CGD 2008 Profiles in Science: Atmospheric Modeling and Predictability
(formerly Climate Modeling Section and Climate Dynamics and Predictability)

In fiscal year 2007, the Atmospheric Modeling and Predictability (AMP) section was created, comprising two former CGD sections, the Climate Modeling Section (CMS) and Climate Dynamics and Predictability (CDP) section. AMP leverages staff capabilities in fresh ways, while maintaining focus on the improvement, analysis and documentation of the CCSM Community Atmosphere Model (CAM) and related studies aimed at improving the understanding of key processes in the climate system. AMP scientists also continue to study the inherent predictability of atmospheric phenomena and utilize the expertise gained through ensemble techniques to address the prediction of climate variations and extreme events. In addition to their contributions to CAM development, AMP staff play an integral role in the development and improvement of the Whole Atmosphere Community Model (WACCM), a comprehensive model of the atmosphere from the Earth's surface to about 150 km, which includes interactive chemistry and physical processes throughout the model column. Additionally, AMP staff contribute directly to national and international activities focused on advancing climate science by coordinating and conducting broad community initiatives.

Project: Predictability and Prediction Studies of Weather and Climate Variations

The studies described below are devoted to the prediction and predictability of climate variations and extreme events, which are integral to our section goals of extending and defining the spatio-temporal domain over which scientifically and societally useful forecasts can be made.

Working in conjunction with Jeff Yin of the Climate Analysis Section, Grant Branstator has undertaken a new project devoted to characterizing the effect of intra-annual and longer time-scale fluctuations in the circulation on the likelihood and strength of extreme near-surface wind events. They have found it useful to subdivide this influence into two categories. One category concerns the simple additive effect of fluctuations in the mean winds whereby the probability distribution of wind speeds shifts without changing its shape. The second category concerns a multiplicative effect whereby the changing low-frequency state changes the character of the statistical distribution of high-frequency perturbations. They have found that both mechanisms are important but their relative importance is highly dependent on the geographical region being studied. Simplifying matters is the fact that the multiplicative effect is largely manifested through a simple change in the variance of high-frequency variations though the shape of statistical distributions can also be affected by low-frequency circulation changes. A promising outcome of their work is that when they compare relationships between low-frequency circulation changes and the statistics of extremes in nature to corresponding relationships in Climate of the Twentieth Century integrations with CCSM3, the relationships are very similar. This includes the regional dependence of the relationships. The verisimilitude of the CCSM3 integrations, together with the large samples made possible by ensemble experiments, will make it possible to derive statistically robust relationships between large-scale circulation states and wind storm extreme statistics. These statistical relationships can then be used to estimate changes in wind storm likelihood and strength in climate change experiments without the need for having large enough ensembles to explicitly derive the changes in the statistics of extremes.

In traditional prediction studies, Joseph Tribbia has been developing and analyzing the ENSO predictive skill of the NCAR CCSM. Over the previous year, he had produced a number of experimental hindcasts demonstrating the skill of CCSM3. This suite of hindcasts was used as a testbed for the further development of CAM3 and CCSM3. A remediation of the errors in the climatology of the simulated interannual variability in CCSM has ensued with the developments in the convective parameterization included in CCSM3.5. New forecast studies are currently underway with the latest version of CCSM to quantify the degree of improvement in ENSO hindcasts and to elucidate the root causes of the remaining deficiencies in the simulation of interannual variability.

One means of characterizing those dynamics of a system that affect its slow (and thus potentially predictable) evolution is to identify preferred or recurring trajectories that the system traverses through phase space. In past years Branstator has characterized these prominent trajectories in long integrations of AGCMs. Recently, working with Christian Franzke (IMAGe) and Andrew Majda (NYU), he has developed a theory for understanding which dynamical interactions can produce the trajectory signatures found in the earlier work. This theory leads to the conclusion that some of the most interesting trajectory features result from subtle departures from Gaussianity in the probability density functions of prominent flow patterns. Thus a necessary condition for forecast models to be able to reproduce these trajectories is that their climates have these same nonGaussian features. A more detailed description is included in Franzke, C., A. Majda, and G. Branstator, 2007: The Origin of Nonlinear Signatures of Planetary Wave Dynamics: Mean Phase Space Tendencies and Contributions from Non-Gaussianity. Journal of the Atmospheric Sciences, 64, 3987-4003.

Project: Diagnostic and Theoretical Studies of Variability and Validation

Within AMP the purpose of diagnostic analyses is twofold: diagnosis is used to test theoretical ideas concerning the mechanisms responsible for climate variations and their relative import and also test (i.e. validate) the behavior of comprehensive climate models like the NCAR CCSM against that of the observed climate system. A particularly insightful example of this type of research is a recent AMP study exemplifying these two types of diagnoses which is detailed below.

Theoretical ideas from physics, in particular statistical physics, are occasionally beneficial in the study of climate. In this vein, Branstator has extended work he has done in collaboration with Andrey Gritsun of the Russian Academy of Science concerning application of the Fluctuation-Dissipation Theorem (FDT) to climate problems. The FDT makes it possible to construct response operators that provide estimates of how a dynamical system will react to an external forcing. In general these operators are more accurate than a simple linearization of the governing equations. Branstator and Gritsun's past efforts have been devoted to producing operators for estimating the response of the mean circulation in an atmospheric general circulation model. During the past year this has been extended to the case of the response of second moments of state variables. For example they have succeeded in constructing operators that give very accurate estimates of how storm track variances and fluxes will change in reaction to any given heat or momentum forcing. These operators can be used for optimal forcing problems in which one finds the most efficient way to excite a response with prespecified attributes. For example they have considered optimal ways to excite the Atlantic storm tracks by tropical heating. Extensive tests of this methodology have been carried out with CCM0, NCAR's original community climate model; now the methodology is being carried over to NCAR's state-of-the-art AGCM, CAM3.

Project: Nonlinear Dynamical and Numerical Model Development Studies

In data assimilation work funded through the NSF Collaboration between Mathematics and Geophysics (CMG) program, Greg Duane (AMP visitor), Jeff Weiss (CU) and Tribbia have been examining the relationship between synchronization and assimilation. In past work they showed that the synchronization approach is equivalent to standard approaches based on least-squares optimization, including Kalman filtering, except in highly non-linear regions of state space where observational noise links regimes with qualitatively different dynamics. In such narrow regions, the synchronization approach is expected to give an improvement to Kalman filtering that will apply in any situation where a computational model is intended to track a physical process. The synchronization approach is used to calculate covariance inflation factors from parameters describing the bimodality of a one-dimensional system. See Duane, G.S., J.J. Tribbia, and J.B. Weiss, 2006: Synchronicity in Predictive Modelling: A New View of Data Assimilation. Nonlin. Processes in Geophys., 13, 601, for more details. In the recent extension of this research, the use of synchronization ideas in parameter estimation has been explored and the promising results for this paradigm are detailed in Duane, G.S.and J.J. Tribbia, 2007: Dynamical Synchronization of Truth and Model as an Approach to Data Assimilation, Parameter Estimation and Model Learning. Advances in Nonlinear Dynamics in the Geosciences, A. Tsonis and J. Elsner, Eds, Springer, 302pp.

In addition to advances in the numerics, the next generation of atmospheric model dynamical cores will in all likelihood span a range of scales which will include those for which the hydrostatic approximation is questionable. This will require new understanding of global non-hydrostatic effects, including the role of the horizontal component of the Coriolis force.

In search of benefits that a more general formulation of the dynamical models for global weather prediction and climate projection will provide, Akira Kasahara continued his research to understand the role of the horizontal component of the Coriolis force, which is neglected on the basis of the "traditional approximation (TA)" in most of the current weather prediction and climate projection models. It has been known that the usual justification of the TA for the atmospheric and oceanic dynamics is too simplistic. Because of a mathematical complication involved in the analysis of the dynamica system without the TA, Kasahara has been focusing the analysis on a very simple yet nontrivial dynamical system, namely linear Boussinesq equations in Cartesian coordinates. The most detrimental effect of using the TA is on the physics of inertio-gravity motions. An accurate description of inertio-gravity motions requires the horizontal component of the Coriolis force as well as its vertical component. Yet, relatively little work has been done to understand the role of this often-neglected partner of the earth's rotation effect in the atmosphere and ocean.

This year Kasahara formulated a numerical model to solve initial-value problems with the linear Boussinesq equations without the TA. Motions are assumed to be horizontally periodic, but bounded vertically at the top and bottom. The time-evolution of the vertical structure of wave motions is calculated from given initial conditions with or without forcing/dissipation under variable thermal buoyancy stratification. This program is intended to serve as a simple numerical laboratory to study the time-evolution of inertio-gravity waves. As one example, the formation of near-inertial currents in the oceans generated by atmospheric storms is investigated in detail. It is shown that under a realistic vertical buoyancy stratification, the non-traditional wave mode is likely to be excited if the forcing is applied near the bottom of the ocean, resulting from, say, an up and down movement of barotropic tide over corrugated topographic features. This phenomenon may provide a new mechanism to energy dissipation of tidal motions unique only by taking into account of a full rotation effect, not a partial effect as done traditionally. More details are included in Kasahara, A., 2007: Initial-value approach to study the inertio-gravity waves without the "traditional approximation". J. Comp. Phys., 225, 2175-2197.

Tribbia continued investigating the limitations of the hydrostatic balance approximation in a different context, that of limited area modeling. With Roger Temam (Indiana University) and Antoine Rousseau (Universit'e Paris-Sud), he has been pursuing the examination of approximate equations which break the strong constraint of hydrostatic balance. The reason for their interest is the well-known deficiency of the hydrostatic primitive equations, ill-posedness as an initial-boundary value problem. The ill-posedness of the system imposes severe restrictions on the applicability of the system for limited area regional climate modeling and the use of adaptive mesh methods. In the recent work they have studied a linear differential system consisting of two coupled scalar evolution equations in one space dimension which was derived from a modal analysis of the Primitive Equations of the ocean. They have shown numerically that, by adjunction of a small viscosity, the system converges to an unusual, unexpected limit system thus producing boundary layers and reflections of waves at the boundary. They have proposed an alternate set of boundary conditions of transparent type for the viscous systems and, in this case, the viscous system does not produce boundary layers or reflections of waves at the boundary. This work is described fully in Rousseau, A., R. Temam, and J. Tribbia, 2005: Boundary conditions for the 2D linearized PEs of the ocean in the absence of viscosity. Discrete and Continuous Dynamical Systems - Series A, 13, 5, 1257-1276. Over the past year, this work has successfully been extended to three spatial dimensions and the manuscript delineating the results is currently under review.

In studies for which the Rouseau, Temam, Tribbia 2005 research noted above should have immediate application, Tribbia is also involved in a project that examines the efficiency of numerical modeling on parallel machines. The collaborative effort with Aime' Fournier (IMAGe), Mark Taylor (Sandia) and Ferd Baer and Houjun Wang (UMd), has developed a spectral element based, locally refined resolution version of CAM. The work is described in Baer, F., H. Wang, J. J. Tribbia, A. Fournier, 2006: Climate modeling with spectral elements. Mon. Wea. Rev., 134, 3610-3624, doi: 10.1175/MWR3360.1 as well as in Wang, H., J.J. Tribbia, F. Baer, A. Fournier and M. A. Taylor, 2007: A Spectral Element Version of CAM2, Mon. Wea. Rev., 135, 3825-3840.

Project: CAM simulations

AMP scientists continue to explore innovative ways to evaluate the quality of CAM simulations. One of the more unique approaches recognizes that Numerical Weather Prediction frameworks can provide an excellent method of examining parameterization methods as it allows direct comparison of the parameterized variables (e.g. clouds, precipitation) with observations from field campaigns such as ARM early in the forecast while the forecast state is still near that of the atmosphere. In collaboration with staff members of PCMDI, Dave Williamson and Jerry Olson have developed the capability to apply the Community Atmosphere Model (CAM) in forecast mode without developing a complete NWP (numerical weather prediction) forecast/analysis system. The forecasts are initialized from reanalyses or operational NWP analyses for the atmosphere with the land spun up to be consistent with the atmosphere. They have completed in-depth studies of the CAM for several ARM IOPs, concentrating on moisture and cloud aspects of the forecasts. These studies have examined the balance of terms in the moisture and temperature prediction equations during the forecasts at the ARM CART site for different synoptic situations. Although these analyses cannot attribute a unique mapping of forecast error to parameterization component deficiencies, they do identify which model components should be further examined to determine the cause of their anomalous behaviors. Studies with these formulations are also being used to examine individual field projects, such as the PACDEX experiment. AMP scientists are also using data assimilation techniques in conjunction with IMAGe scientists to better understand model errors, particularly in the Arctic.

Project: CAM development with very simplified surface conditions

Williamson and Olson have developed versions of the CAM with very simplified surface conditions, allowing examinations of physical parameterization behavior with both the surface and the large-scale dynamical core. These "aqua-planet" experiments in which the surface is specified to be all ocean with a simple, often zonal, specified sea surface temperature distribution is a useful configuration since the atmosphere retains its full complexity, but eliminated the complexities associated with sea-ice, land, orography, and land-ocean contrasts. This work has led to an internationally coordinated activity known as the Aqua-Planet Experiment (APE) which is being conducted under the auspices of WGNE with collaborators Brian Hoskins and Mike Blackburn (University of Reading). The project is intended to provide a benchmark of current model behaviors, and more importantly, to stimulate research to understand the cause of differences arising from different models, different subgrid-scale parameterization suites, different dynamical cores, and different methods of coupling parameterized physics and dynamics. In addition to this simplified configuration of CAM, Williamson explored the questions of convergence with increasing resolution and of equivalent resolutions with different dynamical cores. These studies are being extended to additional dynamical cores.

New Project: Global Model Diagnostics

Section scientists continue to develop a community package to provide the best observations and diagnostic tools for use internally and by the community. We have also been working to integrate advanced techniques for modifying model output and to incorporate and utilize satellite simulators to better represent observations and better evaluate model simulations. These tools are used internally, and available to external collaborators. The effort contributes to other section efforts to evaluate the model.

Project: Simulation quality as a function of horizontal and vertical resolution

Another major evaluation focus in AMP is on simulation quality as a function of horizontal and vertical resolution. Jim Hack, Julie Caron, and John Truesdale have been examining the quality of the mean climate and its variability characteristics using the spectral dynamical core for horizontal resolutions ranging from T31 through T341. This work has demonstrated clear improvements in the simulated mean dynamical circulation when more a more traditional climate resolution of T42 is doubled to T85 (Hack et al., 2006). Variability metrics, however, are generally unchanged at the higher resolution. Furthermore, continued increases in horizontal resolution exhibit weaker responses in terms of simulation improvement, particularly with regard to the most serious systematic simulation errors. This appears to point to deficiencies in the treatment of parameterized physics as the principal source of these systematic errors. The resolution studies have demonstrated significant differences in the interaction of the dynamical core and the parameterized physics package as a function of horizontal resolution. To better explore these issues, Truesdale has incorporated modifications to the CAM so that a single-column model version of the CAM (known as SCAM) can be more seamlessly exploited. Recent work by Truesdale includes an analysis of hurricanes in the high-resolution (T341) CAM runs compared to statistics generated by the Nested Regional Climate Model (NRCM) runs. Studies using the CAM and SCAM at multiple resolutions are ongoing.

Project: Analysis of the Madden-Julian Oscillation (MJO)

In addition to exploring alternative configurations of the CAM for diagnostic purposes, AMP staff continue to extend standard turn-key and other diagnostic capabilities. Caron continues to develop quantitative methods for analyzing the structure of the Madden-Julian Oscillation (MJO) using the eastward propagating MJO-period OLR as an index (as in Wheeler & Kiladis 1999). Recent efforts include recoding, testing, and debugging by Truesdale, who has updated the code in the CAM standard diagnostic package. Caron and Dennis Shea will soon begin work to include some of the more detailed MJO diagnostics in the package as well. Caron continues development of diagnostic techniques for examining the diurnal cycle of warm-season convection in T31, T42, T85, and T170 CAM for the Southern Great Plains as well as other regions around the globe. An important component of this diagnostic work has been a comparison of the individual terms of the warm-season water vapor budget for a point in the Southern Great Plains in CAM simulations with IOP data (from Zhang et al 2001). Caron has also been working in collaboration with Junhong Wang, and Dave Parsons to look at the structure of the diurnal cycle of the Low-Level Jet over the Southern Great Plains compared to results from IHOP. Caron aims to present results of the diurnal structure of the low-level jet and the associated moisture transports in CAM compared to the North American regional reanalyses in an upcoming paper.

Project: The role of Polar Mesospheric Clouds (PMCs)

Andrew Gettelman continues to explore the role of Polar Mesospheric Clouds (PMCs) that form at the summer mesopause (~80km altitude) when temperatures reach below 150K. These clouds are modulated by temperature and by water vapor concentrations, and may be affected by particles acting as nuclei. The variability of these clouds provides a critical test of the coupling of dynamics and chemistry of the mesopause, and sheds important light on variability due to solar cycles, or due to long term climate changes in the upper atmosphere. Gettelman modified the cloud condensation routines in CAM & WACCM to parameterize Polar Mesospheric Clouds (PMC's) which can serve as a sensitive diagnostic of model processes in the mesopause region strongly affecting trace gas chemistry. PMC's are being used as a model diagnostic, as well as for historical climate simulations to look at the sensitivity of PMC's to climate changes over the last 150 years.

New Project: Arctic Climate

AMP scientist Gettelman and CSU postdoctoral researcher Jennifer Kay are working in conjunction with collaborators in CGD and the community to better understand drivers for Arctic climate, and investigate recent sea-ice loss events. Work has started using observations to look at radiative flux anomalies in the Arctic, and attribute recent sea-ice losses to long term trends as well as unforced atmospheric circulation variability. Work has started with observations and is continuing with advanced model diagnoses for the Arctic.

Project: Climate feedbacks

Climate feedbacks in the upper troposphere are some of the largest remaining uncertainties in quantifying the climatic response to anthropogenic forcing. With collaborators at the University of Washington and University of Reading, Gettelman continues to explore the radiation balance of the Tropical Tropopause Layer (TTL) and the implications for understanding upper tropospheric water vapor feedbacks in observations and in climate models. Recent work has investigated differences in humidity between observations and models and is continuing to analyze co-variations of clouds, humidity and temperature to try to understand climate sensitivity. This work suggests that humidity in the upper troposphere increases as the surface temperature increases. Significantly, CAM simulations show a similar picture. Much of this work exploits newly available satellite sensors, in particular from NASA EOS satellites (Aqua and Aura). Extensive work with AIRS and data validation has continued this year. Validation of EOS Aura data (MLS and eventually HIRDLS) is progressing and has led to numerous collaborations on the validation side with in situ data: University of Idaho, NASA-JPL, NOAA, Kyoto University, CNRS (France), IAP-Beijing, as well as with several other groups working with the data, including the University of Washington, and Johns Hopkins University.

Project: The study and improvement of atmospheric transport

Another major area of research in the Atmospheric Modeling and Predictability Section involves the study and improvement of atmospheric transport exploiting observational opportunities associated with tracer transport. Phil Rasch, et al., continue to explore the broad range of transport pathways as well as transport associated with resolved-scale motions, which is integrally coupled to ongoing studies of dynamical cores conducted by CGD and NCAR investigators (Williamson, Chen, Lauritzen, Nair).

Project: Studies of and improvements to the parameterized treatment of physical processes

Section scientists continue to refine and apply the CCPP-ARM Parameterization Testbed (CAPT) method to examine parameterizations. This approach runs the CAM in a forecast mode using an ensemble of historical analyses from very recent years for periods during which high-quality field program measurements exist, such as ARM IOPs. This allows direct comparison of the parameterized variables (e.g. clouds, precipitation) with observations from the field programs. Cécile Hannay, Williamson and Olson evaluated the way the CAM represents regions of persistent stratocumulus with forecast simulations of the 2001 East Pacific Investigation of Climate (EPIC) cruise. Stratocumulus clouds strongly influence the global climate due to their radiative effects and are a crucial factor in the surface and top-of-atmosphere energy balance. Also, because these clouds influence the radiation primarily though their albedo, their diurnal cycle is an important factor on their radiative effectiveness. Hannay et al. (2008) showed that the CAM underestimates the planetary boundary layer (PBL). They also evaluated forecasts with a revised atmospheric boundary layer formulation from the University of Washington (CAM-UW) and they suggest that the CAM-UW physics is superior to CAM for representing stratocumulus topped PBLs. The CAM produces a strong diurnal cycle in the cloud layer and surface fluxes but there are discrepancies in the daily mean, and in the magnitude/phase of the diurnal cycle compared to EPIC observations. This affects the radiative fluxes at the surface and therefore, the surface energy balance. This is especially important in coupled simulations as a misrepresentation of the net flux between the atmosphere and ocean can lead to severe SST biases. (Hannay, C., D. L. Williamson, J. J. Hack, J. T. Kiehl, J. G. Olson, S. A. Klein, C. S. Bretherton, and M. Koehler, 2008: Evaluation of Forecasted Southeast Pacific Stratocumulus in the NCAR, GFDL and ECMWF Models. J. Climate, accepted.)

Project: Radiative effects of aerosols

Examination of the radiative effects of aerosols continue, and involve a large community of NCAR and external investigators, including W. D. Collins, N. Mahowald, P. Rasch, T. Bond, J.-F. Lamarque, C. Zender, M. Flanner, and J. Randerson.

Project: Improving the parameterization of moist convection in the CAM

In collaboration with Rasch, Jadwiga Richter has implemented a parameterization of in-cloud convective momentum transport in CAM. This parameterization improves several of the long-standing model biases, including features in the surface wind field such as (a) the easterly bias in the tropics, b) the westerly bias in the north-eastern pacific, c) and the westerly bias in the 60S jet. Additionally, as a result of added convective momentum transport, the representation of tropical precipitation in CAM is improved in the Indian Ocean and the equatorial Western Pacific. The convective momentum transport has become a part of the next generation CAM/CCSM.

Project: Effects of gravity wave parameterization in CAM

J. Richter has compared the tropospheric climate in CAM to that in WACCM and found that there were several improvements in WACCM as compared to CAM. In particular, the biases in the sea level pressure and surface stresses in the Northern Pacific relative to observations were much smaller in WACCM. J. Richter investigated the cause of the differences and found that all of the following factors in WACCM contribute to the improvements: a different critical Froude number, convective and frontal gravity wave parameterization, and a higher lid. Richter is currently investigating the possibility of an increased top lid in CAM to arrive at a better tropospheric climate in this model. Parameterization comparisons continue with the Colorado State University's cloud-resolving model (CRM) and results are expected to transition to the Advanced Research WRF (ARW) model in the future.

Project: Microphysics parameterization for CAM

Gettelman has been leading a major 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 how cloud drops are affected 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. The current project is a collaboration between MMM, CGD and ACD, along with members of the community. An important component of the broader activity is the development of better satellite data sets of cloud microphysical properties for comparing the CAM simulations to observations. This work has recently been extended to treat the effects of ice clouds and ice cloud aerosol interactions.

Project: Gravity wave research

J. Richter has compared the tropospheric climate in CAM to that in WACCM and found that there were several improvements in WACCM as compared to CAM. In particular, the biases in the sea level pressure and surface stresses in the Northern Pacific relative to observations were much smaller in WACCM. J. Richter investigated the cause of the differences and found that all of the following factors in WACCM contribute to the improvements: a different critical Froude number, convective and frontal gravity wave parameterization, and a higher lid. Richter is currently investigating the possibility of an increased top lid in CAM to arrive at a better tropospheric climate in this model.

Project: Improving the capabilities and simulation fidelity of WACCM / New gravity wave parameterization in WACCM

In collaboration with R. Garcia (ACD) and F. Sassi (CGD), J. Richter has developed a new gravity wave parameterization for WACCM. This parameterization replaces the old, arbitrarily-specified gravity wave source spectrum with a source-oriented approach. The new parameterization explicitly treats gravity wave generation from fronts and convection. This parameterization provides more realistic spatial and temporal distribution of gravity wave sources. As a result, the simulation of the middle atmospheric climate in WACCM is improved as compared to the previous version of the model. The greatest improvement is seen in the improvement of variability and the improved frequency of Sudden Stratospheric Warmings.

Project: Model intercomparison activities

AMP Scientists were also involved in a number of model intercomparison activities. These included several GEWEX intercomparisons, including Case 5 of the Deep Convection Working Group that concentrates on the TOGA CORE period, and the Cross Pacific Intercomparison case, which spans the range from deep convection to stratocumulus (Williamson, Olson, Hannay, Kiehl, and Hack). Gettelman is co-coordinator of an international project to evaluate coupled chemistry climate models, a project conducted under the auspices of the Stratospheric Processes and their Role in Climate (SPARC) project of the World Climate Research Program (WCRP) as well as lead author for a major assessment report on global ozone modeling.

Under the auspices of WGNE, Williamson is organizing an intercomparison (Transpose AMIP) of climate models, and possibly forecast models, applied in the CAPT forecast mode, from common initial conditions. Data from the participating groups have been collected at NCAR. These groups are the Numerical Prediction Division of the Japan Meteorological Agency, GFDL, NCAR, the Hadley Center of the Met Office, and the Experimental Climate Prediction Center of the Climate Research Division, Scripps Institution of Oceanography. Analyses are now underway. Williamson continued to organize the coordinated Aqua-Planet Experiment (APE) under the auspices of WGNE with Mike Blackburn and Brian Hoskins (University of Reading). The project is intended to provide a benchmark of current model behaviors, and more importantly, to stimulate research to understand the cause of differences arising from different models, different subgrid-scale parameterization suites, different dynamical cores, and different methods of coupling the two. Details of the experimental design and of the workshop can be found at http://www.met.reading.ac.uk/~mike/APE/ape_home.html. The second (final) workshop was held November 2007 at the Disaster Prevention Simulation Center, Chiba Institute of Science, Choshi, Chiba, Japan. Plans are underway to produce and atlas of APE results and for a series of papers to be collected for a special journal issue. AMP Scientists also continue to play leadership roles as members or chairs of many national and international committees including CCMVal (Gettelman), SPARC (Gettelman), IGAC (Rasch, Chair), North American THORPEX Committee, WMO (Tribbia), Center for Ocean, Land, Atmosphere Studies, Scientific Steering Committee (Branstator) CLIVAR PSMIP (Hack, co-chair), DOE ASCAC (Hack), and WGNE (Hack).