GEOS-5 atmospheric modeling and diagnostics
October 26, 2012 15:26:12
Description of Problem
An overarching goal of the GMAO atmospheric modeling effort is to develop a single atmospheric general circulation model (GCM) suitable for data assimilation, weather forecasting and climate simulation. Climate simulation includes atmosphere only, coupled ocean atmosphere, and coupled chemistry-climate modes. The model’s collection of physical parameterizations is of central importance to the success of the GMAO’s modeling effort.
Part of this year’s effort was focused on the final model changes that led to improvements in data assimilation mode at higher resolution, and resulted in the release of the model to be used for decadal climate prediction.
Scientific Objectives and Approach
The basic approach is through multiple simulations in climate, weather forecasting and assimilation modes, analysis of model results based on comparison with observations, and new experiments based on hypotheses to explain model-validation inconsistencies. This includes the development of innovative ways in which to compare model and observations, and innovative ways to inform the model’s parameterizations using observations.
The Fortuna 2_5 version of the GEOS-5 GCM was released, for use in the GMAO’s Coupled Model Intercomparison Project -5 (CMIP5) coupled and atmosphere only simulations and for use in the GMAO assimilation system used for mission support during the Discover AQ mission. Fortuna-2_5 was also released for public use through the GMAO web server. Results of coupled atmosphere-ocean climate simulations and of atmosphere only climate simulations are comparable in accuracy to other state of the art GCMs.
A series of 3-year simulations were performed with the Fortuna-2_5 version of the GEOS-5 GCM, aimed at a comprehensive validation and documentation of many aspects of the simulations. A series of experiments was also designed and performed to carefully attribute the differences between Fortuna-2_5 and MERRA AGCM simulations to specific changes in model parameterizations. The behavior of the mean climate as compared to reanalyses and to satellite-based observational estimates of many facets of the simulation are documented in a NASA technical memorandum. The technical memorandum also includes the documentation of the model changes implemented to improve the simulations.
The development effort for the GEOS GCMs physical parameterizations during the coming year will focus primarily on the identification and removal of model behavior which is discontinuous in nature. Analysis of GCM and single column simulations has pointed to errors specifically connected to the discontinuities in the behavior of the turbulence and moist parameterizations. The aspects of the parameterizations that will be specifically addressed include the assumption of a ‘top-hat’-shaped sub-grid scale distribution of water vapor and sudden onset of phase changes in the prognostic cloud scheme, and an abrupt end to buoyant ascent in both the convective and turbulence parameterizations. The model parameterizations will also be examined to remove behavior that is inherently dependent on vertical resolution. Removal of these discontinuities is anticipated to reduce model error and enable a more continuous response to parameter changes during model tuning endeavors. In addition to the correction of these known errors, development during the coming year will include the replacement of the current version of the cumulus parameterization with the more modern RAS-2, which includes the simulation of a cumulus downdraft and a modified mass flux entrainment profile for shallow convection.
In addition to the model development effort, the coming year will be focused on the completion of technical memoranda, and journal papers presently in the review process. These publications include a technical memorandum documenting the changes in parameterizations and the connections to the improved climate simulation. A journal paper will follow, focused on explaining the improvements in tropical precipitation and global stationary wave pattern in the context of existing theory about tropical organization and teleconnections. A journal paper reporting on the study of model subgrid scale variability of total water and the connection to the parameterization of critical relative humidity will also be completed and submitted.
Other Publications and Conferences
Molod, A. (2011), Constraints on the Width of GCM Total Water PDF from AIRS and High Resolution Modeling, poster presented at the Community Earth System Model Workshop, Breckenridge, Co., June 20-23, 2011.
Molod, A. (2011), GEOS-5 AGCM: MERRA to Fortuna, presented at the GEOS-5 MAP Workshop, Greenbelt, MD, June 3-4, 2011.