ESSL LAR 2008: Strategic Goal #1, Priority #1
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, Strategic Priority #1: Exploring atmospheric, Earth system, and solar processes, variability, and change, is defined in the NCAR Strategic Plan as follows: "Developing a better understanding of atmospheric, Earth system, and solar processes, as well as the variability and change associated with these processes....Exploration of these "Priority 1" areas focuses on three key activities: simulating the Earth system's natural variability, investigating the Sun's magnetic-flux eruptions, and understanding effects of gravity waves, including related interactions between the upper troposphere and lower stratosphere."
This NCAR priority, driven by ESSL's themes 2, 5, 6, & 7, is critical to achieving NCAR's first strategic goal.
The section below describes specific research conducted by ESSL staff relevant to Priority 1. The major ESSL activities in this area are studies of paleoclimate, solar dynamo and solar cycle, chemistry and dynamics of the UTLS and middle and upper atmosphere, solar magnetic flux emergence and CME initiation, global air quality, the impact of environmental changes on tropical cyclones, climate variability, and various aspects of the solar interior, the lower solar atmosphere, and the solar corona and wind.
- Paleoclimate - CGD
- High Resolution Dynamics Limb Sounder (HIRDLS) recovery and application - ACD
- Solar dynamo modeling and solar cycle prediction - HAO
- Upper Troposphere and Lower Stratosphere (UTLS) initiative - TIIMES
- Simulations and observations of magnetic flux emergence and CME initiation - HAO
- Globalization of air quality and intercontinental transport - ACD
- Model for Ozone And Related chemical Tracers (MOZART): Global chemistry-transport modeling - ACD
- Hurricanes - MMM
- Climate Variability and Predictability World Climate Research Program (CLIVAR) - CGD
- Convection, flux tubes, and waves in the solar interior - HAO
- Spectro-polarimetric studies of magnetic fields in the lower solar atmosphere - HAO
- MHD physics of the solar corona and wind - HAO
- Chemistry and dynamics of the middle and upper atmosphere - ACD
- UTLS dynamics, trends, and composition - ACD
- Gravity waves - TIIMES
- Weather Research and Forecast model coupled with Chemistry (WRF-Chem) - MMM
- Stratospheric ozone recovery [Whole-Atmosphere Community Climate Model (WACCM)] group - ACD
- WACCM - ACD
- Intense photochemistry in the Antarctic troposphere - ACD
- Severe atmosphere convection - MMM
- Intraseasonal/tropical climate variability - CGD
- RT/MHD modeling of the solar surface layers - HAO
Paleoclimates offer a unique perspective to understand both Earth's climate sensitivity and stability. Observational data tell us that Earth has experienced a wide range of climates over various time scales, and that transitions in Earth's climate can take place rapidly. We know that many of these past climates were determined by changes in external forcing factors. To the extent that climate models can reasonably simulate past warm and cold climates, we gain confidence that the models can be used to study Earth's future climate.
A strong test of the Community Climate System Model (CCSM) is to simulate past climate against records from ice cores, tree rings, and other proxy data. Magnitudes and rates of past change also provide an important context for future climate changes. Within ESSL, we are exploring past changes over many different time periods: from the distant geologic past, with radically different continental configurations, when the Earth's surface temperature and latitudinal gradients were significantly different from present and levels of atmospheric carbon dioxide, methane, and other greenhouse gases reached levels up to ten or more times present levels; the last million years, when the Earth experienced a waxing and waning of ice ages and levels of atmospheric carbon dioxide, methane, and other greenhouse gases during the ice ages were reduced by half or more from present levels; and the last few millennia with colder periods extensively documented in the proxy record associated with solar fluctuations and volcanic eruptions. Each of these geologic periods gives us an improved understanding of the natural variability of the Earth system and our ability to model feedbacks in the climate system. By comparing climate simulations of Earth's past to the data from geological and geochemical archives, we can evaluate the accuracy of climate models such as CCSM that are used to look at Earth's future. At the same time, geologists have started to use CCSM to understand how their specific data can be understood in a more large scale, dynamical context. CCSM has become a valuable partner to field-based geological research.
CCSM has been applied to all these different time periods. The Late Ordovician (445 Ma) was a time of elevated carbon dioxide, but extremely cold climates. The first major extinction on Earth took place at this time. The CCSM3 has been configured to simulate the climate of this time period, in order to study the role that climate played in the mass extinction. The model will also be used to explore the growth of large continental glaciers on Gondwana, a large land-mass that occupied the southern hemisphere at this time. A CCSM3 simulation of the warm mid Cretaceous (100 Ma) has also been carried out to study how well the model can replicate the pole to equator gradient in surface temperature for this time period. Proxy records for the deglaciation that started 21 thousand years ago indicate events with large freshwater inputs to the Atlantic Ocean basin as iceberg discharges into the high-latitude North Atlantic, Laurentide meltwater input to the Gulf of Mexico, or meltwater diversion to the North Atlantic via the St. Lawrence River and other eastern outlets. The climate responded, in the North Atlantic region and globally, to these freshwater events, but the responses varied among the events and are not completely understood. The sensitivity of the climate system to the magnitude and location of freshwater input into the North Atlantic has been studied using the fully coupled version of CCSM3 for glacial conditions. The results suggest that the response of the North Atlantic meridional overturning circulation is proportional, though not linearly, to the size of the freshwater added. On the other hand, the southward migration of the ITCZ over the tropical Atlantic displays a threshold response to the amount of freshwater forcing and hysteresis in the response versus recovery from the event. This has implications for detecting freshwater events using the Cariaco Basin records.
2009 and Beyond
Future plans include further CCSM3 simulations of the climate of the Late Ordovician, a time of great cold and a time of the earliest mass extinctions of life. Future plans also include deep time simulations of the Latest Cretaceous a time period just prior to the massive asteroid impact that led to the demise of the dinosaurs. A CCSM3 run will be carried out of the first synchronously coupled transient ocean-atmosphere-dynamic vegetation GCM simulation of the past 21,000 years. Under the auspices of the Paleoclimate Modeling Intercomparison Project (PMIP), a CCSM3 simulation of the mid-Pliocene will be carried out. This time period is possibly the closest paleo analog for an equilibrium climate with current CO2 levels. There will also be a focus on simulating the magnitudes and rates of past climate change on many time scales using the planned NCAR Earth System Model, which will allow us to explore more completely feedbacks with vegetation and ice sheets, atmospheric chemical changes, and the carbon and nitrogen cycles.
The High Resolution Dynamics Limb Sounder (HIRDLS) is a 21 channel infrared limb scanning radiometer, jointly developed by ACD, the University of Colorado, and the Physics Department of Oxford University. It is designed to make observations of temperature, ozone, water vapor, and 8 other trace species, as well as PSC's, aerosols and cirrus clouds, from the upper troposphere to the mesosphere, with higher vertical resolution than has previously been available from space observations. HIRDLS was launched on the Aura spacecraft in July 2004. Despite an obstruction that limited the view to the atmosphere to a small fraction of the width of the optical beam, HIRDLS scientists demonstrated that there is recoverable atmospheric information in the signals, and worked hard to develop algorithms to correct for the effects of the obstruction, as discussed under Goal 5, section 4.
As a result of these efforts, Version 4 of the data, providing high resolution retrievals of temperature, ozone, nitric acid, CFC 11, CFC 12, aerosol extinction plus cloud top location and types was made available to the community. Five papers describing the validation of V3 data were published. V4 data has improved cloud detection, leading to more reliable ozone data in the UTLS region. In addition, the mean accuracy of the temperature and ozone data has improved.
An example of what can be seen is shown in Figure 1.a, which shows a latitude-potential temperature cross-section along a HIRDLS scan track near 122°W on 1 April 2006. This shows a region of low ozone at 380K in mid latitudes, originally from low latitudes that has been injected across the tropopause, but here has been separated by atmospheric motions, believed to be related to baroclinic waves in the troposphere. Here it remains as a thin layer that is associated with a potential vorticity (PV) contour of 6 10-6 m2/s K/kg, (6 PVU) as shown in the PV map on the 380K surface in Figure 1.b. Following this air mass shows that the low ozone air remains close to the 6PVU contour for several days, before the PV contour relaxes to a lower altitude, leaving the air to mix with its surroundings. This can only be seen because of the high vertical resolution of the HIRDLS data.
Summarizing the accomplishments, the radiance correction algorithms were improved, adding the CFC's and aerosol extinction to be retrieved, as well as improving the accuracy and usefulness of temperature and ozone. A study of gravity waves was published, and several talks on UT/LS processes and strat-trop exchange were presented. An anomaly with the HIRDLS chopper prevented participation in START08, or the ARCTAS field experiment.
The plans for FY09 call for increased emphasis on the application of these unique, high resolution data to a range of scientific applications, as well as continuation of work to further refine the correction and retrieval algorithms to retrieve additional species, and extend the altitude range of the results.
This work was supported by NASA and the NSF.
The magnetic fields that are ultimately the source of the activity that takes place in the solar atmosphere have their origin inside the Sun, where convective, rotational, and other flows of highly electrically conducting plasma contribute to the operation of the dynamo. Work by HAO scientists has shown that the meridional circulation, a large-scale flow directed from the equator to the N and S poles at the solar surface and completing the circuit from the poles to the equator near the bottom of the convection zone, plays a fundamental role in in the dynamo process. It continuously transports poloidal magnetic fields from the surface to the tachocline at the base of the convective envelope where they are differentially stretched by the rotational shear therein to produce new toroidal magnetic fields. Buoyant magnetic flux tubes formed from these fields become the source for new poloidal fields at the surface, thus completing the dynamo cycle. HAO researchers have pioneered the development of the so-called flux-transport dynamo model that is based on this physical picture, and have had remarkable success in applying it to the problem of simulating and predicting solar cycle amplitudes.
During the last year, HAO scientists made significant progress in efforts to understand the physical processes contributing to dynamo action in the Sun, and to further develop the predictive capabilities of the flux-transport dynamo model. M. Miesch, together with J. Toomre, B. Brown (both CU), A. S. Brun (CEA-Saclay), and M. Browning (CITA), continued to develop global three-dimensional models of convection and dynamo processes in the Sun and in other stars. Their work focused mainly on simulations of rapidly-rotating solar-type stars which exhibit modulated convection patterns wherein columnar convective cells group together in one or more localized longitudinal patches with relatively quiescent flow elsewhere . Magnetohydrodynamic simulations of such stars show strong dynamo action, with prominent, persistent bands of toroidal magnetic fields and quasi-periodic polarity reversals. The recent solar simulations of Miesch and collaborators have demonstrated that the presence of a tachocline of rotational shear has a profound influence on dynamo action in the convective envelope, promoting mean-field generation. Strong toroidal flux structures are formed in the tachocline through turbulent pumping and shear that then feed back on the poloidal field component, enhancing and stabilizing the dipole moment.
M. Dikpati, P. Gilman, and G. de Toma investigated the well-known "Waldmeier effect", that is, the anti-correlation between the magnitude of the peak in the sunspot number of a cycle, and the time from minimum to reach that peak. It has been suggested that the Waldmeier effect can be used to predict the peak of a cycle shortly after the onset of that cycle. They have shown that the Waldmeier effect is not present in the sunspot area data. Hathaway et al. (2002) had previously shown that the Waldmeier effect is much weaker (correlation r = -0.34) in sunspot group number. Thus the Waldmeier effect may be specific to only the Wolf sunspot number. Given the near coincidence of solar minima in the two data sets, the main differences in rise-time of spot number data and spot area data occur from the differences in timing of maxima (see the positions of the green and yellow crosses in Figure 2). Dikpati, de Toma, and Gilman have also evaluated the relative skill of the polar magnetic flux, the magnetic flux crossing the solar equator, and the toroidal magnetic flux in the tachocline as predictors of solar cycle amplitude, using observations as well as a calibrated flux-transport dynamo model. On the verge of the upcoming solar cycle 24, these three cycle indicators have all received attention. Dikpati, de Toma, and Gilman have shown that in the context of a flux-transport solar dynamo, the (n-1)th cycle's tachocline toroidal flux and the (n-1)th cycle's equator-crossing flux can be good predictors of the nth cycle's peak, but the polar flux of (n-1)th cycle correlates poorly with the peak of the nth cycle. In flux-transport dynamos, the polar magnetic flux is a follower of the cycle rather than being a precursor to it.
During the next year, Miesch and collaborators will continue global-scale, 3D simulations of magnetized convection in a rotating spherical shell with an underlying stable region, in order to further study how dynamo action is affected by the presence of a tachocline. He will also add a surface poloidal magnetic field source term to create a 3D Babcock-Leighton-type dynamo which will be used to study the origin of magnetic cycles, the competition between magnetic pumping and transport of fields by circulation, and the interactions between the Babcock-Leighton source term and the convective generation of mean fields. Dikpati and collaborators will work to further refine the flux-transport dynamo-based prediction tool she has developed. At present, the onset timing and amplitude of a future cycle are predicted separately by applying a very simplified data-assimilation technique, "data-nudging", over the entire simulation run. In order to simultaneously predict the amplitude, timing, and shape of a cycle, a more sophisticated "sequential" data-assimilation technique, rather than just "data-nudging", will be developed. As in atmospheric weather prediction models, the dynamo model output at a certain cycle phase can be compared with the observed cycle at the same phase, and then the model can be updated with adjusted time-varying dynamo ingredients, to proceed forward in time. Dikpati and co-workers will also continue work on a 3D kinematic flux-transport dynamo, splitting the axisymmetric and non-axisymmetric flow and field components and representing the the non-axisymmetric components as a Fourier decomposition in longitude with a few low-order longitudinal modes. Thus this model will focus only on the large-scale, 3D, magnetic field evolution of the Sun, such as the evolution of "active longitudes."
The upper troposphere and lower stratosphere (UTLS) is a sensitive region of Earth’s climate, because water vapor, ozone, cirrus clouds and aerosols in this region strongly contribute to radiative forcing of the climate system. The dynamical processes of broad range of scales, from deep convection and gravity waves, to synoptic weather systems and the stratospheric large-scale circulation, redistribute the chemical species and create unique conditions for microphysical, chemical and radiative processes. Studies of the UTLS seek to determine the role of distinct processes and feedbacks in this region, and how the system will evolve in a changing climate.
During the last 5 years, the UTLS initiative has successfully conducted the Stratosphere-Troposphere Analyses of Regional Transport (START-05 and START-08) experiments using the Gulfstream V (GV). These data in combination with satellite and model analysis provide a new level of quantification and characterization of transport pathways across the tropopause and their linkage to the synoptic scale weather system. These field experiments have also set the stage for a large, unique field campaign focusing on the chemical transport and processing of continental convective storm systems, Deep Convective Clouds & Chemistry Experiment (DC3). START-08 and DC3 are further discussed under Goal 5: Planning of DC3 field program & Planning of START08 field program.
Simulations and Observations Of Magnetic Flux Emergence, CME Initiation and Evolution, and Interplanetary Consequences
Solar-driven space weather can have significantly adverse consequences for the Earth and near-Earth environment. Coronal mass ejections (CMEs) are the principal solar drivers of space weather. Using 3D magnetohydrodynamic simulations of the coronal magnetic field driven by the emergence of a twisted flux rope, HAO scientists have made significant advances in understanding the origin and dynamic evolution of CMEs and what makes a CME geoeffective.
Connecting interplanetary coronal mass ejections (ICMEs) to their coronal pre-eruption source requires a clear understanding of how that source may have evolved during eruption. In previous work S.Gibson and Y. Fan, (2006a,2006b) presented a three-dimensional numerical magnetohydrodynamic simulation of a CME, which showed how, in the course of eruption, a coronal flux rope may writhe and reconnect both internally and with surrounding fields in a manner that leads to a partial ejection of only part of the rope as a CME. They have now (Gibson and Fan 2008) explicitly determined how such evolution during eruption would lead to alterations of the magnetic connectivity, helicity, orientation, and topology of the ejected portion of the rope so that it differs significantly from that of the pre-eruption rope. Moreover, because a significant part of the magnetic helicity remains behind in the lower portion of the rope that survives the eruption, the region is likely to experience further eruptions (Figure 1). These changes complicate how ICMEs embedded in the solar wind relate to their solar source. In particular, the location and evolution of transient coronal holes (Figure 2), the topology of magnetic clouds ("tethered spheromak") (Figure 3), and the likelihood of interacting ICMEs would differ significantly from what would be predicted for a CME which did not undergo writhing and partial ejection during eruption.
The formation of twisted magnetic flux ropes in the solar corona which contains free magnetic energy and drives solar eruptions has its origin from the solar interior through the emergence of twisted active region magnetic flux. To understand the origin of CME precursor structures in the solar corona, Fan is carrying out 3D numerical simulations of the dynamic emergence of a twisted magnetic flux tube from the top layer of the solar convection zone into the solar atmosphere and the corona. She is investigating how the emergence, the resulting photosphere evolution, and the coronal magnetic field depend on the properties of the subsurface emerging tube. The simulations show that it is difficult for a twisted flux tube to emerge bodily into the corona as a whole due to the heavy mass trapped at the bottom concave parts of the twisted field lines. However, it is found from the simulations that on the photosphere, after a brief stage of shearing motion during which the two polarities of the bipolar region become separated, significant rotational motion develops within each polarity (Figure 4a), similar to the observed sunspot rotations, which transport significant amount of twist into the corona. The rotational motions of the two polarities are found to twist up the inner field lines of the emerged fields such that they change their orientation into an inverse configuration (i.e. pointing from the negative polarity to the positive polarity over the neutral line). As a result, a flux rope with sigmoid-shaped dipped core fields forms in the corona (Figure 4b). Sunspot rotation has been observed in many events preceding X-ray sigmoid brightening and the onset of eruptive flares. The simulations show that such rotational motions take place during flux emergence as a result of the propagation of torsional Alfven waves along the flux tube which transport significant twist from the interior portion towards the expanded coronal portion. These results provide insight into the nature of the observed processes that lead up to solar eruptions.
Transpacific Pollution Transport during INTEX-B: Spring 2006 in Context to Previous Years
We analyze the transport of pollution across the Pacific during the NASA INTEX-B (Intercontinental Chemical Transport Experiment) campaign in spring 2006 and examine how this year compares to the time period 2000-2006. In addition to aircraft measurements collected during INTEX-B, we include multi-year satellite observations of CO from the Measurements of Pollution in the Troposphere (MOPITT) instrument. We integrate these measurements with simulations from the chemistry transport model MOZART-4. Model tracers are used to examine the contributions of different regions to pollution levels over the Pacific and to estimate the O3 production from NOx sources in Asia to O3 loadings over the Pacific and North America. Additional modeling studies are performed to separate the impacts of inter-annual variability in meteorology and dynamics from changes in source strength. Figure 1 shows the variability in the tropospheric CO burden (relative deviation from mean and absolute amounts) over the Pacific and the contiguous US as derived from MOPITT data and two model simulations, one with emissions that vary by year (MozVar) and one with constant emissions (MozConst).
This work was funded by NASA and NSF. This work will be published in FY09 and similar studies made in the analysis of the NASA ARCTAS aircraft experiment.
Impacts of the Fall 2007 California Wildfires on Surface Ozone: Integrating Local Observations with Global Model Simulations
In this study we quantified the impact of the fires in California in fall 2007 on regional air quality and in particular on surface ozone by analyzing surface observations of ozone concentrations together with MOZART simulations (Pfister et al., GRL, in press). The simulations include a synthetic tracer providing information about the amount of ozone produced from the fires. A clear increase in observed ozone is found when the model predicts a strong impact of pollution from the fires, where measured afternoon 8-hour concentrations increased, on average, by about 10 ppb. The findings demonstrate that intense wildfire periods can significantly increase the frequency of ozone concentrations exceeding current U.S. health standards, and might cause violations also during photochemically less active seasons. The study also demonstrates the far-reaching impact of ozone production from the fires.
Figure 1 shows observed and modeled 8-hour O3 concentrations for 10-18 LT, 11-19LT and 12-20 LT binned by the model fire tracer O3FIRE. Shown is the deviation from the mean concentrations (ppb) separately for rural, urban and suburban sites, and for the two main fire periods (Sep 1-30 and Oct 8 - Nov 8).
This work was funded by NSF and NASA. In FY09 similar studies will be made of the extensive California fires of Summer 2008 as part of the analysis of the NASA/ARCTAS-CARB aircraft experiment.
Contribution of fires to ozone from Mexico City pollution
During the MIRAGE experiment during March 2006, numerous wildfires were evident in the hillsides surrounding Mexico City, complicating the characterization of the urban emissions and resulting pollution. Using the MOZART global chemical transport model run at a horizontal resolution of 0.7 degrees, the ozone burden resulting from the total emissions in the Mexico City region, as well as only from open fires, has been simulated. By tagging the NO emissions from a single type or region of sources in the model, the ozone produced from that source can be quantified. In the middle of the MIRAGE experiment (on March 20) a period of rainy weather began, significantly dampening the fire activity around Mexico City. Figure 2 shows the ozone produced from all of the NO emissions from Mexico City and its environs (urban and fires) and ozone from only the fire emissions, for the periods before and after the rain event.
These results show that, for ozone, the open fires in the vicinity of Mexico City are a fairly small, but non-negligible contribution (20%) to the regional ozone pollution. Other studies have shown, however, that other components of Mexico City pollution, in particular particles, have significant contributions from fire emissions. Some of these sources could be cooking, heating and trash burning, which are not included in the tagged fire emissions in the MOZART simulations.
This work was funded by NSF and NASA. Analysis of the MIRAGE observations using MOZART will continue in FY09.
As the current high level of Atlantic hurricane activity continues to cause major disruption and damage, research at NCAR is both expanding and becoming of greater societal importance. Research and applications work within ESSL and in collaboration with our colleagues in RAL and CISL, the Willis Research Network, Georgia Tech, SUNY, RSMAS, Cal Tech, and Mississippi State has:
• Expanded our understanding of cyclone formation in both tropical
and subtropical conditions, wind-pressure relationships, hurricane structural
evolution and the impacts of climate variability and change;
• Led to several improvements in the Advanced research Hurricane WRF (AHW);
• Enabled participation in the NOAA Hurricane Intensity Forecast Improvement Program (HIFIP) with the Advanced Hurricane-research WRF (AHW); and,
• Commenced a study to improve the communication of forecast information to vulnerable communities.
High-resolution simulations of the 2005 hurricane season have enabled a demonstration of an hypothesis that tropical cyclone formation can be enhanced by wave accumulation processes when easterly waves move into a region in which the easterly flow is decreasing westward. The simulations have also indicated that mesoscale interactions may be an important contributor to enhanced cyclogenesis. This represents a move away from earlier assumptions that such interactions were largely stochastic and could equally enhance or reduce cyclogenesis potential. These factors are considered to be major contributors to the highly active 2005 season. The role of upper-trough interactions and in subtropical development of tropical cyclones has been further investigated. An important conclusion is that strong vertical shear enhances such developments, quite the opposite of the requirement for low shear in equatorial developments. Tropical cyclones moving into higher latitudes and environments with stronger vertical shear were also investigated. Emerging frontal structures, asymmetric rainfall patterns, and responses to the deleterious structural effects of vertical shear were simulated and analyzed. A major field program to further study this process has been proposed. NCAR also participated in the TPARC field program for cyclone development in the western North Pacific, and the WRF modeling system will be used to develop a comprehensive reanalysis for subsequent research.
A new wind-pressure relationship has been derived that updates an older relationship used widely in developing synthetic hurricane climatologies for design and planning of coastal and offshore structures. This relationship was able to confirm an earlier study that major hurricanes in the 1950s were overestimated in intensity, and to extend this to include the period through to the 1970s.
An NCAR Breakthrough Science (BTS) study used AHW for extremely high resolution simulations of an idealized hurricane to assess the impacts of increasing resolution on structure and intensity (Fig. 1). This showed that there was only a slight variation of intensity with resolution below 1 km. However, at an unprecedented horizontal resolution of 62 m, the AHW was able to resolve turbulent structures (Fig. 1), The subsequent analysis found a marked sensitivity of intensity to the specified horizontal mixing length in axisymmetric models and showed that earlier studies of potential increases in hurricane intensity by migration of high-entropy surface air in the eye into the eye wall were incorrect. One additional outcome was the development of a revised hurricane maximum potential intensity relationship that did not require the need for approximations employed in earlier versions.
The AHW has been further improved based on experience with real-time and research simulations over the past several years. This included upgrades to the cloud microphysics and boundary-layer parameterizations an ocean mixing and upwelling parameterization based on a 1-D configuration, and further developments of the data assimilation system. The ocean parameterization successfully reproduces much of the negative feedback associated with oceanic cooling during a cyclone passage. Preparations have been completed for participation in the NOAA HIFIP program. AHW will use the Ensemble Kalman Filter data assimilation and parameterized ocean that reproduces much of the observed negative feedback arising from cooling by upwelling and mixing associated with the cyclone passage. NCAR will be a full participant in HFIP, with post-event forecasts of all the cases recommended by NOAA.
Recent experience has shown quite clearly that coastal residents do not respond adequately to the risk from an approaching hurricane. Building on preliminary investigations, a substantial study has now commenced at NCAR to examine the way in which such communities interpret warning information and develop improved methods of communication and response to the hurricane threat.
Climate Process Teams (CPTs)
ESSL scientists remain actively involved in leadership of the Climate Variability and Predictability (CLIVAR) initiative of the World Climate Research Programme (WCRP) through membership on various national and international CLIVAR panels, as well as through research contributions to CLIVAR goals and objectives. The purpose of CLIVAR is to investigate climate variability and predictability on time-scales from months to decades, as well as the response of the climate system to anthropogenic forcing. CLIVAR, as one of the major components of the WCRP, started in 1998 with a 15-year charter, which focuses on the role of the coupled ocean and atmosphere within the overall climate system, with emphasis on variability, especially within the oceans, on seasonal to centennial time scales. CLIVAR aims to explore predictability and improve projections of climate variability and climate change using existing, reanalyzed, as well as new global observations, enhanced coupled ocean-atmosphere-land-ice models, and paleoclimate records.
A major effort of the U.S. CLIVAR program has been the introduction and fostering of Climate Process Teams (CPTs). A CPT is a team of theoreticians, observationalists, process modelers, and coupled climate modelers formed around specific issues or uncertainties. CPTs aim to link process-oriented research to modeling for the purpose of addressing key uncertainties in coupled climate models. Expediting the incorporation of new parameterizations into ocean models and assessing their climate impacts are among their primary goals. Within ESSL, major ocean model developments are proceeding under the auspices of the CPTs on both gravity current entrainment and eddy mixed layer interaction in collaboration with the external university and laboratory community.
For the CPT on the eddy mixed layer interaction, we implemented a new submesoscale physics parameterization in the ocean component of the Community Climate System Model version 4 (CCSM4), following Fox-Kemper, Ferrari, and Hallberg (2008, J. Phys. Oceanogr., v38, 1145-1165). The submesoscale represents the range of scales between the mesoscale (typical scales of a month and 100 km) and the fine-scale (typical scales of inertial or faster time and up to hundreds of meters). The submesoscale dynamics are dominated by the development of fronts and the ageostrophic circulations associated with the fronts. Some recent studies indicate that both the depth and the stratification of the surface mixed layer are significantly modified by these ageostrophic circulations. Fox-Kemper, Ferrari, and Hallberg (2008) present a parameterization scheme to represent the mixed-layer stratification associated with these frontal instabilities and frontogenesis. Inclusion of this new physics in the ocean model leads to some improvements compared to a control integration without this parameterization. In particular, due to the restratification by the parameterized mixed layer eddies, generally deep bias of the simulated mixed layer depths are significantly reduced, thus producing more favorable comparisons with the available observations in most regions of the globe (see Figure 1).
Some preliminary results from this study are reported in Fox-Kemper, Danabasoglu, Ferrari, and Hallberg (2008, CLIVAR Exchanges, v13, 3-5).
The results from the successful implementation of an overflow parameterization for the Mediterranean overflow (PMO; Parameterized Mediterranean Overflow) through the Strait of Gibraltar in CCSM3 were documented in Wu, Danabasoglu, and Large (2007, Ocean Modelling, v19, 31-52). This parameterization, based on the marginal sea boundary condition scheme of Price and Yang (1998, in Ocean Modeling and Parameterization, Kluwer Academic, 155-170), represents exchanges through narrow straits / channels, associated entrainment and intrusion of overflow product water into the open ocean. These overflow processes occur on very small spatial scales, essentially prohibiting their explicit representation in ocean circulation models used in climate studies. Therefore, their effects must be parameterized. The PMO is applicable only to overflows from enclosed seas.
Therefore, we have developed a new parameterization that can be used in open-ocean overflow applications such as the Denmark Strait and Faroe Bank Channel overflows.
A particularly novel aspect of the parameterization is a new treatment of the baroclinic / barotropic mode split. This new open-ocean overflow parameterization was first implemented and tested in the Parallel Ocean Program version 1.4 (POP1.4). It has now been incorporated in the new version of the ocean model (POP2.0). Newly obtained results show that inclusion of the Denmark Strait Overflow (DSO) and Faroe Bank Channel overflow via this new scheme significantly reduces some long standing model biases in the North Atlantic. Potential temperature difference distributions at a depth of 2000 m (see Figure 2) clearly show that the CONTROL case warm bias is significantly reduced with the new parameterization which is used to represent only the DSO in this particular experiment.
A primary purpose of these CPTs is to document climate impacts of these new parameterizations using fully coupled simulations. To this end, we have already completed several simulations that also include CFCs. In FY2009, the final year of these projects, we will perform additional integrations to complete the suit of necessary experiments and document any climate impacts due to these new parameterizations.
The observational MLD data are based on the Polar Science Center Hydrographic Climatology (PHC2) data sets (a blending of the Levitus et al. 1998 and Steele et al. 2001 data). The MLD is defined as the depth at which the local density is higher then the surface density by 0.125 kg / m^3. The experiments SUBMESO and CONTROL are forced with the Coordinated Ocean-ice Reference Experiments (COREs) Normal-Year atmospheric forcing data from Large and Yeager (2004). The winter-mean represents January-March and July-September means in the Northern and Southern Hemispheres, respectively. In SUBMESO, the root-mean-square MLD difference from observations is reduced by more than 20% compared to the CONTROL - OBS difference.
The figures clearly show the substantial elimination of the model warm bias of the CONTROL case with the new parameterization in the DSO experiment.
The energy liberated through the nuclear burning of hydrogen in the core of the Sun is transported outward by radiative diffusion for radii less than about 0.7 R_sun, and by convection within that portion of the interior located between 0.7 R_sun and the photosphere. The structure and dynamics of this outer convective envelope, the nature of the interface between it and the underlying stably stratified radiative interior, and the hydrodynamical and magnetohydrodynamical (MHD) processes that take place within these layers are critical to understanding the operation of the solar dynamo, the transport and emergence of dynamo-generated magnetic fields, and the properties of the Sun's differential rotation and meridional circulation. During 2008, HAO researchers made substantial progress on plans to study the dynamics and evolution of buoyant magnetic flux ropes in the convection zone, and to investigate the nature and properties of the instabilities and waves that can exist in the the solar tachocline and radiative interior.
Y. Fan carried out a set of 3D, spherical-shell, MHD simulations of the buoyant rise of active region flux tubes in a model solar convective envelope. The results of these computations put new constraints on the initial twist of the flux tubes in order for them to emerge with tilt angles consistent with the observed Joy's law of the mean tilt of solar active regions. Asymmetric stretching of an Omega-shaped rising tube by the Coriolis force causes the leading side (leading in the direction of rotation) to have a stronger field and to thus be more cohesive compared to the following side (Figure 1). There is also significant asymmetry between the twist and the upward helicity flux along the leading and following legs of the emerging tube. The values of the local twist of the magnetic field lines in the leading leg show modest variations with height in the convection zone, while the field lines in the following leg are frayed and show large fluctuations and mixed signs of twist (Figure 2). In addition, in the upper half of the convection zone, the upward helicity flux along the leading leg is significantly greater than that in the following leg (Figure 3), a property which has been reported in a recent observational study of emerging active regions by Tian and Alexander (2008).
M. Dikpati, P. Gilman, M. Miesch, and P. Cally (Monash University) have used a 3D, thin-shell model of the tachocline to investigate the occurrence and properties of axisymmetric (m=0), MHD instabilities inside the Sun. They find that bands of toridal magnetic fields become unstable to axisymmetric perturbations for solar-like field strengths (100 kG) with e-folding times that can be months, or even a few hours if the field strength is a million Gauss or higher, as might occur in the solar core, white dwarfs, or neutron stars. These instabilities exist with and without rotation, although differential rotation has a stabilizing effect. The onset of an m=0 instability occurs from the poleward shoulder of banded toroidal field profiles. Unlike the non-axisymmetric instability which tips or deforms a band, in the axisymmetric instability, the fluid can roll in latitude and radius, and can break up bands in the radial direction. The velocity produced by this instability in the case of low-latitude bands crosses the equator and hence can provide a mechanism for hemispheric coupling. T. Rogers and K. MacGregor continued their study of internal gravity waves (IGW) in the radiative interior of the Sun. These disturbances are generated by the overshoot of flows from the convection zone into the underlying stably stratified layers of the solar interior. They used numerical simulations to investigate the interaction of IGW with a layer of toroidal magnetic field, located just below the base of the convection zone. Their results indicate that the wave energy present in the deep radiative interior is severely diminished by the reflection and absorption of downwardly propagating waves in the magnetic layer. It is found that for field strengths in the range of 1-100 kG, the wave energy in the radiative zone is decreased by 4-5 orders of magnitude, independent of field strength. This poses significant challenges to models which rely on angular momentum transport by IGW to account for the uniform rotation of the solar radiative interior, as well as for observational searches for g-modes.
In the coming year, simulations of magnetized convection in a 3D, rotating, spherical shell with an underlying stable region will be carried out, in an effort to study how dynamo action is affected by the presence of a tachocline. Studies of magnetic flux concentrations in the convection zone will continue with simulations of the formation of flux tubes by the occurrence of the magnetic buoyancy instability in dynamo-generated mean fields. These simulations will be conducted in 3D, rotating, spherical geometry, with distributions of poloidal and toroidal fields obtained from the dynamo model of M. Rempel for different solar cycle phases. Further studies of MHD instabilities of tachocline fields and rotation will include exploration of the linear growth and nonlinear evolution of instabilities in stars, particularly those having anti-solar differential rotation. The investigation will be directed toward determining whether the poleward side of a band of toroidal field is stable in the case of anti-solar differential rotation rather than the equatorward side as in the case of the Sun. Examination of gravity waves in the solar radiative interior will proceed with additional simulations intended to further clarify mode properties and background interactions in the presence of buoyancy, magnetic, and rotational forces.
The Lower Solar Atmosphere (LSA) section studies the evolution of the solar magnetic field and its interactions with the plasma from its emergence through the photosphere up to the chromosphere. The magnetic flux rises from the interior due to buoyancy and becomes directly measurable at the photospheric level by means of spectro-polarimetric observations. In this layer the dynamical state is dominated by convective plasma motions and the magnetic field is forced to follow these flows. As it moves upwards into the upper photosphere and lower chromosphere, a transition occurs to a completely different physical regime in which magnetic forces take over and dominate the dynamics. Understanding the implications of this transition is the main challenge of the LSA section, which is heavily driven by observations of the polarimetric signatures imprinted by magnetic fields on photospheric and chromospheric spectral lines.
The most significant achievement of the LSA during the past year has been the release of the MERLIN code for the inversion of spectro-polarimetric observations of the solar photosphere. This code interprets the polarized solar radiation in the two Fe I lines at 630.2 nm, and outputs the vector magnetic field and the thermodynamic properties of the observed solar region. The development of the code and of its web client was supported through the CSAC initiative. Through the CSAC website it is now possible to access pre-inverted spectro-polarimetric data selected from a continuously updated database of observations from the SOT/SP instrument on-board the Hinode spacecraft. Currently these observations represent the best available data for synoptic measurements of the vector magnetic field on the Sun, thanks to the high spatial resolution, stability of image quality, and extended time coverage attainable from space. The MERLIN web client allows a web user to access these pre-inverted observations, or to initiate a completely new inversion of a solar region of choice. The user is notified by e-mail when the inversion is completed, and the results ready for display and download (see figure 2).
For next year we plan to work towards the release of the next community-inversion code LILIA on the CSAC website. This code will allow a significantly more refined analysis of spectro-polarimetric data than to MERLIN, including the derivation of magnetic-field and thermodynamic gradients in the solar atmosphere. This type of information will prove essential for comparing observations to recent advances in the magneto-hydrodynamic convection modeling of the solar photosphere (M. Rempel). With the completion of SPINOR, there will also be an effort towards adding observations taken with this instrument to the CSAC database. We expect to achieve a complete debugging of ProMag, successful deployment of the instrument, and start of science operation within the next year.
The solar corona, heated by various mechanisms to million-degree temperatures is fully ionized. Embedded with a magnetic field of about 10 Gauss at the coronal base, this atmosphere is an excellent conductor of both heat and electric current. Outward thermal conduction of heat, aided by MHD and plasma waves, drives the outer corona to expand continuously into the solar wind, filling interplanetary space. High electrical conductivity enables the magnetic fields to store significant amounts of energy which is released through the resisitive dissipation of thin sheets of electric current. Such an MHD process heats the corona ubiquitously and produces the impulsive flares, but we still have much to learn about its basic physical nature. The corona is not static, of course. Its magnetic field changes in time, reversing its global polarity once every 11 years in concert with dynamo action in the solar interior. Thus, the dynamo rejuvenates the corona in each cycle, producing flares and sending daily coronal mass ejections into the more slowly varying solar wind. This is the dynamical origin of space weather. The NCAR program described below investigates the basic MHD of the corona as an essential component of national space-weather research.
Under coronal conditions of extremely high electrical conductivity, magnetic fields evolve with no change in their field topologies unless the ordinarily weak effect of electrical resistivity is enormously amplified via the spontaneous formation of current sheets, as described by the theory of Parker (1994). Plasma parcels embedding distinct magnetic flux systems cannot mix freely, and current sheets would develop as tangential field discontinuities at the boundaries between the flux systems. The nonlinear mathematical problems posed by this fundamental effect are formidable in general, but have been found to be tractable for the topologically untwisted magnetic fields defined by Low (2006a, b). Ase Marit Janse (ASP) and B. C. Low (HAO) presented two explicit examples of current-sheet formation in this special class of magnetic fields. The first example considers a topologically untwisted field inside a cylinder of perfectly conducting fluid (Janse & Low 2009). The field is anchored by its magnetic footpoints fixed at the cylinder ends, so that its topology is invariant to a continuous change in the length of the cylinder. Whereas this field of a fixed topology may have a continuous equilibrium state, as a potential field, for a particular length of the cylinder, no continuous equilibrium state is available to the field when the cylinder is given other lengths. In the latter case, magnetic discontinuities or current sheets must form throughout the field, whose dissipation can then change the field topology into one compatible with a continuous state. An important aspect of this demonstration is that magnetic neutral points are not essential to the process, as implied by the Parker theory.
The second example treats the topological change brought about by the dissipation of a pre-existing current sheet embedded under equilibrium conditions in a topologically untwisted field (Low & Janse 2009). This dissipation produces a reconnected potential field whose field topology implies that other current sheets, in addition to the sheet in the initial state, must have formed and dissipated throughout the field in order to arrive at its potential state. These demonstrations offer basic physical explanation of the ubiquitous heating observed in both the quiescent corona and energetically explosive events like the flares. The Parker theory needs to be incorporated into the MHD simulation models in use in solar and space-weather research.
Natasha Flyer (IMAGe), Bengt Fornberg (University of Colorado), Ken Miller (Wichita State University),and Low investigated the turbulently-relaxed state of a magnetic structure following a flare-like release of magnetic energy. The theory of Taylor (1974) points out that the formation and dissipation of current sheets in this relaxation is subject to an approximate conservation of the total magnetic helicity under the condition of extremely high electrical conductivity in the corona. Extending this idea originally developed for a contained laboratory plasma to an open astrophysical atmosphere is essential but has been neglected in current solar research. This extension was carried out by solving a free-boundary problem in a two-dimensional atmosphere. A numerical solver was developed to construct the relaxed equilibrium state, seeking as an unknown the boundary separating the flared magnetic structure from the surrounding part of the atmosphere not involved in the flare. The flared structure contains the initial flaring magnetic field as well as the surrounding field that has reconnected with it, subject to the conservation law on magnetic helicity. A hydromagnetic implosion effect first pointed out by Hudson (2000) is cleanly demonstrated by these numerical computations (Miller et al. 2008). As the stored magnetic energy is transformed into escaping radiation and high-energy particles during a flare, the magnetic pressure of the flaring magnetic field is reduced, leading to an inward collapse as the surrounding plasma and field push in with their superior pressure. This work motivates observations to look for the signatures of the Hudson implosion, and paves the way for numerical modeling of coronal relaxed structures in realistic 3D modeling.
Flyer, Mei Zhang (National Astronomical Observatory, Beijing & NCAR Affiliate Scientist) and Low continue with their theoretical study of magnetic helicity accumulation in an open hydromagnetic atmosphere. Progress has been made towards an analytical proof of an upper bound on the total magnetic helicity contained in a global force-free magnetic field. Exceeding this upper bound is a sufficient condition for a loss of equilibrium that must open up the field to let trapped magnetic twist escape. In this manner, the total helicity can be brought down to within the bound for equilibrium to be re-established. This possible hydromagnetic origin of coronal mass ejections has been investigated by Zhang & Flyer (2008).
A two-year effort initiated by Piotr Smolarkiewicz (MMM/IMAGe) and Low to simulate 3D MHD evolution leading to the formation of current sheets has produced interesting first results. The challenge of numerically describing sheet formation developing as singularities in a 3D magnetic field is handled by describing the field in terms of its flux surfaces as opposed to the usual Eulerian specification of the field vector as a function of space. Joined by Ramit Bhattacharyya (ASP), Smolarkiewicz and Low investigated a periodic field in an incompressible, viscuous, electrically perfectly-conducting fluid. The field is drained of its free energy as its Lorentz force drives a flow whose kinetic energy is viscously dissipated. The equilibrium end-state so produced requires the existence of a set of global magnetic flux surfaces each containing a constant fluid pressure. Although the initial field is untwisted with well defined global flux surfaces, it is the nature of 3D fields that, in general, it is topologically not possible for any set of these flux surfaces to be arranged into global isobaric surfaces. This impossibility manifests in the formation of magnetic tangential discontinuities. The above initial-value simulations show a first-stage extended smooth evolution followed by an evolutionary change identifiable with the formation of current sheets and their unavoidable artifical dissipation via the loss of numerical spatial and temporal resolution. These simulations provide the most direct illustration of sheet formation via the explicit representation of magnetic flux surfaces. The numerical codes developed by Smolarkiewicz (2006) have opened the way to further developments, to treat compressibility; boundary conditions of interest to solar coronal magnetic fields; and, twisted magnetic fields in the description of Low (2007a) using two pairs of flux surfaces. A problem of fundamental interest to be addressed is whether magnetic discontinuities can form in finite time (see Kerr & Brandenburg 1999) and (Grauer and Marliani 2000).
In an ongoing effort, Janse and her collaborators Oeystein Lie-Svendsen (Norwegian Defence Research Establishment & University of Oslo) and Ruth Esser (University of Tromsoe) investigated the heating of minority ions (carbon, oxygen and silicon) in the solar wind, using a multi-fluid numerical model developed at the Institute of Theoretical Astrophysics, University of Oslo. The goal is to compare the different solar-wind solutions, produced by varying the theoretical heat input in the model corona, for comparison with observations.
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ACD scientists have worked with data from the TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) satellite to investigate the dynamics and chemistry of the middle atmosphere. Much of this research has been done in collaboration with US and international colleagues.
The diurnal tide gives large amplitude variations over 24-hours of temperature and winds in the mesosphere. ACD scientists have worked on the characterization of variations in the tidal amplitudes and phases using temperature and wind data from TIMED and ground-based radar. The results indicate that there are large variations of tidal amplitude on semi-annual and annual timescales. The tide at the equator also varies on a multi-year timescale in concert with the quasi-biennial oscillation in equatorial lower stratospheric winds. This study will continue, using small differences in the tides to derive the eddy diffusion rate in the mesosphere.
Up to now, the chemistry of the mesosphere has been poorly constrained by observations. This is beginning to change, and analysis by ACD scientists is contributing. A new measurement from the SABER instrument on TIMED gives the distribution of atomic hydrogen. These global, multi-year measurements show a drop in the hydrogen in the summer mesosphere that is inconsistent with transport. However, a closer look indicates that this drop is consistent with the vertical redistribution of hydrogen due to the freezing of water in polar mesospheric clouds and subsequent vertical displacement and sublimation of the ice particles.
Ozone observations by SABER also reveal some unusual aspects. At the location of the ozone secondary maximum near 95 km, SABER observations show occasional very high mixing ratios of over 20 ppmv, and as high as 50 ppmv, during night. Analysis indicates that these are associated with unusually large amplitudes of the diurnal tide. The large tide leads to low nighttime temperatures at 95 km at the equator; the low temperatures affect the photochemistry of ozone in such a way that the ozone is high. The magnitude and occurrence of the high ozone fit well with current understanding of ozone photochemistry.
Stratospheric temperature trends
ACD scientists helped organize an updated assessment of stratospheric temperature trends, based on radiosonde, satellite and lidar measurements. This work was performed over the last several years in collaboration with a group organized under the WCRP SPARC Program. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit (MSU) covering 1979-2007 and the Stratospheric Sounding Unit (SSU) covering 1979-2005. Radiosonde results were compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ~0.5 K/decade over much of the globe for 1979-2007. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO2 increases. The results show mean cooling of 0.5-1.5 K/decade during 1979-2005, with the greatest cooling in the upper stratosphere near 40-50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995-2005. These observations will be utilized for detailed comparisons to model results within the SPARC CCMval Project.
Forcing of the tropical Brewer-Dobson circulation
The Brewer-Dobson upwelling in the tropical lower stratosphere is a dynamically-forced phenomenon, with a pronounced annual cycle leading to seasonal variations in stratospheric temperature, water vapor and other constituents. ACD scientists used diagnostic studies to quantify the dynamical forcing of large-scale upwelling in the tropical lower stratosphere, based on circulation statistics from ERA40 and NCEP/NCAR reanalysis data. Zonal mean upwelling derived from momentum balance and continuity (so-called downward control) was found to be in reasonable agreement with independent calculations based on thermodynamic balance. The detailed momentum balances associated with the dynamical upwelling were investigated, in particular the contributions to climatological wave forcing (EP flux divergence) in the subtropics. Results showed that the equatorward extension of extratropical waves (baroclinic eddies and, in the NH, quasi-stationary planetary waves) contribute a large component of the subtropical wave driving near the tropical tropopause. Additionally, there is a significant contribution of forcing from equatorial planetary waves forced by tropical convection. The observed balances demonstrate that the strong annual cycle in upwelling across the tropical tropopause is forced by subtropical eddy momentum flux convergence associated with waves originating in both the tropics and extratropics. Additionally, tropical upwelling is found to systematically increase in simulations of future climate. (link)
Transport and chemistry of the Asian monsoon anticyclone
The Asian monsoon anticyclone is a region of persistent pollution in the upper troposphere during Northern Hemisphere summer, resulting from vertical transport of surface pollution in deep convection, and confinement by the strong anticyclonic circulation. This circulation extends into the lower stratosphere, and may be an important mechanism for troposphere-stratosphere coupling. ACD scientists used short-lived chemical species measured by Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) to explore chemical behavior of the Asian monsoon anticyclone. The climatology of carbon monoxide (CO) measured by ACE-FTS shows a local maximum in the Asian monsoon anticyclone region (Fig. 1). Other short-lived species measured by ACE-FTS, such as hydrogen cyanide (HCN), ethane (C2H6) and acetylene (C2H2), which have common sources of biomass burning, show maximum enhancement inside the anticyclone near the tropopause (Fig. 2a). The photochemical age of air was estimated by the ratio of C2H2/CO, indicating that air inside the anticyclone is relatively young (Fig. 2b), i.e. this air has was recently transported from lower altitudes. Ongoing work is aimed at understanding the transport pathways within the monsoon region, and quantifying the effect of convective transport and large-scale circulation in the UTLS region.
Microphysics parameterization for CAM
ACD and CGD scientists have been leading a collaborative development effort for a new microphysics parameterization for CAM. The goal of this effort is to develop an advanced microphysics package which can represent the size of cloud drops, and simulate how cloud drops are influenced by the distribution of aerosols. The ultimate goal is to quantify aerosol indirect effects in CAM and CCSM. This work dovetails with other studies in ACD, MMM and CGD of cloud microphysics, both in observations and in models. An important component of the broader activity is the development of better satellite data sets of cloud microphysical properties for comparing the CAM and WACCM simulations to observations. This work has recently been extended to treat the effects of ice clouds and ice cloud aerosol interactions.
Evaluating model simulations of the Tropical Tropopause Layer
ACD scientists and collaborators also analyzed the representation of the Tropical Tropopause Layer (TTL) in global models. Comparisons of a number of diagnostics with observations indicate that global models are capable of representing TTL structure and variability well on many scales. TTL structure in 13 different chemistry - climate models has been analyzed, and the performance of the models and their representation of the past and future explored. Comparisons indicates that convection within the TTL itself is not critical for TTL structure, though convection below the TTL is important. Most models predict the tropical tropopause will get higher and slightly warmer in the 21st century. ACD scientists are continuing this work on evaluating tropopause behavior in global models within the SPARC CCMval Project.
ACD scientists are also leading an effort to develop a community model diagnostic package for use internally and by the community for coupled chemistry climate models. This includes developing advanced techniques for modifying model output, and utilizing satellite simulators to better represent observations and evaluate model simulations.
Transport from troposphere to stratosphere associated with the secondary tropopause
It has long been recognized that the temperature lapse rate based thermal tropopause definition produces breaks and multiple tropopauses in the extratropics. Recent analyses using global high resolution GPS data has shown that the area of double tropopause is much more extensive than previously realized. ACD scientists used newly available satellite data from HIRDLS to explore the connection between the occurrence of the secondary tropopause and chemical transport from troposphere to stratosphere. Figure 4 shows an example of low ozone layer above the primary extratropical tropopause and below the secondary tropopause, which is an extension of the tropical tropopause. A similar structure is also found in the potential temperature lapse rate, derived from the NCEP/GFS meteorological analyses, for the same cross section. (Pan et al., submitted, 2008)
This observation of relationship between the chemical structure and the meteorological field made it possible for the development of a forecast tool for aircraft observations, which in turn allowed successful in situ observations of this intruding tropospheric layer during the START08 experiment. Figure 5 shows an example measurement from the NCAR GV research flight on April 18, 2008, including meteorological analyses and in situ measurements. As indicated by Figure 4, the flight successfully observed an extensive layer of intruding tropospheric-like air between the two tropopauses. The large suite of chemical species measured will help characterizing the chemical impact of this type of event. The analyses of these new data will be the focus of next year.
Linking pollution with aerosols and clouds
In collaboration with Jonathan Jiang (Jet Propulsion Laboratory), ACD scientists analyzed MLS and MODIS data to quantify relationships between CO in the upper troposphere (an indicator of pollution) and cloud characteristics (MLS ice water content and MODIS effective cloud radii). Figure 7 shows the relationships between MODIS ice effective radii and MLS ice water content (IWC) for clean and polluted clouds over the Amazon during the wet and dry seasons. “Clean clouds” are those for which (by definition) MLS CO at 215 hPa is less than 120 ppbv, while “polluted clouds” have CO greater than 240 ppbv. MODIS aerosol is fairly constant during the wet season for the full range of MLS CO (since precipitation process scavenge aerosol). In the dry season MODIS aerosol increases as MLS CO increases. Therefore, differences in the dry and wet season panels are expected and observed. The results show that MODIS effective radii are systematically smaller in polluted clouds, demonstrating the so-called aerosol indirect effect.
HIRDLS cloud measurements
ACD scientists also led validation and analysis efforts for measurements of clouds from HIRDLS data. Figure 6 presents a comparison of cloud frequency in the tropical upper troposphere from HIRDLS and CALIPSO measurements. The magnitudes and geographical distributions are very similar, which demonstrates that both experimental data sets can be used to determine the occurrence and seasonal variations of cirrus near the tropopause in a convincing manner.
The goal of the NCAR Nested Regional Climate Model (NRCM) is to study processes across a wide spectrum of scales in the climate system. In the current NRCM setup, the NCAR Weather Research and Forecast (WRF) model is one-way nested in the NCEP reanalysis between 45S-45N, with vertical domain between the ground and ~10hPa. The horizontal resolution of the model is 36km (with two level nesting over Indonesia archipelago at horizontal resolution of 12km and 4km). Afforded by the high spatial resolution, we would like to explore gravity waves generated from the NRCM simulations, especially their spatial distribution and seasonal variability, and compare them with observations.
Han-Li Liu (HAO/TIIMES) and Jimy Dudhia (MMM) analyzed one year of NRCM (Columbia) simulations. Using wavelet method, they studied the global distribution and seasonal variability of the potential energy density, momentum flux and energy flux of gravity waves with zonal scales between 100-1600km. They also compare these results with those derived from satellite observations (GPS, SABER, CRISTA, and HIRDLS), and found good agreement between model results and observations. For example, strong gravity waves are identified over Western Pacific/South East Asia, Indian Ocean, West Africa, and Central America at low latitudes, similar to those revealed by GPS measurements and most likely related to tropical convection. Peak momentum flux of ~2x10-3 Pa over the Western Pacific/South East Asia is consistent with estimate from HIRDLS measurement. Their analysis also shows that the gravity waves display clear scale dependence: The potential energy density of gravity waves at low latitudes increases with zonal scale while those associated with orographic waves at mid-latitudes decreases with zonal scale. They also found that the quality of the simulation results deteriorates above ~20km because of the rapid decrease in vertical resolution and significant wave reflection from the model top.
They plan to compare the gravity wave sources from NRCM with those obtained from physics based gravity wave parameterizations recently implemented in WACCM. This will help improving gravity wave parameterization used in global models.
Gravity waves are believed to significantly impact the mesosphere and lower (MLT) thermosphere and drive it off radiative equilibrium. This has been predicted by theoretical and general circulation models, but direct measurement or inference of gravity wave forcing prove to be a challenge.
In this study by Han-Li Liu (HAO/TIIMES), Dan Marsh (ACD), Qian Wu (HAO), Chiao-Yao She (Colorado State University), and Jiyao Xu (Chinese Academy of Sciences), the wind balance in the mesosphere and lower thermosphere is revisited. Using simulation results from the NCAR Whole Atmosphere Community Climate Model (WACCM), they demonstrate geostrophic balance is no longer valid in the zonal direction due to the large zonal gravity wave foricng. As a result, the zonal mean geostrophic meridional wind is significantly different from the actual zonal mean meridional wind, and the residual mean merdional circulation derived from geostrophic winds is much weaker than that derived from model winds. It is also shown that the ageostrophic contribution in the MLT at middle and high latitudes comes primarily from gravity wave forcing, and it is possible to infer gravity wave forcing in the MLT from wind measurements. As a proof of concept, this wind balance relationship is applied to both the TIMED/TIDI winds and wind climatology obtained from the CSU Na-lidar to derive gravity wave forcing (Figure 2).
The next step in the research is to obtain wind climatology from multiple radar site at middle and high latitudes and also from more comprehensive satellite measurements to obtain a more comprehensive climatology of gravity wave forcing at middle and high latitudes.
The Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem) has been and continues to be developed by NOAA scientists, in collaboration with the WRF community including NCAR/ESSL scientists. The model is used for investigation of regional-scale air quality, field program analysis, and cloud-scale interactions between clouds and chemistry. ESSL scientists and staff provide support by integrating and maintaining the chemistry components in the evolving WRF modeling system, as well as contributing new code in the development of WRF-Chem. Models such as WRF-Chem are used to further the understanding of precipitation and chemical processes, including multiscale atmospheric chemical constituent transport, dispersion and transformations. Because WRF-Chem is able to simulate the coupling between dynamics, radiation, chemistry and aerosols, science issues that depend on these interactions are being pursued.
In April 2008, version 3 of WRF-Chem was released to the community. This version included new modules provided by NCAR/ESSL scientists. These modules are the Model of Emissions of Gases and Aerosols from Nature (MEGAN) which allows scientists to study interactions between the biosphere and atmosphere with impacts on air quality and climate, and the photolysis rate module, fast TUV, which performs a simplified, but accurate, radiation calculation of actinic fluxes. With this addition, WRF-Chem now has 3 options for the photolysis rate calculation. ESSL scientists are currently evaluating these 3 options with the MIRAGE field campaign measurements and are improving the coding to be more efficient.
Continued development of new modules for WRF-Chem addresses several aspects of the model. These include evaluating the new dust module, creating a framework to test different secondary organic aerosol chemistry schemes from complex to simple, developing a chemical tracer package that allows more flexibility for investigating sources of key chemical constituents, and creating a new and implementing old parameterizations of lightning-production of nitrogen oxides to be used at both the cloud scale and continental scale. In addition, the chemistry mechanism from the NCAR/ACD global model MOZART is being incorporated in WRF-Chem. This will allow us to contrast regional-scale and global-scale analyses of field campaigns and chemical weather using the same chemistry at both scales.
Simulations performed with WRF-Chem making use of the version 3 modules and ongoing development are focused on megacity impacts on the regional-scale air quality, the effects of thunderstorms on the upper troposphere composition, and the impact of wildfires on regional air quality. MIRAGE field campaign analysis with WRF-Chem has shown the importance of dust in cleansing the atmosphere via uptake of nitric acid and the significant role of secondary volatile organic compounds on ozone production downwind of Mexico City. Simulations of thunderstorms and chemistry have shown that air mass thunderstorms simulated for the northern Alabama region transport boundary layer CO throughout the free troposphere while more organized storms simulated for Oklahoma and northeastern Colorado transport CO directly to the upper troposphere. These simulations are being done in preparation for the Deep Convective Clouds and Chemistry field experiment. Other WRF-Chem simulations are addressing deep convection during the North American monsoon, wildfires in Greece affecting European air quality, and air quality in the Shanghai region to prepare for a future mega-city field campaign. This work is supported by the NSF, NCAR-MIRAGE strategic initiative funds, and NASA.
A study using WACCM and other models with interactive chemistry looked at the predictions for climate change in the Southern Hemisphere. In particular, the study addressed the predictions for winds in the SH middle and high latitudes. There is a clear difference in the climate predictions of models that do not include interactive stratospheric chemistry and those that do. The difference can be traced to the simulation of the ozone hole. When interactive chemistry is not included, winds continue to accelerate in a manner consistent with observations in recent years. However, in model simulations with interactive chemistry and a full stratosphere, the Antarctic ozone hole recovers during the 21st century due to the predicted drop in halogens in the stratosphere. This changes the thermal structure since ozone is an important radiative gas. With ozone recovery, polar temperatures warm and winds decelerate such that the overall prediction for the climate in the southern high latitudes is significantly different.
ACD work using the WACCM model involves an extensive set of collaborators from other ESSL divisions and externally from US and international institutions.
The model was used for a global simulation of dust particles from meteors hitting the Earth's atmosphere. The simulations show that the summer high-latitude mesosphere is relatively depleted in these particles due to large scale upwelling. The number of particles may still be sufficient to supply the necessary condensation nuclei to account for the formation of polar mesospheric clouds.
Another set of simulations used WACCM to assess the impact of the insertion of sulfur into the stratosphere: one of several suggested "geo-engineering" activities to counteract the predicted climate change from greenhouse gases. The sulfur would form particles that absorb and reflect sunlight, thereby reducing the solar heating at the Earth's surface in a manner similar to large volcanic eruptions. As shown by the WACCM simulations, the sulfate particles do delay the warming at the surface compared to other simulations without the injected sulfur. However, the sulfur would also accelerate the ozone loss that occurs in late winter over Antarctica and would increase the ozone loss over the northern polar region.
According to WACCM simulations, a regional nuclear conflict could also have severe consequences for stratospheric ozone. In this case, the primary effect would be the heating of the lower stratosphere due to thick and persistent smoke. The heating leads to photochemical loss of ozone. A conflict involving 100 bombs could lead to ozone losses of 20% globally, with predictions of more than 50% at high latitudes.
Antarctic Tropospheric Chemistry Investigation (ANTCI) was a collaborative four-year study of the sulfur chemistry in the antarctic atmosphere, including two antarctic summer field seasons in 2003-04 and 2005-06. The overall project involved thirteen principal and senior investigators at seven institutions. The broad based goal of this program was to enhance our understanding of the processes that control tropospheric levels of reactive hydrogen radicals, reactive nitrogen, sulfur, and other trace species over the Antarctic continent for the further purpose of improving the climatic interpretation of sulfur-based signals in antarctic ice core records. ANTCI was designed to address the many questions arising from the surprising findings of two earlier studies: Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) and Sulfur Chemistry in the Antarctic Troposphere Experiment (SCATE). The former effort involved two field investigations (i.e., 1998 and 2000) at the South Pole (SP); whereas the latter study was carried out at Palmer Station on the coast of Antarctica. The general picture that emerged from these earlier studies was that although the oxidation of sulfur at coastal sites was a major component of the overall chemical system, one of the critical steps (e.g., involving the intermediate oxidation product dimethyl sulfoxide, DMSO) was largely controlled by heterogeneous processes. By contrast, at SP reactive gas phase sulfur was a rather minor chemical player with its oxidation being controlled by intense HOx/NOx photochemistry prior to reaching the pole. In fact, based on the ISCAT studies, much of the chemical environment at SP appeared to be dominated by fast photochemical processes. ANTCI was designed to further explore the mechanisms controlling the oxidation of sulfur under the quite different environmental conditions presented by these two sites. However, it was equally important to gain a more complete understanding of the detailed atmospheric processes controlling reactive nitrogen and HOx radical levels, and to determine the extent of the enhanced levels of these compounds.
ANTCI 2003 was the first of two studies designed to address the above issues. The first phase of ANTCI 2003 had both a large ground-based chemistry component and a limited set of aircraft chemistry measurements. The second phase (e.g., ANTCI 2005, to be discussed in later publications) was largely aircraft based with a far more complete set of aircraft photochemistry measurements, but it also had a ground-based winter-over sampling component at the SP. ANTCI 2005 was designed to study HOx/NOx chemistry over a substantial portion of the Antarctic plateau and coast under a range of different conditions. An additional goal was to explore possible primary sources of reactive nitrogen to the plateau.
Results from the ANTCI 2003 study were published in 2008 in Atmospheric Environment. The papers have a strong focus on the oxidative characteristics of the plateau's near surface atmosphere because it is this oxidation process that determines the products of reactive nitrogen, sulfur, mercury, and other gases that will characterize the trace composition of the Antarctic atmosphere, snow, and ultimately the ice core record. Model results show amazingly high OH concentrations for this dry, remote area with high solar zenith angle and 24 h of continuous solar activity. These are mainly driven, as previously shown at the SP during ISCAT, by highly elevated NO levels. The latter were a major surprise as were the enhanced concentrations found for other OH/HOx precursors, including H2O2 and CH2O. In each case, these precursors were found to be emitted from the snow surface. The aircraft observations supplied the first extensive horizontal and vertical distributions for NO on the plateau. In so doing, they demonstrated that the major enhancements observed at SP are not an artifact of the station area but rather tend to be typical of a significant region of the plateau. They also provided the first upwind history of NO emissions, allowing estimates of how NO and O3 concentrations can grow during transport to the SP.
Severe convective weather, including tornadoes and severe wind gusts, impacts life and property throughout the world. In the United States, severe convective weather results in hundreds of deaths every year. ESSL scientists study the processes by which thunderstorms produce severe weather with the goal of understanding and better predicting their occurrence.
Several ESSL scientists participated in the 2008 Spring Experiment, which is held annually in Norman, Oklahoma. This program brings together researchers and forecasters to evaluate experimental severe weather forecasting techniques in real-time. In support of the experiment, ESSL scientists developed a real-time WRF forecasting system using a 3-km grid. A significant advancement this year was the use of mesoscale data assimilation based on the WRF-3DVAR system, which allows for detailed representation of severe convection in the model’s initial conditions. Results show that inclusion of mesoscale data assimilation improved many forecasts, even in the 24-36 hour timeframe. However, in some cases the modeling system initialized storms that were too intense and lasted too long, which had a detrimental effect on some forecasts. Further work is planned to identify and eliminate the sources of this anomalous convection.
Through the STEP (Short Term Explicit Prediction) program, ESSL scientists collaborated with EOL and RAL scientists to develop new methods for producing 0-12 hour forecasts of high-impact weather. For example, one method being investigated is ensemble forecasting with numerical models that produce convective storms explicitly. One of the recent highlights for the STEP program was the IHOP retrospective study, in which experimental nowcasting and forecasting systems were demonstrated for a one-week period of active weather in June 2002. Results show that the ensemble forecasting system is able to accurately predict the regions where supercell thunderstorms occurred.
The production of severe convective winds at night remains a forecasting challenge. One reason is the formation of strong stable layers near the surface caused by radiative cooling. A study has investigated severe convective systems in these environments by analyzing observations and by conducting idealized numerical simulations. The study found that near-neutral layers above the stable layer are favorable for severe wind production. These near-neutral layers help increase conditional instability, but they also allow for the formation of cold pools above the stable layer. In some numerical simulations, cold pools develop downward from these near-neutral layers, sometimes leading to severe winds at the surface. The simulated convective systems are structurally similar to severe bow echoes even if they do not produce severe winds, which helps to explain the difficulty in identifying severe convective storms at night. Further research is planned to clarify the environmental conditions that can be used to better predict these events and the numerical model settings needed to accurately simulate them.
ESSL scientists have been collaborating with other scientists at NCAR, NOAA,
universities, and private companies to plan the Verification of the Origins
of Rotation in Tornadoes Experiment 2 (VORTEX2). This field experiment in
the US Great Plains in May and June of 2009-2010 will investigate tornadogenesis,
near-ground winds in tornadoes, relationships between tornadic storms and
their environments, and numerical weather prediction of supercells and tornadoes.
In addition to going to the field, ESSL scientists will be supporting this
experiment through real-time high-resolution WRF numerical weather prediction.
This year has seen considerable progress in understanding the interaction between organized precipitating convection and the large scales of motion. Our approach involves cloud-system resolving models (CRM), dynamical models, and observational analysis. The following summarizes work in which the organization of precipitating convection is a common factor.
a) Parameterization of convective organization in global models
The parameterization of convective organization in global models is a problem that has confounded the parameterization community for over three decades. It will be at least another decade before computational facilities are sufficient to enable convection to be represented explicitly in contemporary climate models, so parameterization is necessary. The new hybrid parameterization for next-generation global models (grid-spacing ~10 km) reported last year is relevant in the context of the Community Atmospheric Model (CAM) and the Community Climate System Model (CCSM). In the hybrid approach, mesoscale convective organization is represented explicitly by grid-scale circulations while small-scale convection is parameterized. Along with CRM analysis, this hybrid approach is a basis for constructing a parameterization of mesoscale convective organization. As a first step, the plan is to estimate the free parameters in the parameterization framework of Moncrieff and Liu (2006) using CRM simulations. A proposal has been written (PIs: Yaga Richer, CGD and Mitch Moncrieff, MMM) for support to develop and test the mesoscale parameterization. This is a collaborative effort between MMM, CGD, and the TIIMES Water Systems Program. This work is supported by NSF and, if the proposal is successful, will be supported by NOAA.
b) Stratospheric gravity waves generated by multiscale organized tropical convection.
The objective here is to numerically simulate and model stratospheric gravity waves generated by organized precipitating systems, contrasting with previous work on wave generation by single clouds. This year the effects of tropospheric shear on convective organization and stratospheric gravity waves were investigated. The objective of this new study is to examine how mesoscale momentum transport (MMT) by organized convection relates to momentum transport by convectively-generated gravity waves in the stratosphere. (Gravity wave energy absorption is a major uncertainly in models of the deep atmosphere, such as WACCM.) Figure 1 shows that the convective momentum transport in the troposphere has the same sign as the gravity-wave transport in the troposphere. Consequently, when organized convection is represented in global models (see above item); the gravity-wave response in the stratosphere can be predicted. This research is collaborative with Todd Lane, The University of Melbourne, Melbourne, Australia. This work is sponsored by NSF.
c) Madden-Julian Oscillation (MJO) and convectively-coupled waves
A new study of explicitly simulated tropical convection over idealized warm pools aims at quantifying the relationship between the spatial pattern of organized tropical convection and sea-surface temperature (SST), and explaining the observation that the strongest convection is not located where SST is highest. The reason hinges on the effects of cloud-radiative interaction and surface friction on convective organization. Figure 2a, the control simulation contains both cloud-interactive radiation and surface friction. In Fig. 2b, where the radiative heating is horizontally uniform, organized convection shown by the cloud outlines is far displaced from the maximum SST in the center of the domain. In Fig. 2c, where surface friction is omitted, the convection occurs over the warmest SST. This shows that spatial pattern of convective organization is sensitive to the parameterization cloud-radiation interaction and surface friction. A paper by Liu and Moncrieff has been submitted to J.G.R - Atmospheres.
Propagating rainfall episodes associated with convectively-coupled waves over the Bay of Bengal analyzed from satellite data indicate multiscale convective organization similar to the MJO, suggesting a scale-invariance between the MJO and the episodes in the Bay. Figure 3 shows multiscale convective organization over the Bay during the monsoon season. The rainfall streaks consist of organized rainfall episodes (mesoscale convective systems) originating over the Indian continent and subsequently propagating southward over the Bay. Irregularly, the propagating episodes cluster into larger-scale systems morphologically similar to the multiscale structure of the MJO. Interestingly, the propagation direction of the MCS over the Bay of Bengal is parallel to the shear vector, which contrasts with the perpendicular orientation of MCSs at large. The shear-parallel organization stems the hydraulic effect of the convectively-generated pressure gradient on propagating systems, somewhat similar to density-current dynamics. The results were published in Liu et al. (2008).
The above research was supported by NSF.
d) Year of Tropical Convection (YOTC)
Incomplete knowledge and practical issues relating to tropical convection
severely disadvantage the skill of numerical weather prediction models and
climate models Furthermore, tropical convection has long-range effects on
stratospheric-tropospheric exchange, the large-scale circulation of the upper-atmosphere,
and the variability of weather and climate around the world. In order to address
this major challenge, WCRP and WWRP/THORPEX proposed a year of coordinated
observing, modeling, and forecasting, which led to the Year of Tropical Convection,
YOTC. The theme of YOTC is the role of organized tropical convection at large
scales. Together with accompanying research activities, the YOTC seeks to
advance knowledge, diagnosis, modeling, parameterization, and prediction of
multi-scale tropical convection and two-way interaction between the tropics
and extra-tropics. The YOTC project will exploit the vast amounts of existing
and emerging observations, the expanding computational resources, the new,
high-resolution modeling frameworks, and theoretical insights (see Fig. 4).
This activity involves unprecedented collaboration between international programmatic
activities, the operational prediction, research laboratory, and academic
communities. Global databases of satellite data, in-situ data, and high-resolution
model analysis and forecasts will be constructed. Emphasis is on timescales
ranging from days-to-months; that is, the intersection of weather and climate.
The following objectives were achieved this year: i) the ECMWF T799 (25 km)
global analysis, forecast products, and special diagnostics being archived
at ECMWF will be available to the community; ii) the YOTC Science Plan was
drafted; iii) the YOTC Implementation Plan is in the first stage of development.
Started in May 2008, YOTC will contribute to the Asian Monsoon Year (AMY),
the THORPEX Pacific Area Regional Campaign (TPARC), the United Nations Year
of Planet Earth, and the International Polar Year. The scientific basis for
YOTC is published in Moncrieff et al (2007), and a brief summary is published
in Waliser and Moncrieff (2007). The YOTC work is collaborative between MMM
The solar photosphere is a transition region in which the primary energy transport mechanism switches from convection to radiative transfer. At the same time the plasma becomespartially ionized, due to the lower temperatures requiring a more complicated equation of state. Also, the role of the magnetic field is changing: while the interior of the sun is dominated by the gas pressure, in or above the photosphere the magnetic pressure becomes the dominant contribution. Due to the rather short density scale height the photosphere is a highly stratified medium in which convective motions easily steepen up to supersonic flows and shock waves. The combination of all these conditions make numerical modeling of the photosphere challenging, but also extremely interesting due to the strong interaction between convection, magnetic field; and radiation, and the possibility for in depth comparison with high resolution observations.
During the past year,HAO, in collaboration with the Max-Planck Institute for Solar System Research (MPS) in Germany, modified the MURaM MHD code to specially deal with the numerical challenges encountered in regions of strong magnetic field in the photosphere. This new code has been used to study the fine structure of sunspots, especially the filamentary structure of the penumbra. The numerical simulations have identified a magneto-convective origin of penumbral filaments in which expansion and flux expulsion due to overturning convective motions leads to a filamentary structure characterized by reduced magnetic field strength, increased inclination angle and horizontal outflows. The simulated penumbral structure corresponds well to the observationally inferred interlocking-comb structure of the magnetic field with Evershed outflows along dark-laned filaments with nearly horizontal magnetic field and roll-type perpendicular motions, which are embedded in a background of stronger and less inclined field.
Future research in this field will focus on high resolution simulations that allow for more detailed comparison to observations such as Hinode. Simulations of full sunspots in larger domains will address the global structure of sunspots and their surroundings. Results from these simulations are also essential for the development of more advanced helioseimic inversion methods.