SGER: Upgrading Community Atmosphere Model (CAM) Cloud Macrophysics

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A climate model is a complex software system whose parts must work properly together. This is a particular challenge for its atmospheric 'moist physics' parameterizations - turbulence, cumulus convection, cloud formation, cloud-aerosol interaction and precipitation, because these processes tightly interact on length scales much smaller than a climate model grid cell. The Community Atmosphere Model (CAM) is the atmospheric component of the world's leading community-developed climate model. Coordinated efforts by several research groups, loosely organized into a CAM Microphysics Initiative, are completely revamping most of the moist physics parameterizations. Their aim is to reduce CAM's cloud and precipitation biases and to enable credible global simulation of the effect of human-produced aerosol on clouds. During these efforts, some important shortcomings have been found in the CAM formulation for 'cloud macrophysics'- the parameterizations that determine the fractional cloud cover and the masses of liquid and ice cloud particles that condense and evaporate in each model grid cell. These shortcomings involve inconsistencies between the calculation of cloud fraction and cloud condensate that often result in 'empty clouds' (cloud area with no cloud particles), especially in regions of shallow cumulus convection. In high-latitude regions, CAM cloud predictions are sensitive to arbitrary changes to the order of calls to different parameterizations. The goal of this research is to implement and test a set of detailed code changes that will: (1) Better treat the interaction between cumulus and stratiform cloud cover, ameliorating issues of 'empty clouds' with finite cloud fraction but no condensate.(2) Ensure that the cloud macrophysics is operating on an appropriate state. (3) Use a more justifiable closure to calculate the rate of condensation /evaporation in the cloudy part of grid cells. (4) Use a relationship between cloud fraction and relative humidity that is analogous to schemes used by other leading climate centers. This research must be completed within the next several months in order to be evaluated in time to be included in the next generation Community Climate System Model 4 (CCSM 4), to be frozen in early 2009 and applied to the Intergovenmental Panel on Climate Change (IPCC) fifth assessment.Intellectual merit. The model changes will improve the internal consistency of the treatment of cloud processes in CAM. They will allow all moist processes to contribute to a consistent and physically realizable subgrid spatial distribution of cloud condensate within each grid cell. All changes are based on recommendations from the published literature. Broader Impacts. Many NCAR, DOE and university scientists use the CCSM as their primary climate model. This research will provide a solid basis for further code improvements in the microphysics, cumulus convection, and moist turbulence parameterizations that are needed to produce a cutting-edge simulation of human-induced aerosol effects on clouds and climate and better projections of 21st century climate change. These changes will be ready to use for model simulations in support of the 5th IPCC Assessment, for which better simulation of cloud-aerosol feedbacks is emerging as an important thrust.