ways to better model natural processes and working with university partners to devise ways of tackling scientific questions."
This NCAR priority, driven by ESSL's themes 2, 3, 4, 6, & 7, is critical to achieving NCAR's first strategic goal.
The section below describes specific research conducted by ESSL staff under projects relevant to Priority 2. The major ESSL activities in this area are studies of the global and regional carbon, nitrogen, and water cycles and their coupling, feedbacks, and interactions. Additional major activities focus on climate change and variability, extreme weather events, and the impacts of climate, weather, and urbanization on society and ecosystems.
- Regional carbon cycle - TIIMES
- Landfall impacts of hurricanes - MMM
- Global and regional water cycle - TIIMES
- The impacts of climate and weather on society and ecosystems: Polar climate - CGD
- Megacities Impacts on regional And global Environments/Megacity Initiative: Local and Global Research Observatories (MIRAGE/MILAGRO) - ACD
- Biosphere-Atmosphere Exchange of Aerosols with Cloud, Carbon, and Hydrologic cycles including Organics and Nitrogen (BEACHON) program objectives and plans - TIIMES
- Global biogeochemical cycles - TIIMES
- Bioemissions and photochemical processing - ACD
- Ecosystem - biogeochemistry - climate interactions - CGD
- Numerical simulation of turbulence - MMM
- Exploring the role of aerosols - ACD
- Convection organization: Observational analysis and resolved simulations - MMM
- Atmosphere/ocean interactions - MMM
- Long-term climate change in the thermosphere - HAO
- The impacts of climate and weather on society and ecosystems: Climate change - probabilistic climate change, and solar forcing of climate - CGD
- Land-atmosphere coupling - MMM
- Emission inventories and application - ACD
- The impacts of climate and weather on society and ecosystems: Water cycle - CGD
- Parameterization - MMM
- Role of the oceans in climate - CGD
- Structure and evolution of clear and cloudy atmospheric boundary layers - MMM
- Fine mesh land model - TIIMES
- NASA African Monsoon Multidisciplinary Analysis (NAMMA) - MMM
- Integrated Land Ecosystem-Atmosphere Processes Study (iLEAPS) contributions - ACD
- Texas air quality study - ACD
TIIMES measurement and instrument development projects have advanced our understanding of regional carbon fluxes, particularly in the mountain west, and provided key NCAR contributions to the multi-agency North American Carbon Program (NACP). A central component of this effort is the Regional Atmospheric Continuous Carbon Dioxide Network in the Rocky Mountains (Rocky RACCOON).
In order to improve our understanding of regional carbon fluxes in the Rocky Mountain West, TIIMES scientists have developed and deployed autonomous, inexpensive, and robustanalyzers (AIRCOA) at six sites throughout Colorado, Utah, and Arizona over the past three years (http://raccoon.ucar.edu). Analysis of the diurnal cycles in concentration and variability at these sites provide insight as to when and under what conditions mountaintop signals are regionally representative, as well as first-order constraints on boundary-layer heights and flux rates for use in evaluating model fidelity (Figure 1).
Comparisons between the RACCOON measurements and estimates of free-tropospheric background concentrations reveal regional-scale CarbonTracker flux estimation system as well as the GlobalView data product used by modeling groups around the world.flux signals that are generally consistent with one another and our expectation of peak uptake in mountain forests during spring (Figure 2). Combining these differences with information on boundary-layer mixing can lead to quantitative estimates of monthly regional fluxes. These data have also been used in the NOAA
The RACCOON observations at Fraser Experimental Forest have occurred while the trees in the St. Louis creek drainage have experienced widespread mortality due to mountain pine bark beetle infestation. Themeasurements at the base of this valley show large increases in at night as the valley drainage flow pools respiration from a large area. This nocturnal build-up has decreased over the past three years (Figure 3), suggesting a decrease in ecosystem respiration in response to the insect outbreak. This decrease indicates that the reduction in autotrophic respiration is greater than any short-term increase in litter fall, and will be a valuable test of models predicting the impact of the recent outbreaks on regional scale carbon fluxes.
Future plans to address regional carbon fluxes include in depth analysis of ACME-07 and Rocky RACCOON data and synthesis with modeling efforts to:
- define regional-scale monthly to interannual carbon fluxes for the U.S. Central Rocky Mountains and Southwest,
- assess key drivers of variability and trends including drought, fires, and insects, and
- optimize community observational efforts across the Mountain West.
The 2005 Atlantic hurricane season is a vivid reminder of the economic and societal consequences of landfalling tropical cyclones. Improved forecasts of hurricane structure, intensity change, and time-extensions of skillful track prediction are vital for evacuation strategies. Progress requires solving difficult problems such as the inner-core hurricane dynamics and how it affects intensity, quantifying the net enthalpy flux from the ocean in high-wind-speed conditions, and incorporation of a variety of remotely sensed data into model initial conditions. The purpose of ESSL/MMM research in hurricane simulations is to create the next generation hurricane-prediction system, and a community hurricane-prediction model that can be used for process and predictability studies.
During the past year, analysis of the real-time forecasts for the 2007 North Atlantic hurricane season revealed that intensity prediction is currently limited by poor initialization of the hurricane vortex. Experimentation with the Ensemble Kalman Filter data assimilation technique shows this could be key in creating suitable initial conditions and improved intensity prediction. A breakthrough science project allowed a simulation of an idealized hurricane to be conducted at 62m grid spacing. At this resolution, the model was able to resolve turbulent eddies. The 1-minute mean wind field form this simulation showed lower maximum winds than a more coarse resolution simulation that was unable to resolve turbulence.
In a new application, WRF simulations of the major U.S. landfalling hurricanes of 2004 and 2005 were used as inputs into insurance-industry loss models to evaluate the potential utility of high-resolution winds for improving hazard assessment and loss estimation. The project revealed WRF can provide information on asymmetries and small-scale details of the wind field that are currently not included in most insurance loss models.
Nested Regional Climate Model simulations have been explored using a tropical-cyclone-detection algorithm. The model captured the general global and seasonal distributions of tropical cyclones but consistently overproduced the number of tropical cyclones. Two-way nesting down from 36km to 12km over the Atlantic region for the 2005 season improved both the number and spatial distribution. This dataset has been used to show the importance of the background flow in modulating easterly waves thereby creating favorable conditions for tropical cyclogenesis, and higher-resolution simulations of genesis have shown vortices merging during the period of genesis.
In the coming year, new ground will be broken with the Ensemble Kalman Filter
technique by running this cycling assimilation system for an entire North
Atlantic hurricane season to create initial conditions for high-resolution
hurricane simulations. These simulations will make an important contribution
to NOAA’s Hurricane Forecast Improvement Project by providing a statistically
significant comparison of the effect of high-resolution (~1km in the horizontal)
on the accuracy of hurricane-intensity forecasts. Data from T-PARC will be
analyzed and cycling assimilation will be run retrospectively for the period
of the campaign. Finally, trends in hurricane activity in the North Atlantic,
Caribbean and Gulf of Mexico regions will be examined through ensembles of
high-resolution climate-change simulations using the Nested Regional Climate
Model driven by boundary conditions from the Community Climate System Model.
This ambitious project is providing for the first time, decadal climate simulations
of hurricanes at a resolution sufficient to identify key intensity and structural
details. It also provides a genuine test of the requirements for future supercomputing
systems to enable decadal predictions of the regional detail required for
planning and adaptation strategies. The work has strong community involvement
and support, including financial support from the offshore oil industry.
The Water Cycle Program has conducted research related to the Regional and Global water cycle since 2001. Using the diurnal cycle of precipitation as a focus, research has shown that current climate models do not accurately simulate the frequency, intensity, and timing of summer time convection over much of the globe, including continental regions, despite reasonable simulations of precipitation amount (see figure). This model deficiency greatly hampers climate models’ ability to predict future changes in intense storms, flash floods, tornados, hurricanes, and other severe weather events that likely have the largest impact on the society under global warming. This model deficiency results from a variety of factors, including: 1) a lack of realistic representation of atmospheric convective inhibition processes, and 2) a poor representation of propagating systems of convection in the lee of major mountain ranges. This later deficiency is reflected in the high degree of uncertainty of current climate model runs in these regions.
The goals of the Water Cycle program are: 1) to reduce this uncertainty through focused research on the physical mechanisms leading to the onset of moist convection and the propagation of convective systems in the real world, 2) to test and improve new parameterizations of convection that improve simulations of these phenomena, 3) to improve our understanding of the coupling between the land surface, atmospheric boundary layer and convective parameterizations in climate models, and 4) to improve the representation of the cool season water cycle over complex terrain in climate models.
Current research has focused on the following areas supporting the goals mentioned above:
Further quantifying precipitation characteristics (frequency and intensity and their dependence on data resolution, temporal and spatial correlative structures, etc.) on global and large scales using high-resolution satellite data, and apply them to evaluate global models (Dai, Laing, Trier, Davis, Carbone, Trenberth, Wang).
Studying Upper Colorado Basin snowpack under projected climate change in the 21st century using high-resolution regional models (the Colorado Headwaters Project) (Rasmussen, Gochis, Chen, Liu, Ikeda, Arsenault, Houser, Liang, Dudhia, Yu, Tewari,Thompson).
Improving convective parameterizations in climate models for better simulations of the frequency, intensity, and diurnal timing of precipitation and the propagation of convective systems (Dai, Li, Neale, Liu, Moncrieff, Tulich, Grabowski, Davis, Rasmussen, Mapes, Neelin, Wu).
Conducting high resolution model simulations using boundary conditions with short waves removed. Investigate the role of the mountain-plains circulation in maintaining and initiating the propagating convection as well as other mechanisms (Trier, Wang, Tuttle, Carbone).
Percent of precipitation over the United States attributable to propagating convection. Twelve year radar dataset does not reveal a correlation of the intensity or phase of the propagating convection to ENSO (Tuttle, Carbone).
Diagnosing the water cycle in climate models and observations and in retrospective datasets (Dai and Trenberth).
Improving the hydrological cycle in CLM3 (Oleson and Lawrence).
Examine the role of the Great Plains nocturnal jet in modulating convection in CAM (Caron).
Examining the role of the land surface in modulating propagating convection (Chen, Tewari, LeMone).
Results to date show that short waves are not the dominant factor in initiating convection. Research is now focused on other mechanisms such as the mountain/plains diurnal circulation. Examination of the amount of precipitation attributed to propagating precipitation over the continental U.S. suggests that up to 70% of central U.S. precipitation is due to propagating systems. Research on the role of land surface impacts suggest that the land surface plays a secondary role in the formation and propagating of long-lived convective systems. Research related to the low-level jet over the central U.S. shows a strong coupling of the corridors of propagating convection to the low level jet. In support of improved simulation of these propagating systems in climate models, a new cross-NCAR convective parameterization scientific interest group has been formed and has been holding monthly meetings to discuss new work related to convection parameterization. The Colorado Headwaters project started, and was awarded 500,000 GAUs through the Accelerated Science Discovery competition.
The Water Cycle Program will focus on the following areas during the next few years:
Continued emphasis on improving convective parameterizations in climate models, including the testing of various candidate schemes (including the new Moncreiff and Liu scheme developed under Water Cycle sponsorship) and testing different treatments of convective inhibition in models, with the aim to reduce the model bias related to precipitation frequency, intensity and diurnal cycle. The Convective Parameterization group will continue to meet and likely hold workshop.
Finalize work related to mechanisms that lead to the initiation of propagating systems.
Continue with Regional and Global Diagnostic studies of the water cycle.
Evaluate how to improve the coupling of land surface, boundary layer, and convective schemes in climate models.
Initiate and carry out the Colorado Headwaters cool season water cycle over complex terrain project.
Over the past several decades, Arctic sea ice extent has been steadily shrinking. Due to the ice-albedo feedback, this reduction in ice cover has contributed to an observed amplification of Arctic warming relative to the rest of the world. Observed sea ice extent in September 2007 smashed the previous record low. Climate models project that sea ice decline will continue into the foreseeable future, with the possibility of summer ice-free conditions being reached later this century. Considerable effort is underway to examine these observed and projected changes in the sea ice system and the consequences of a seasonally ice-free Arctic ocean for the climate and ecological systems. An analysis of projected changes in the future ice cover suggests that gradual, linear changes are unlikely; rather, sea ice decline is likely to be punctuated by abrupt transitions such as that seen in 2007. Integrations with the Community Climate System Model (CCSM) exhibit abrupt reductions in the future summer sea ice cover, with the most extreme event going from 80% September ice coverage to 20% coverage in approximately 10 years (Figure 1). The mechanisms responsible for these transitions include: 1) an increased efficiency of open water production as the Arctic ice thins, 2) rapid increases in ocean heat transport that trigger the events, and 3) the surface albedo feedback, which accelerates the ice retreat. An analysis of additional models participating in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) indicates that about 50% of them exhibit similar abrupt reductions in future Arctic summer ice cover for some future forcing scenarios, although the length and magnitude of the events vary. The presence of seasonally ice-free conditions has potential impacts on the Arctic and global systems and CCSM is being used to assess the impacts on global atmospheric circulation, the hydrological cycle as well as ocean and terrestrial conditions.
Along with sea ice loss, there are strong indications that the permafrost is warming and thawing over large scales. Thawing of permafrost is likely to induce a number of feedbacks to the hydrologic and carbon cycles of the Arctic system. Of particular concern, especially from a global perspective, is how permafrost thaw will affect the carbon balance in the Arctic. The future carbon balance of the Arctic remains one of the largest uncertainties in climate change science.
Changing sea ice and permafrost conditions have important implications for the Arctic hydrological system change. Because of the proximity to deep water formation regions within the northern North Atlantic, this in turn can modify the global thermohaline circulation.
Finally, while changes in high northern latitudes are considerable, the Antarctic region generally remains quite stable. There are numerous and varied reasons for this different behavior in the southern high latitudes, including changes in the atmospheric circulation and ocean heat uptake that appear to mute an anthropogenic warming signal. However, the interactions and importance of these processes for the future Antarctic climate remain unclear.
In 2008, we made substantial progress on a variety of research topics, ranging from an evaluation of the role of external versus internal forcing of sea ice transitions, an assessment of the impact of sea-ice loss on polar bear habitat and on permafrost degradation. We also continue to evaluate the role of changes in freshwater forcing on the thermohaline circulation. We also made substantial progress improving the representation of polar processes in CCSM in preparation for CCSM4. For more information, see the following topics:
- Natural versus external forcing of rapid ice loss [M. Holland Profile]
- Polar bear habitat loss [M. Holland Profile]
- Accelerated terrestrial warming during rapid sea ice loss and permafrost degradation [C. Deser Profile] [M. Holland Profile] [D. Lawrence Profile]
- Role of atmospheric circulation in sea ice decline [C. Deser Profile] [H. Teng Profile]
- Impact of freshwater forcing on thermohaline circulation [G. Meehl Profile] [B. Otto-Bliesner Profile]
- Development and improvement of polar processes in CCSM4 [D. Lawrence]
2009 and Beyond
CCSM experiments exhibiting periods of abrupt sea ice loss and rapid permafrost thaw raise the question as to whether or not sea ice or permafrost loss exhibit characteristics of a so-called tipping point in the climate system. Experiments are underway to evaluate whether or not sea ice can thin to the point that its further loss is no longer dependent on further warming. Related work on the stability of seasonally ice free conditions is also in progress. Experiments are also planned to evaluate whether permafrost is sustainable or not once the ground has reached a thermal state in which talik, a perpetually unfrozen layer between seasonally frozen ground above and permafrost below, has formed.
Work is also underway to incorporate a dynamic wetland model that is capable of simulating the anticipated changes in wetland distribution associated with permafrost thaw induced soil subsidence. Additional efforts will focus on an integration of the CLM organic soil representation with the prognostic soil carbon calculated in the CLM carbon cycle model. We also intend to evaluate how a conversion of tundra to woody arctic shrubland will affect the carbon cycle and surface energy budgets, with an emphasis on the relative importance on these budgets.
Within the context of our longer term efforts to understand how and why permafrost degrades so rapidly in CCSM, we are conducting a series of prescribed snow experiments that will isolate how projected changes in snowfall, snow depth, and snow-season length affect the rate of soil temperature change in the model.
We also intend to continue to evaluate the climate response to Arctic sea ice loss, with a particular emphasis on the seasonal atmospheric, oceanic, and terrestrial response. The primary goal of this proposed project is to investigate the mechanisms underlying the seasonal response of the climate system to Arctic sea ice loss within the context of anthropogenic climate change. Additional studies on the marine ecosystem response to a changing sea ice melt season length are also planned.
The mechanisms involved in the relative stability of the Antarctic sea ice are also the subject of ongoing investigation. This includes studies on the influence of changing sea ice buoyancy forcing for ocean conditions and work that investigates the different sea ice behavior and feedbacks in the two hemispheres.
Megacities Impacts on Regional And Global Environments / Megacity Initiative: Local And Global Research Observatories (MIRAGE/MILAGRO)
MIRAGE is a NCAR Strategic Initiative designed to improve the understanding, numerical modeling, and predictability of the chemical and physical processes that occur when urban plumes are dispersed over larger geographic regions. Future urbanization of the global atmosphere could have wide-ranging consequences for human health and cultivated and natural ecosystems, visibility degradation, weather modification, changes in radiative forcing, and tropospheric oxidation (self-cleaning) capacity.
In the first phase of MIRAGE, NCAR organized and led a large field campaign to examine the atmosphere in and near Mexico City, with coordinated aircraft and ground-based measurements supported by extensive modeling and satellite observations. More than 400 researchers from 8 countries and 50 universities (35 U.S.), government labs, and other institutions participated in the intensive observational period during March 2006. Many others are collaborating in the ongoing analysis, interpretation, and modeling of the results.
ACD scientists plan to continue the analysis, interpretation, and modeling of the 2006 Mexico measurements, in collaboration with researchers from academia and government labs. This work is proceeding along three broad lines:
- Characterization of the atmosphere in Mexico City and surroundings, including better quantification of different sources (urban, industrial, biomass burning, long-range transport), prevailing chemical regimes (VOC and NOx limitations), and radiative impacts of various aerosols.
- Model evaluation and improvement, particularly for the state-of-the-art chemistry transport models WRF-Chem (regional) and CAM-Chem (global). The Mexico data base is being used extensively in this activity. Areas of critical uncertainty include the formation of secondary aerosols, budget and partitioning of nitrogen oxides, daytime and nighttime boundary layer processes, plume transport, and interactions with background air. We are also using detailed process models to examine some of the more problematic processes, including the SGMM model for the formation of organic aerosols, and the TUV model for radiative closure at visible and ultraviolet wavelengths.
- Coordination and synthesis of the community-wide effort in data analysis and modeling.
Concurrently, we are planning to extend the MIRAGE initiative to other megacities that are representative of specific environments and economic development. In view of rapid growth in China and India, we are focusing our next efforts on these regions (Figure 1). We have established a collaboration with the Shanghai Meteorological Bureau to implement the WRF-Chem model for their region and to participate with chemical instrumentation in ground-based field campaigns scheduled for September 2009 and (tentatively) May 2010. This will allow us to evaluate and improve models in a very different environment (relatively shallow PBL, humid-polluted, with large regional contributions) with different impacts on oxidant formation, aerosol composition and sizes, radiative budgets, and pollution-cloud interactions. ACD will bring to the field campaigns instruments to measure NOx, O3, VOCs, and spectral actinic fluxes, and will lead an intercomparison with instruments currently used by Chinese scientists, in order to help build capacity and enhance the quality of their long-term data collection. Longer-term plans include an aircraft campaign (likely with extensive ground-based components), carried out in India. This campaign would aim at investigating convectively influenced outflow of polluted air from a mega-city. Aspects of convective outflow are the processing of air masses in clouds, the modification of local convection and weather by the pollution emitted from the mega-city itself, and the convective lifting of the pollution into the upper troposphere with its implications for global transport of pollution and its impact on UT/LS chemistry. This work is funded by NSF/NCAR.
Biosphere-Atmosphere Exchange of Aerosols with Cloud, Carbon, and Hydrologic cycles including Organics and Nitrogen (BEACHON) program objectives and plans
The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen (BEACHON) project is a broadly collaborative and interdisciplinary research effort being developed by researchers at NCAR and the university community. BEACHON will improve predictability of earth system behavior over the time scale of months to a decade based on a better understanding of the coupling between water, energy and biogeochemical cycles in a multi-scale modeling framework. This will be accomplished through coordinated modeling, observations and process studies that explicitly address the coupled water, energy and biogeochemical cycles at multiple temporal and spatial scales. The main goal of the BEACHON project is to provide a detailed and quantitative characterization of biosphere-hydrosphere-atmosphere interactions and to use that characterization to improve regional and global models of the earth system. A major focus will be measurement and interpretation of surface fluxes of energy, aerosols, CO2, water, and organic and nitrogen compounds. Investigations will also address other fundamental processes including atmospheric aerosol production and growth processes, oxidant and cloud processes; landscape heterogeneity impacts on hydrological and biogeochemical cycles; and the response of ecological, hydrological, and physiological processes to ecohydrological disturbances. BEACHON is focusing on water-limited ecosystems but will work with international programs, such as IGBB-iLEAPS (see www.ileaps.org), to compare and contrast over a large range of water availability.
ESSL scientists detected the first significant fluxes and concentrations of the signaling compound, methyl salicylate (MeSA), in a real forest atmosphere using micrometerological techniques in combination with highly sensitive mass spectrometry. These observations during the BEACHON CHATS study show that MeSA can make a major contribution to the total biogenic volatile organic compound (VOC) flux from a forest comes as a surprise. This finding may help to explain previous studies indicating that there is a missing source of biogenic VOCs. Due to the semivolatile nature of MeSA and similar plant hormones, these findings will influence future directions on biogenic VOC research in atmospheric sciences, for example the impact on secondary organic aerosol formation. The results also provide tangible proof that plant to plant communication occurs on the ecosystem level and connect two separate scientific communities on plant volatile research that have coevolved over the past years: One has historically focused on biogenic VOCs important for atmospheric chemistry (e.g. isoprene, monoterpenes); the second community has targeted the ecology of plant volatiles (e.g. floral scents). We expect this study will transform the research approaches of these separate scientific communities and result in more integrated and multi-disciplinary studies.
An initial BEACHON workshop was organized in FY08 for the purpose of developing a detailed implementation plan and was attended by more than 100 participants including more than 50 atmospheric scientists, ecologists and hydrologists from the university community. The workshop and following discussions defined four major scientific themes for the BEACHON project and identified NCAR and university leaders:
Biogenic aerosol, clouds, and water availability
Eco-hydrological disturbances: bark beetles and forest fires
Moisture and nutrient limitations of eco-hydrological processes
Landscape heterogeneity: topography and canopy structure
More than 50 scientists, including representatives of 12 universities, participated in the initial BEACHON study (July-September 2008) at the U.S. Forest Service Rocky Mountain Research Station MEF observatory (see www.tiimes.ucar.edu/beachon/srm/activities.html). Investigations focused on:
- The formation and growth of atmospheric nanoparticles
- Emissions and reactions of BVOCs and other important trace gases and oxidants
- The formation and hygroscopic properties of biogenic SOA and primary particles
- Terrestrial and canopy controls on the emission and deposition of water, nutrients and aerosols
- Characterize transport in canopy and from canopy to cloud
- Modeling of Aerosol-Cloud-Emissions Interactions
BEACHON FY09 objectives include the development of specific scientific and implementation plans for each of the four scientific themes. These efforts will include instrument and model (1D, LES and regional) development and application, long-term observations and intensive campaigns at BEACHON field sites, and process studies in NCAR and university laboratory facilities. FY09 activities will also include analysis of the CHATS and MEF-08 field study data. BEACHON instrument development will include airborne and tower flux measurements systems, Time-of-Flight Chemical Ionization Mass Spectrometry systems (TOF-CIMS), and aerosol measurement systems. Model development will include significant advances in land surface model parameterizations and implementation of improved biogenic emission, aerosol formation and growth, and cloud microphysics in the WRF regional model. Field observations will be used to characterize the processes controlling biosphere-atmosphere exchange and to evaluate model simulations of these processes. Laboratory studies will emphasize observations that will improve quantitative descriptions of atmospheric and ecological drivers (e.g., drought stress, insect infestation, climate driven ecosystem changes, solar radiation, and temperature) of biogeochemical cycling and their impact on atmospheric distributions of trace gases and aerosols.
As is clearly articulated in the Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC, 2007), there is increasingly strong motivation to examine terrestrial and oceanic carbon fluxes on regional to continental scales, to understand the coupling of the carbon cycle to the climate system and to other biogeochemical cycles, to understand the processes responsible for present uptake of anthropogenic carbon, to predict future trends in these fluxes under various climate change scenarios, and to assess potential strategies for increasing carbon uptake and storage into the future. The challenges of scaling up from local measurements and scaling down from global constraints are being addressed in TIIMES through the development and application of advanced observational and modeling tools.
As highlighted by the report of Stephens et al., (2007), there exists less than 100% uncertainty in localizing the terrestrial uptake of anthropogenic carbon to specific latitudinal zones and this uncertainty is directly linked to vertical transport biases in the coarse-resolution atmospheric transport models used in CO2 inversion studies. Comprehensive measurements of atmospheric CO2 and related tracers, particularly at altitude and in previously undersampled regions, are needed to challenge these models and improve our understanding of global carbon cycling. This is the aim of the project HIAPER Pole-to-Pole Observations of Atmospheric CO2 and Related Tracers (HIPPO), a Harvard-NCAR-Scripps-NOAA collaboration to measure cross sections of atmospheric concentrations approximately pole-to-pole, from the surface to the tropopause, five times during different seasons over the next three-years (see Figure 1).
A comprehensive suite of tracers of the carbon cycle and related species will be measured: CO2, O2:N2 ratio, CH4, CO, N2O, 13CO2:12CO2, H2, SF6, COS, CFCs, HFCs, HCFCs, black carbon, and selected hydrocarbons. HIPPO will transect the mid-Pacific ocean and return over the Eastern Pacific. The program will provide the first comprehensive, global survey of atmospheric trace gases, covering the full troposphere in all seasons and multiple years. HIPPO will quantify the sources of major carbon cycle and greenhouse gases by region at the global scale. Hypotheses to be tested include, as examples:
Northern mid-latitude terrestrial ecosystems are a major sink for CO2
The Southern Ocean is a major sink for CO2 and the driver for global seasonality of the O2:N2 ratio
Amazonia is a major source region for CH4, CO, and N2O
Upper tropospheric data can be used to challenge models used to derive inverse analysis of the global carbon cycle
TIIMES scientists are supporting several key systems on these flights, including the NCAR Airborne Oxygen instrument (AO2, see Figure 2) and the MEDUSA flask sampler. These systems flew during the START-08/pre-HIPPO campaign in April-June of 2008. In this campaign, AO2 made the first successful airborne measurements of oxygen variations. This vacuum-ultraviolet absorption instrument is based on an existing NCAR laboratory instrument, but has been designed specifically for airborne use to minimize motion and thermal sensitivity and with a pressure and flow controlled inlet system. AO2 has a precision of +/- 2 per meg on a 4-second measurement which is the equivalent to detecting the removal of one O2 molecule from 2.5 million molecules of air. Such measurement are very useful in discriminating various influences on atmospheric CO2 (see Figure 3). The flasks collected during this campaign are being analyzed at Scripps for O2, Ar, and isotopes of CO2. Both AO2 and MEDUSA are planned for long-term availability to community researchers.
Stephens et al., Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO2, Science, 316, 22 June 2007.
Chemicals produced by the biosphere include volatile compounds that are emitted into the air where they can have a substantial impact on the chemistry of the atmosphere. These biogenic gases are dominated by volatile organic compounds (VOCs) both in total mass and number of compounds. The important role of biogenic VOCs in controlling global oxidant (e.g., OH and ozone) distributions has been demonstrated using global models while regional air quality models have shown that it is necessary to included biogenic VOC emissions in modeling efforts to develop regional ozone pollution control strategies. ACD and TIIMES scientists are investigating the processes controlling biogenic emissions and their role in tropospheric photochemistry and are developing numerical schemes for including this information in regional and global earth system models.
Observations of OH and other trace gases at rural and remote sites suggest that OH losses are considerably higher than what can be accounted for by the measured OH sinks. This may indicate that atmospheric chemistry models are missing some of the chemical species that are needed to accurately describe photochemical processing. In FY2008, ACD/TIIMES scientists collaborated with Prof. Yoshizumi Kajii's research group from Tokyo Metropolitan University to investigate OH lifetime and sinks associated with plants growing in NCAR foothills laboratory growth chamber and at the ESSL BEACHON research site in the Manitou Experimental Forest. Emissions and ambient concentrations of a wide range of compounds were measured to quantify their contributions to OH loss. The initial results suggest that observed compounds in the ambient air can account for some, but not all, of the missing OH sinks. The compounds emitted from enclosed vegetation can, at least in some cases, explain the observed OH reactivity (see Figure 1).
Tropical landscapes are thought to be responsible for about 80% of global biogenic VOC emissions and yet are among the least understood. Global chemistry and transport models often perform poorly when using the biogenic VOC emission rates recommended by current emission models. This could be due to uncertainties in emissions but could also be a result of inaccurate characterization of boundary layer meteorology and/or chemistry. ACD and TIIMES scientists participated in the January-March 2008 AMAZonian aerosol characterization experiment (AMAZE) in central Amazonia along with an international team of scientists that included Scot Martin (Harvard University), Jose Jimenez (U. Colorado) and Tony Prenni (Colorado State University). The NCAR scientists quantified the magnitude and variation of VOC, ozone and NOx fluxes and concentrations during the study and are using these observations to understand the processes controlling aerosol formation and growth in this region. ACD and TIIMES scientists also participated in the Oxidant and Particle Photochemical Processes (OP3, see http://badc.nerc.ac.uk/data/op3/) study in the south-east Asian tropical forests of Borneo. NCAR scientists measured biogenic VOC emissions using enclosures and tower based measurements.
FY2009 work will continue laboratory and field investigations of factors affecting biogenic emissions and their impact on oxidants. This work was funded by NSF/NCAR and EPA.
Laboratory Kinetics Studies
ACD scientists have been studying the reaction kinetics and mechanisms of hydroperoxy radicals, HO2, using a number of complementary techniques. HO2 radicals are a member of the HOx family, which is comprised of OH and HO2. Hydroxyl radicals, OH, are responsible for the oxidation of many organic and inorganic pollutants which are emitted into the troposphere. Following a series of reactions, HO2 is usually produced, and then cycled back to OH by its reaction with nitric oxide, NO. However, measurements of radical concentrations in forested areas have shown unexpectedly high levels of OH, suggesting that some recycling of radicals is taking place. Normally this requires the presence of NO, but in clean areas other mechanisms may dominate.
The reactions of HO2 radicals with organic peroxy radicals are being investigated in the laboratory using a combination of techniques (infrared spectrometry, gas chromatography and high performance liquid chromatography). These reactions were initially thought to form solely organic hydroperoxides, reasonably stable compounds that terminate the oxidative chain reactions. However, in a 2004 study in collaboration with Fresno State University [Hasson et al., 2004] it was shown that OH radicals could be produced in substantial yield from the reaction of HO2 with acetyl peroxy radicals.
HO2 + CH3C(O)O2 -> CH3CO2 + OH + O2
The potential for recycling of HO2 to OH in these reactions may be able to explain apparently high levels of OH radicals in relatively clean, forested areas. A systematic study of the OH yields from reaction of HO2 with oxygenated peroxy radicals is currently underway.
In the lower stratosphere, OH and HO2 radicals play a large role in controlling the concentration of ozone.
OH + O3 -> HO2 + O2
HO2 + O3 -> OH + 2O2
These oxidation chains can be terminated via the self reaction of HO2 radicals, which forms hydrogen peroxide as a product.
HO2 + HO2 -> H2O2 + O2
The rate coefficient for this particular reaction depends on both the temperature and pressure, and water vapor concentration. Scientists in ACD have measured the variation of the rate coefficient with temperature at low pressure. The HO2 radicals are produced by a pulse of ultraviolet laser radiation, which dissociates chlorine gas; the chlorine atoms then initiate the chemistry which forms the HO2. The radicals are then detected using tunable diode laser spectroscopy. The technique allows the decay of the radicals to be followed in real time over a time span of tens of milliseconds, from which the rate coefficient can be determined.
There have been 4 previous determinations of the temperature dependence of the rate coefficient at low pressure. Whereas 3 measurements in the 1980s (Thrush and Tyndall, 1982; Kircher and Sander, 1984; Takacs and Howard, 1986) showed a dependence on temperature, a more recent one (Christensen et al, 2002) suggested that the reaction rate is independent of temperature. However, the new measurements indicate an increase in the rate coefficient at low temperature, in agreement with the earlier studies. A stronger temperature dependence to this reaction reduces the concentration of HO2 radicals in the upper troposphere and lower stratosphere, and consequently reduces the amount of ozone depletion due to HOx radicals.
Laboratory studies of reaction kinetics and mechanisms will continue, with a focus on the formation mechanisms of organic nitrates. This work was funded by NSF/NCAR and NASA.
As described by Friedlingstein et al. (2006), feedbacks between the climate and the carbon cycle have the potential to modify, perhaps enhance, climate change. In ESSL, we study these feedbacks to help understand projections of future climate. Our primary tool is CCSM, with the addition of parameterizations of terrestrial and marine biogeochemical cycles. The work happens within the context of the Biogeochemistry Working Group, whose overall goal is to improve our understanding of the interactions and feedbacks between the physical climate and biogeochemical systems under past, present and future climates.
CCSM scientists continued to develop models of the global carbon cycle, including oceanic and terrestrial ecosystems. Model experiments found that the coupling between the terrestrial carbon and nitrogen cycles significantly alters the carbon cycle-climate feedback. The nitrogen cycle leads to increased carbon storage on land under radiatively-forced climate change, and an overall negative climate-carbon cycle feedback. The primary mechanism responsible for increased land carbon storage is fertilization of plant growth by increased mineralization of nitrogen directly associated with increased decomposition of soil organic matter under a warming climate.
Additional analysis of these experiments has revealed deficiencies in the oceanic uptake of trace gases, including anthropogenic CO2 and CFCs, a class of purely anthropogenic chemicals that are well sampled in the ocean. In order to address this model bias, oceanic mixing parameterizations have been enhanced to include processes that were previously omitted. These efforts have significantly reduced the bias in CFC uptake and will presumably reduce the bias in anthropogenic CO2 uptake as well.mes more computational resource than the 2 degree run. The 0.5 degree version of the new CCSM 4 will be used to make the short-term climate projections for the next IPCC assessment.
2009 and Beyond
Continued development of the terrestrial carbon cycle model, including additional capabilities to simulate dynamic vegetation, anthropogenic land cover change, croplands, wildfire, and biogeochemical cycles is currently underway.
As the model matures, a primary scientific theme to examine will be natural and human-mediated changes in land cover and ecosystem functions and their effects on climate, water resources, and biogeochemistry.
Large-Eddy Simulation (LES) has been widely used to examine turbulent processes in the PBL, however most LES applications have been limited to PBLs over horizontally uniform surfaces or idealized strip-like heterogeneous surfaces, which are uncommon in nature. The most logical way to expand our LES study for more complex and realistic PBLs is by nesting an LES code inside a regional mesoscale model. However, nesting a turbulence-resolving model (like LES) inside a turbulence-parameterizing model (like mesoscale or climate models) is difficult because at the nest boundaries the simulated flow fields change abruptly from non-turbulent to turbulent flows, or vice versa. We plan to explore the capability of nesting WRF-LES inside WRF-mesoscale domains.
Another complex and important PBL regime is the one under cumulonimbi. We know precipitation can generate cold pools in the PBL due to evaporation, but how these cold pools affect turbulent transport in the PBL and how the change in the PBL affects the life cycle of deep convection remain uncertain. As part of the research funded by the new NSF Science and Technology Center for Multiscale Modeling for Atmospheric Processes (CMMAP), which is based at Colorado State University and led by Dave Randall, we began exploring the interaction between deep convection and the PBL. A benchmark LES of a tropical deep convection system (over a domain of about 205 km x 205 km x 27 km with a grid mesh of 100 m x 100 m in the horizontal and 50-150 m in the vertical) was performed by a CMMAP colleague (Marat Khairoutdinov at the Stony Brook University) in which deep and shallow convection, as well as energy-containing turbulent eddies are all resolved. A spectral analysis (Fig. 1) of the benchmark flow in the PBL shows that the vertical-velocity w variance peaks at the energy-containing turbulence scale (which is about 500 m, the average PBL depth), the moisture q variance peaks at the cloud-system cold-pool scale (on the order of 50 km), and the co-spectrum of w and q peaks at both scales. A systematic application of a low-pass filter separates the flow field into cloud-system scales and turbulence scales, and shows that about half of the moisture flux in the PBL is carried by turbulent motions smaller than 1-2 km and the other half by cloud-scale motions. The goal of this work is to find a way to represent the effects of turbulence inside a cloud-system-resolving model that has a grid mesh on the order of 1-2 km.
The effects of aerosols on climate represent the single largest source of uncertainty in our understanding of global warming. In order to reduce the uncertainty of the role of aerosols in climate and weather, ESSL scientists are conducting both experimental and modeling studies of aerosol formation, composition, and direct as well as indirect radiative forcing.
During 2008, ACD scientists collaborated with Peter McMurry (U. Minn.) to investigate the formation and growth of nanometer-sized particles in the atmosphere. These minute particles form from nucleation of low-volatility vapors, and grow by condensation by processes that are, as yet, not well understood. It's essential that we understand the processes by which particles form and grow in the atmosphere because nucleation is main global source of particle number, and subsequent condensational growth makes these particles important to the earth system by allowing them to be cloud condensation nuclei (CCN). Measurements performed at the ground site in Tecamac, Mexico, during the 2007 MILAGRO campaign suggest that current models that do not account for growth due to organic species most likely underpredict the growth of particles formed by nucleation, and thus underpredict the impact of new particle formation on climate. Figure 1 elucidates this point. The plot in Figure 1a shows a vigorous new particle formation event that occurred on March 16, 2007. The black trace on the plot shows the diameter analyzed using NCAR's Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), which can measure the chemical composition of size-selected particles as small as 8 nm. Figure 1b shows the results of TDCIMS measurements during the event with species broadly classified as sulfate, nitrate, and organics. The result is that about 84% of the detected ions are organic, comprising of organic acids, hydrocarbon-like species, and nitrogen-containing organic compounds. Particulate sulfate which arises from the condensation of sulfuric acid vapor constituted only 10% of the detected ions, and nitrate comprised 6%. Results such as those in Figure 1b suggest a prominent role of organic species in particle formation processes: unraveling the mechanism by which these organics create highly nonvolatile species through gas and particle phase chemistry is a major research goal in 2009.
During March - April 2008, ACD scientists led a field study to evaluate the chemical composition of the atmospheric aerosol, investigate primary biological particles in the atmosphere, and to explore relationships between the aerosols and clouds at a high altitude site. The Storm Peak Aerosol and Cloud Characterization Study (SPACCS) was funded in part by a unique two-year program, organized by scientists in ACD, to foster US/Nordic collaboration among early career scientists in the field of biogenic secondary organic aerosol.
Funded by: NSF, NOAA, USEPA, DOE, U. Nevada/DRI.
The propensity for deep, moist convection to organize and project onto larger spatial and temporal scales requires numerical simulations spanning convection-resolving scales to continental scales. Furthermore, simulation studies must be closely constrained by observational analysis of the organizing properties of convection. Prediction of tropical and warm-season higher-latitude convection, and the response of the synoptic-scale and planetary-scale flow is vital for increasing our ability to anticipate significant weather events more than a day in advance. It is also vital for credible representations by models of regional climate.
Last year, simulations of convection episodes in E. Asia and the U.S. continued and simulations over Africa commenced. Initial results suggest that the Advance Research WRF was able to simulate the proper modes of African convection, though the latitude of maximum convective activity was biased southward during a 12-day simulation. Simulations of convection over North America, forced by only the mean diurnal cycle on the boundaries, showed a reasonable representation of a corridor of propagating convection with essentially no mesoscale perturbations in the initial state. An improved double-moment microphysics algorithm was tested in ARW and showed improved simulations of the trailing stratiform region of MCSs.
The Taiwan Island Mesoscale Rainfall Experiment (TIMREX) was conducted and obtained data on several cases of heavy coastal rainfall in Taiwan. Theoretical research on orographic rainfall showed a strong dependence of rainfall location on the impinging flow speed (FIG XX), but not on the nondimensional mountain height. Simulations of maritime convection in an idealized context showed the organized convection occurs over the gradient in sea-surface temperature, not where the temperature is greatest.
In the coming year, simulations of TIMREX cases will occur, focusing on mechanisms determining whether rainfall occurs on the coastal plain or over higher terrain. Analysis of TIMREX dropsonde and rawinsonde data will be performed to verify numerical simulations and enhance the thermodynamic interpretation of heavy rainfall cases. TIMREX work will also integrate the recent theoretical work on orographic rainfall. Simulations of convection episodes over North America and Africa will continue, emphasizing the diurnal cycle in the ARW model.
Air-sea interaction occurs over a wide spectrum of scales ranging from millimeters (spray droplets and air bubbles) to hundreds of kilometers (synoptic-scale storms) and even larger (global climate). A goal of marine surface-layer research is to identify and quantify coupling mechanisms that connect the atmospheric and oceanic boundary layers (the ABL and OBL) and surface waves. Some of the specific problems of interest in the ABL include the effects of wave age, swell, surface roughness, and wind-wave misalignment. In the OBL, waves may induce mean currents and turbulence. Wave influences on the OBL are of particular importance under high-wind conditions. Turbulence-resolving simulations, and in particular large-eddy simulation (LES), with its ability to perform systematic process studies, play an important role in air-sea interaction research. LES has provided new insights into the couplings between imposed ocean waves and turbulence.
In the past year, MMM air-sea interaction research was directed towards developing LES models of the marine atmospheric and oceanic boundary layers applicable to high-wind regimes. For the atmospheric boundary layer MMM scientists began building an LES model to simulate turbulent flow over a general time-dependent moving wavy surface, i.e., over a spectrum of imposed surface waves. This algorithm is a generalization of the scheme used previously to simulate turbulent flow over 2-D waves. The wave field imposed at the lower boundary of the computational domain is constructed from a general set of plane waves; the wave heights can be chosen to match empirical spectra, e.g., the equilibrium Pierson-Moskowitz wave height spectrum, or can be smoothed wave-height measurements. The future turbulence simulations will be computationally intense as they require fine-mesh resolution and small time steps to resolve both small and large-scale surface waves. The basis of the wavy surface LES is a recently developed massively parallel algorithm. This parallel code, which utilizes a 2-D domain decomposition based on the Message Passing Interface, has been exercised on different machine architectures and can use as many as 16,384 processors (see Figure AA). The long term goal of the research is to utilize LES with resolved surface waves to help in the interpretation of observations collected in the High-Resolution Air-Sea Interaction (HRES) field campaign. Of particular interest is the pressure distribution over a spectrum of waves and the identification of the scales that support momentum transfer from the atmosphere to the ocean as a function of wind speed and wave age. HRES is sponsored by the Office of Naval Research and is tentatively planned for Spring 2010 off the California coast.
Anthropogenic changes in "greenhouse gases" can increase temperature in the lower atmosphere due to their ability to absorb infrared radiation, but also affect the upper atmosphere through radiational cooling. Since the prediction by Roble and Dickinson  that a consequence of increasing CO2 levels would be to decrease the temperature of the upper atmosphere, NCAR researchers have been studying this effect and its consequences for the neutral density of the near-Earth space environment.
The thermosphere goes through natural, cyclical changes in density driven by the Sun's 11-year activity cycle, heating and expanding at solar maximum, cooling and contracting at solar minimum. Detection of the smaller and more gradual changes occurring due to increasing anthropogenic emissions in this context is therefore difficult, but during the last several years, three different groups have been able to measure thermospheric density changes by observing the effect of atmospheric drag on satellite orbits. These studies agreed that thermospheric density is systematically decreasing by several percent per decade near 400 km altitude, in general agreement with simulations conducted at the NCAR High Altitude Observatory using an extended and updated version of the Roble and Dickinson model. The modeled secular change under solar minimum and solar maximum conditions is variable, with larger trends at solar minimum and smaller change during solar maximum, and there is now observational support for these systematic differences.
A key question for thermosphere/ionosphere physics is how these changes in the thermosphere affect ionospheric densities and dynamics. Observations of secular trends in the E and F1 regions of the ionosphere indicate that electron densities have increased, and that the height of the E-region peak has decreased, during the past several decades. Detection of trends in the upper ionosphere through analysis of F2-layer parameters has been more complex and controversial. In order to facilitate observational detection of long-term trends in the ionosphere, simulations were performed by Qian et al.  using CO2 concentrations for the year 2000 and projected for the year 2100. Results show that increased CO2 concentration increases electron density in the lower regions of the ionosphere, but decreases electron density in the upper ionosphere. The transition altitude occurs slightly below the F2 peak altitude. The proximity of this peak to the transition altitude may explain why different analyses of long-term trends in F2 peak density have shown both positive and negative trends.
The impacts of climate and weather on society and ecosystems: Climate change - probabilistic climate change, and solar forcing of climate
NCAR was one of the first centers to study anthropogenic climate change with global coupled climate models starting in the late-1970s. Consequently, the earliest climate change experiments done at that time were pioneer projects at a national and international level. Few groups were doing climate change modeling as it was considered to be a sidelight to other more highly regarded modeling problems. NCAR climate change modeling (funded by DOE and NSF) was prominent in the DOE State-of-the-Art climate change assessments in the late 1980s, and in the first IPCC assessment in 1990 and the 1992 IPCC update since only four groups in the world (including NCAR) had functioning global coupled climate models that were being used for climate change projections.
Since then, climate change modeling has become a very prominent activity at NCAR, most recently through the Community Climate System Model effort, and is now a headline activity for ESSL. As climate change modeling evolves to include more complexity, we are moving toward an earth system model-type activity. This crosses division boundaries in ESSL and requires close cooperation with the other science divisions since such earth system models will include not only the basic atmosphere-ocean-land surface-sea ice global coupled model, but also components of chemistry, aerosols, dynamic vegetation and carbon cycle.
CGD scientists and collaborators have been involved with research that has directly influenced and characterized national and international assessment activities. For example, there has been a growing awareness that anthropogenic black carbon aerosols, with their properties of both absorbing and reflecting solar radiation, may be contributing to significant climate change in the Indian monsoon region of south Asia. To address this problem, a six member ensemble of 20th century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model was analyzed to study the effects of black carbon (BC) aerosols on the Indian monsoon. The BC aerosols act to increase lower tropospheric heating over south Asia and reduce the amount of solar radiation reaching the surface during the dry season. The increased meridional tropospheric temperature gradient in the pre-monsoon months of March-April-May, particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months (Fig. 1). With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau (Fig. 2). During the summer monsoon season, the model experiments showed that BC aerosols have likely contributed to observed decreasing precipitation trends over parts of India, Bangladesh, Burma, and Thailand.
2009 and Beyond
Future research priorities in climate change modeling include further studies of extremes, studying decadal variability including understanding the relative contributions of inherent decadal variability and forced response in 20th century climate change, and mitigation scenario simulations of future climate change.
Over the last forty years, atmospheric research has shown that land-atmosphere
coupling is of critical importance for weather and climate prediction. Due
to its diverse nature, a tall canopy's influence on turbulent exchange is
extremely complex, e.g., because of their spatial distribution, seasonal variability,
flexibility, porosity, etc. Within the layer of the atmosphere directly influenced
by the canopy, turbulence varies dramatically depending on the detailed structure
of the roughness elements and cannot be described by traditional similarity
relationships. Where vegetation covers the surface, it becomes the important
momentum sink and a key player in distributing sources and sinks of moisture,
heat and trace atmospheric constituents. Parameterization of turbulent transport
in and above tall canopies remains somewhat elusive but is essential for accurate
weather and climate prediction. Large-eddy simulation (LES) is an important
tool for studying the coupling between microscale and mesoscale motions. LES
can also incorporate the influence of vegetation on momentum, energy, and
scalars. Because observing three-dimensional and time-dependent fields of
all quantities of interest is difficult, LES has become a direct link between
currently observable quantities and larger-scale models which are forced to
parameterize all of the turbulence.
LES needs to be validated and improved to deal with complex flows, especially for surface layers where dependence on the subfilter-scale (SFS) model increases. To address this issue, NCAR, in collaboration with several university groups, carried out three pioneering observational studies to improve subfilter-scale parameterizations over flat terrain with short sparse vegetation (Horizontal Array Turbulence Study, HATS, over the ocean (Ocean HATS; OHATS), and within and just above a tree canopy (Canopy HATS, http://www.eol.ucar.edu/rtf/projects/CHATS/isff/). These studies provide an observational basis for testing and improving closure approximations used in LES and they have substantially increased our confidence in parameterizations developed using LES as their basis.
At this point, the character of within-canopy SFS motions is not known, nor the role that the eddies shed in the lee of the plant elements play, nor how these wake-scale motions affect scalar and momentum transport. CHATS has provided detailed measurements of SFS variables in a complex environment linking the biosphere, geosphere, and the atmosphere that will allow study of the fundamental interaction between vegetation and atmospheric turbulence, and validation of currently utilized SFS models and improvement of parameterizations representing this critical regime.
Analysis currently underway includes: studying the impact of vegetation on sub-filter scale momentum/scalar fluxes and dissipation as a function of stability; establishing whether pressure correlates with canopy-scale coherent structures and evaluating the pressure destruction term in the scalar-flux equation; investigating heat storage within the canopy and the time-evolution and vertical variation of within-canopy stability; estimating horizontal length scales at the canopy-top using both helicopter and sonic-anemometer array data; studying canopy and stability influences on turbulent diffusion; and investigating sub-canopy processing and transport of biogenic reactive species (relate leaf-level to above-canopy fluxes); and exploring fine-scale turbulent coherent structures above the canopy using the Raman Eye-safe Aerosol Lidar (REAL) that was deployed in CHATS to delineate boundary-layer structure and motion. These studies will continue into the following year and beyond.
Trace gas and aerosol emissions into the atmosphere are the major drivers of the chemical composition of the atmosphere. There is widespread concern about the effect of human activities on these emissions and their impact on atmospheric chemical composition. Changes in human activities are the underlying cause of the current increase in pollutant levels on regional and global scales. In some cases, changes in trace gas emissions are due to obvious pollutant sources including many technological sources. Other sources, including biomass burning and biogenic sources, have a natural component but are strongly influenced by humans. In order to understand these increases and to predict future changes, ACD/TIIMES scientists are quantifying emissions from various sources and improving our understanding of the natural and human influenced processes that control emissions.
ACD/TIIMES scientists have completed a new version of the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is a modeling system for estimating the net emission of gases and aerosols from terrestrial ecosystems into the atmosphere. It is driven by landcover, weather, and atmospheric chemical composition. MEGAN is a global model with a base resolution of ~ 1 km. A stand-alone version of MEGAN is now available on the NCAR community data portal during the past year and has already been downloaded by > 100 users from more than 20 countries. MEGAN has also been incorporated as an on-line component of several regional and global models including MOZART, CCSM-CLM, GEOS-CHEM and WRF-CHEM. Continued development of MEGAN has resulted in a version that includes a detailed canopy environment model that will enable more realistic estimates of the response to landcover and climate change. MEGAN regional and global estimates of isoprene emissions are being evaluated by comparisons with satellite observations of HCHO which is a product of isoprene oxidation.
ACD/TIIMES scientists have also continued to improve a North American wildfire emission model and have used the model to forecast fire emission estimates for the NCAR MIRAGE field campaign. The model estimates daily emissions from fires for all of North America at a 1km resolution. More recently, emissions of mercury were included in the fire emissions model, and the first, state- and monthly resolved mercury emission estimates from fires have been produced. ACD/TIIMES scientists teamed with Jason Neff (U. Colorado) to quantify monthly, state-level CO2 emissions from fires and discuss the potential policy implications of these emissions (see figure).
FY2009 work will include continued improvements of MEGAN and the fire emissions model and enhanced support for the communities using these models. In addition, efforts will be focused on evaluating the model results and quantifying model uncertainties. The emission models will be used in regional chemical transport modeling studies to investigate the radiative impact of aerosols from fires and biogenic sources, interactions between direct particulate fire emissions and secondary organic aerosol formation, and mercury deposition. This work is funded by NSF/NCAR and EPA.
As part of the Water Cycle Program, which involves scientists across ESSL and other national lab and university colleagues, various aspects of the water cycle have been examined in both in observations and climate models. The focus is on precipitation, atmospheric water vapor, and land surface water fluxes, with the goal to improve our understanding and thus modeling and prediction of atmospheric moist convection, precipitation processes, and land surface hydrology on a broad range of time scales. The diurnal cycle of warm-season precipitation over the U.S. and other parts of the world has been employed as a means to systematically examine precipitation characteristics (onset, diurnal timing, frequency, intensity, duration, amount, type, etc.) in data and models, thus allowing a diagnosis of deficiencies in weather and climate models.
The Water Cycle Program also interacts with other ESSL programs such as the Biogeoscience Program - as well as leverages other NSF, NASA, and NOAA-funded studies related to the water cycle. dData sets and model evaluation work produced under this project are helpful for improving the Community Climate System Model (CCSM) and other climate models.
Recent Water Cycle accomplishments include 1) quantifying the temperature and pressure dependence of the snow-rain phase transition (Figure); 2) determining water and energy budgets in hurricanes; 3) analysis of the hydrological effect of Mt. Pinatubo's volcanic eruption and its resultant implications for geo-engineering solutions to global warming; 4) creation of an updated, global data- set of continuous, monthly river outflow for community use in quantifying decadal and long-term changes in continental freshwater discharge; and 5) analyses of satellite-observed and model-simulated precipitation and other hydrologic fields. These studies have resulted in a number of refereed publications.
For example, the phase of precipitation is important for weather forecasts, land hydrology and remote sensing. To quantify the temperature and pressure dependence of snow frequency (F, in %) when precipitation occurs, we have analyzed 3-hourly weather reports of surface air temperature (Ts, °C) and pressure (Ps), and snow and rain occurrences from over 15,000 land stations and available ship observations from 1977 - 2007. It is found that the phase transition occurs over a fairly wide range of temperature from about -2°C to +4°C over (low-elevation) land and -3°C to +6°C over ocean. The F-Ts relationship can be represented by a hyperbolic tangent: F(Ts) = a [tanh (b (Ts - c)) - d], with the slope parameter b close to 0.7 over land and 0.4 over ocean. The pressure-dependence is only secondary and reflected in the parameters. Results show that snow occurs often (F > 50%) for Ts = 1.2°C over land and Ts = 1.9°C over ocean, and are non-negligible (F > 5%) for Ts = 3.8°C over land and Ts = 5.5°C over ocean. This "warm bias" results from the falling of snowflakes into warmer surface layers, which is especially true over ocean. The warm bias is higher when air pressure is below ~750 hPa because snow falls faster in thin air.
Other major findings from recent ESSL work include: a) changes related to human influences on climate since 1970 have increased sea-surface temperatures (SSTs) and water vapor, and this may have altered hurricanes and increased associated storm rainfalls, quantified to be 6-8% higher than the baseline b) major unintended adverse effects, such as reduced water resources and increased drought, may occur from proposed geo-engineering solutions to mitigate global warming through emulating volcanic eruptions by injecting large amounts of aerosols into the Stratosphere; c) continental freshwater discharge into global oceans shows a slight decrease from 1949-2004, in contrast to the notion that continental discharge has increased as the global hydrological cycle intensifies under global warming; and d). large increases in the discharge into the Arctic Ocean during 1949-2004 are not accompanied by increases in precipitation; instead, increased runoff resulting from melting of soil ice in Eurasia may be a significant contributor.
2009 and Beyond
Future plans call for further work to quantify characteristics of precipitation frequency and intensity using high-resolution satellite observations, to improve simulations of these two quantities in CAM by modifying the cumulus parameterizations, and to analyze more comprehensively ofthe potential impacts of global warming on atmospheric water vapor and snowpack over the Colorado headwater regions.
Representation of cloud microphysical processes in models of various complexity (from small-scale to global) is a challenging aspect of numerical weather prediction and climate modeling. This is mostly because of the disparity between scales at which cloud microphysical processes operate (i.e., millimeters and centimeters) and scales resolved by models and observations. With the advent of convection-permitting numerical weather prediction using the Weather Research and Forecasting (WRF) model and application of the superparameterization approach to climate modeling representation of cloud microphysics emerges as the next “key problem”, similarly to the “convection parameterization problem” in the past. The superparameterization approach to climate modeling is the focus of the NSF's Science and Technology Center for Multiscale Modeling of Atmospheric Processes (CMMAP) at Colorado State University. Several NCAR scientists are members of the CMMAP team and are actively involved in the CMMAP research.
MMM scientists have developed a new comprehensive double-moment bulk-microphysics scheme to represent warm-rain and ice processes. These schemes are designed to include information about the atmospheric aerosols that affect cloud formation, the cloud condensation nuclei (CCN) and ice-forming nuclei (IN). Much of the research to improve cloud microphysical schemes is driven by suggested effects of CCN and IN on weather and climate. These are referred to as the indirect aerosol effects. Their uncertain role in climate and climate change was highlighted by the 2007 report of the Intergovernmental Panel on Climate Change. The warm-rain scheme is currently being applied in simulations of shallow convective cloud fields based on observations in BOMEX and RICO. The goal is to investigate indirect aerosol effects in shallow tropical convection. The warm-rain and ice scheme was applied in a study concerning effects of aerosols on precipitation from deep organized convection. In general, aerosol effects on deep convection are difficult to assess because of complex interactions between cloud microphysics and cloud dynamics. As a first step, a 2D kinematic (prescribed-flow) model mimicking a mature squall line was combined with the double-moment microphysics scheme and applied in a large set of sensitivity simulations. For each set of model parameters (such as environmental sounding, convective/mesoscale updraft strength, collision efficiencies, etc), a pair of simulations was performed featuring CCN characteristics of either pristine or polluted environment. In general, results in each pair differ insignificantly (see the figure). These results imply that CCN can affect precipitation from deep organized convection only through the microphysics-dynamics feedback (i.e., affecting the flow pattern), which can only be investigated in the dynamic cloud model.
Work is in progress to include the new scheme in WRF model and to apply it to several observationally based test cases.
Covering 71% of the Earth, the oceans absorb the majority of the solar energy reaching the surface. The heat capacity of the upper three meters of the oceans exceeds that of the entire atmosphere, and the oceans contain approximately 50 times greater inventory of CO2 than the atmosphere. The phase change from liquid to vapor by evaporation at the sea surface is the dominant source of moisture to the atmosphere, and the phase change from liquid to solid in the formation of sea ice has a strong effect on the reflectivity of the Earth's surface. Ocean currents, accomplish roughly equivalent energy transport from the tropics to polar latitudes as the atmosphere, and the meridional transports of sea water and sea ice close the global water cycle.. Ocean currents and turbulent mixing also transport nutrients from the deep ocean to the sunlit upper layers to support marine ecosystems. Through this capacity for storing and transporting energy, water, and radiatively and biologically active chemical species, the oceans act to moderate, modulate, and initiate climate variability and climate change. Instances of abrupt climate change observed in the paleo-climate record and in simulations of future climates with increasing greenhose gases arise from the interactions of sea ice, the hydrologic cycle and the ocean thermohaline circulation. A comprehensive understanding of, and the ability to predict, the behavior of the climate system must therefore be based on an understanding of the physical, chemical, and biological processes operating in the oceans and their interaction with other components of the Earth system.
Through research to develop an understanding of ocean processes, and using this understanding in improving their representation in ocean models, ESSL ocean scientists support a broad spectrum of ESSL scientific objectives. These include: prediction of the Earth's energy, water and biogeochemical cycles, and understanding natural and human influenced climate variability, including high impact variations such as sea level rise. In turn, the ESSL objective of understanding two-way scale interactions within the Earth system is central to improving our understanding of how thermodynamic processes such as sea-ice formation and ocean circulation features such as coastal upwelling zones, western boundary currents, and meso-scale eddies are affected by and affect the basin- to global-scale ocean circulation and the large-scale climate system.
Significant increases in computational resources together with improved physics and greater confidence in CCSM climate models at both modest and high resolution have allowed ocean model developments to be evaluated in fully coupled models for their effects on the climate system as a whole. In a number of cases, these coupled model results have been much more profound and widespread than anticipated from consideration of effects on the ocean in isolation. The key factor is for the ocean model changes to produce small, but persistent, changes in near surface ocean temperatures or sea-ice coverage, then for the coupled model to react to these signals in such a way as to amplify the changes.
A concerted effort was made move the parameterization of diapycnal mixing (the small scale three dimensional turbulence across surfaces of neutral density) from ad hoc prescriptions to a more physically based foundation. A wide range of processes not explicitly represented in the ocean general circulation model need to be considered as potential sources of energy for this turbulence. New parameterizations of the deep ocean mixing from breaking internal tides and tidal mixing in shallow seas have been guided by offline calculations with tide models, theoretical developments, and observations. Incorporation of tidal mixing in the shallow seas of the Indonesian Throughflow region was found to improve the simulation of SST in that region with a large consequent impact on precipitation in this important region for generating atmospheric variability. Microstructure measurements of ocean turbulence and theory have revealed a systematic latitudinal dependence of the interior diapycnal mixing resulting from breaking internal waves, with a maximum in subtropical latitudes, decreasing towards both the equator and polar regions. Incorporating the theoretically predicted latitudinal dependence in CCSM3.5 shows that the changes in the mean ocean state project onto longstanding precipitation biases. Gravity currents entrain and mix with overlying waters in their descent of the continental slope from deep water formation regions in marginal seas to the abyssal ocean. A parameterization of this process previously developed specifically for the outflow from the Mediterranean Sea was generalized and applied to the key deep water passages connecting the North Atlantic with the Greenland-Iceland-Norwegian Seas. A new parameterization of mixing by sub-mesoscale eddies, a process connecting the largely isopycnal mixing by mesoscale eddies, with the diapycnal cross-frontal mixing in the surface boundary layer was incorporated and shown to improve the representation of ocean ventilation processes. The ability to investigate the impacts of these changes in interior mixing have required an increase in vertical resolution such that the intended variations are not overwhelmed by implicit numerical mixing in the advective transport algorithm.
Corresponding changes toward more physically based representations of subgrid scale thermodynamic and radiative processes in the sea ice model have been made. These include the incorporation of a new radiative transfer scheme that makes use of inherent optical properties to define the scattering and absorption properties for snow, sea ice and included absorbers. An explicit melt pond parameterization has been incorporated which relates the pond evolution to the surface ice and snow melt water flux. Additionally, aerosol deposition and cycling within the sea ice component for black carbon and dust species is now included. Taken together, these improvements provide a more sophisticated and complete sea ice albedo treatment.
The improvements achieved in the representation of ocean transport processes in the most recent versions of CCSM have motivated the decision to carry out fully interactive carbon cycle experiments at higher horizontal and vertical resolution than was used in CCSM3. The computational burden of running the higher resolution carbon cycle model thousands of simulated years in order to bring it into equilibrium remains a significant roadblock however. Mathematical techniques based on Newton-Krylov non-linear equation solvers are being investigated to circumvent the need to carry out an explicit time-dependent integration of the model and obtain the equilibrium solution directly.
2009 and Beyond
Nearly all of the ocean model developments completed and underway have been evaluated in a coupled climate context, but typically individually. The effort over the next year will be to bring them together to more fully understand their interactions and collective impact on the climate system. Comparisons with ocean tracers, both observed and as simulated by very high resolution models, will provide the metrics for judging the impact of these parameterizations on ocean transport and ventilation. Diapycnal mixing occurs on scales of centimeters to meters and will remain unresolved, and require parameterization, in global ocean climate models for many generations to come. However, coupled climate simulations that do resolve ocean mesoscale variability, on scales of 10s to 100s of kilometers are now on the horizon. Early results from prototype integrations in this class, as illustrated in the accompanying figure, reveal intriguing new classes of ocean-atmosphere interaction. The emerging availability of high resolution remote sensing products for ocean winds, surface temperature, and surface currents will facilitate an assessment of the performance of CCSM in this resolution regime, and the application of CCSM to furthering our understanding of the processes and scale interactions connecting the ocean mesoscale to the global climate system.
Work has continued on analysis of the chemical behavior of dimethyl sulfide (DMS) and scalar variance budgets in the marine boundary layer (MBL) using DYCOMS-II data from the NCAR C-130 aircraft, in collaboration with Ian Faloona (University of California, Davis). The scope of the effort has expanded somewhat to also include analysis of DMS and SO2 data from the PAcific Sulfur Experiment (PASE) carried out in summer 2007, also flown on the C-130 aircraft. Thus far, the effort is mainly concentrated on documenting the overall structure of the MBL in the equatorial region where PASE was conducted. This effort will continue during FY2009.
This past year, a study of vertical velocity w spectra in the convective planetary boundary (CBL) has been carried out using Doppler lidar data collected during the Lidars in Flat Terrain (LIFT) experiment over flat farmland in central Illinois during summer 1996. This is a continuation of previous analyses that dealt with the 2-point turbulence statistics of w. The NOAA High Resolution Doppler Lidar (HRDL) was pointed straight up for over 100 hrs, providing 11 different cases of a midday convective boundary layer. This takes advantage of the lidar’s capability to obtain range-resolved radial measurements, from which a two-dimensional field of w can be obtained by assuming that the field of turbulence is “frozen” as it advects past the lidar. Measurements of w were obtained from a height of z ? 390 m above the surface to near the CBL top with an unprecedented vertical resolution of 30 m. Considerable day-to-day variability was found in the spectral shape, as shown in the figure, and previous models of the w spectra were not particularly good at describing the observations. Some of this variability was found to be linked to mean CBL structure, including wind speed, shear across the CBL top, and processes at, and just above the capping inversion.
Work will continue over the next year on analysis and interpretation of profiles of higher-order moments of w, from LIFT, including variance, skewness and kurtosis, which will be compared with large-eddy simulation results.
The goal of the Fine-Mesh Land Model project was to develop and implement a framework for modeling land surface processes using NCAR community land models at scales several multiples finer than those of parent atmospheric models. The approach taken was to enhance and implement the fine-mesh model of Hahmann and Dickinson (2001) within the Community Atmosphere Model - Community Land Model (CAM - CLM) climate model.
A paper summarizing the results from the fine-mesh modeling work within CAM/CLM has been written and submitted. Results from simulations with sub-grid topography showed significant changes to both the general circulation and the model surface hydrological fields compared with those where only subgrid land use and land cover was specified. The principal changes were an alteration of the Northern Hemisphere wintertime mid-latitude wave train in the North Pacific. A relative increase in the strength of the Aleutian low combined with changes land surface elevations result in widespread changes in the amount of total precipitation and partitioning between rain and snow over the parts of western Canada and Alaska. Effects in other complex regions appeared to be of lesser magnitudes. Changes in rain-snow partitioning also impart a lag-effect on the surface hydrology by altering the timing and location of snowmelt and runoff from cool season precipitation. Implementation of a precipitation disaggregation scheme into the fine-mesh model which stochastically determines the sub-grid area occurrence and intensity of precipitation based on the coarse grid model precipitation was also completed. Results from simulations including the subgrid precipitation aggregation show modest changes in the global circulation as well as the surface hydroclimate though regional changes were sometimes quite substantial. In the U.S. Great Plains there appears to be a distinct strengthening of the low-level moisture transport from the Gulf of Mexico occurring with a regional increase and redistribution of rainfall and surface evaporation.
We reported on our research last year as part of the NASA-sponsored African Monsoon Multidisciplinary Analyses (NAMMA) campaign. This project was a field research investigation conducted during August-September 2006 based in the Cape Verde Islands, 350 miles off the coast of Senegal in west Africa. We have made considerable progress on the analysis of these data. With Cynthia Twohy of Oregon State University, we have shown that the Saharan dust aerosols acquire a sulfate coating and act efficiently as cloud condensation nuclei. The net effect is that exceedingly high cloud-droplet concentrations are produced in the dust-perturbed clouds. Dr. Twohy has submitted an article to Nature Magazine, with us as coauthors.
We have further found that an appreciable portion of the cloud droplets are lofted high into the NAMMA clouds in vigorous convection. These freeze via spontaneous ice nucleation at temperatures near -40C, thereby producing high concentrations of radiatively reflective small ice particles. A paper reporting on these observations is in preparation.
In November and December 2007, we conducted the ICE-L (ice in clouds experiment- layer clouds). The primary goal of ICE-L was to show that under given conditions, direct ice nucleation measurement(s), or other specific measurable characteristics of the aerosol, can be used to predict the number of ice particles forming by nucleation mechanisms in selected clouds. The target clouds were primarily lenticular mountain-wave clouds. These clouds can act as a laboratory-type setting because they are relatively stationary, permitting repeated penetrations at multiple levels. Flying along or against the wind is laboratory-like in that distance along the wind corresponds to particle-growth times.
We used the NCAR C-130 aircraft for ICE-L. A dozen or so university investigators, sponsored by NSF, participated in the experiment and many brought instruments that brought a new level of detail to how ice forms in clouds. We had eight probes to measure droplet and ice-particle-size distributions, three of which measured sub-50 micron particles with an open-path design to mitigate shattering which has contaminated earlier measurements of ice-particle concentrations. Incredibly interesting and valuable data were acquired with the onboard upward- and downward-viewing cloud radar (94 GHz) and upward-viewing lidar. We had two particulate mass spectrometers and a CVI to get at the composition of the ice nuclei, ice-nucleus measurements, two CCN spectrometers and a photometer to sense the volatility of the ice nuclei.
Nine flights were conducted in lenticular mountain wave clouds (see Figure 1a). Up to a dozen penetrations were made in individual wave clouds (Figure 1b). Our early analyses indicate that there is no mismatch between ice-nucleus concentrations and ice-crystal concentrations. Ice-nucleus and ice-crystal concentrations show very little dependence on temperature. This contrasts with parameterizations of ice-nucleus concentrations currently used in WRF and most cloud and mesoscale models that show that these concentrations increase strongly with temperature. We plan to parameterize the ICE-L results for incorporation into WRF.
ESSL scientists are participating in the Integrated Land Ecosystem-Atmosphere Processes Study (iLEAPS), a new ten-year land-atmosphere project of the International Geosphere-Biosphere Programme (IGBP). The goal of iLEAPS is to understand how interacting physical, chemical, and biological processes transport and transform energy and matter through the land-atmosphere interface. The project is designed to study interactions and feedbacks on scales from molecules to the entire globe, and from minutes to centuries, both past and future. The project brings together multi- and cross-disciplinary scientists to collaborate, distribute ideas and results rapidly, and increase scientific relations with developing countries. The iLEAPS International Project Office is based at the University of Helsinki in Finland and promotes international research projects studying essential phenomena related to global climate change.
In FY2008 ESSL scientists participated in the iLEAPS expert workshop on "The relevance of surface and boundary layer processes for the exchanges of reactive- and greenhouse gases" in Wageningen, Netherlands and the iLEAPS workshop on "Process-based description of trace gas emissions in land surface models" in Helsingborg, Sweden. ESSL scientists will continue to contribute to iLEAPS activities in FY09 and will propose the BEACHON project for sponsorship by iLEAPS. This work is funded by NSF/NCAR.
The Texas Air Quality Study (TEXAQS 2006) was a NOAA led field campaign out of Houston, Texas, that focused on providing a more complete understanding of the sources and processes responsible for photochemical pollution (ozone) and regional haze (aerosols) during the summer in Texas. Ozone and aerosols both play important roles in air quality and climate change as a result of their chemical and radiative properties. A significant fraction of ozone in the troposphere is produced photochemically from precursors that have both anthropogenic and natural sources. Major urban centers are a large source of these precursors, which results in elevated levels of ozone on local and regional scales from production and transport. These elevated levels can be a human health hazard in urban and surrounding areas and can impact ecosystems in both urban and rural areas. Enhanced aerosol production in urban areas can also impact human health and ecosystems. Exposure to high levels of aerosols can lead to respiratory problems for humans and can acidify ecosystems as they are removed by rain. On the NOAA P3 aircraft, ACD scientists measured PANs and collected whole air samples for analysis of non-methane hydrocarbons, halocarbons, alkyl nitrates, and oxygenated VOCs. One interesting result from the measurements made during TexAQS2006 is illustrated in Figure 1. One goal of the TexAQS 2006 campaign was aimed at investigating the changes in photochemical ozone production over Houston as a result of emission reductions implemented between 2000 (the first TexAQS campaign) and 2006. Of high concern were in 2000 the episodes of very efficient ozone production from highly reactive NMHC like ethene, propene, and butenes which are emitted from petrochemical facilities. The measurements made on this flight on October 6, 2007, show that this is still a problem. The plot shows 1-second time resolution measurements of PAN and APAN (both tracers of photochemical ozone production, along with ozone mixing ratios. APAN is a unique product from the oxidation of butadiene, an extremely reactive hydrocarbon used to make butyl rubber. PAN is being produced from many hydrocarbons which produce ozone in the troposphere. During the flight, four passes downwind of major butene emitters (blue dots on map, the size of the dots is proportional to the butene emission magnitude) were made. Ozone is enhanced by almost 80 ppb only 1.5 hours downwind of the source of emissions. Even though dilution sets in for passes #3 and #4 (spiral profile) further downwind ozone is still 60-70 ppb enhanced over the background. This is a very large ozone production rate and cases like this contribute significantly for the overall large ozone problem in the Houston area. The thick black arrow points out where additional ozone production was observed which does not involve butadiene as PAN and ozone are enhanced here but APAN is not. This illustrates the advantage of measuring several (in our case, 5) different PAN species quasi-simultaneously and how it allows us to identify specific hydrocarbon species as they contribute to the photochemical production of ozone.
Observed PAN/PPN ratios were also analyzed in air masses which were transported downwind of the city of Houston into the suburban areas. An increase of the [PAN]/[PPN] ratio was observed during transport of the Houston plume away from the city. PPN is more thermally stable than PAN which works to decrease this ratio if no more production of either of the species took place. However, the precursors for PPN generally react away faster with OH compared to the PAN precursors, shifting the ratio to increasing values while air masses are still production dominated. This generally faster depletion of PPN precursors alone was found to be inadequate to explain the overall observed increase. Our calculations show that isoprene emissions from the surrounding areas are a major source of PAN downwind of Houston and cause much of the observed increase of the [PAN]/[PPN] ratio (see Figure 2).