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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D1, 4035, doi:10.1029/2002JD002523, 2003

A modified formulation of fractional stratiform condensation rate in the NCAR Community Atmospheric Model (CAM2)

Minghua Zhang

Institute for Terrestrial and Planetary Atmospheres, State University of New York, Stony Brook, New York, USA


Wuyin Lin

Institute for Terrestrial and Planetary Atmospheres, State University of New York, Stony Brook, New York, USA


Christopher S. Bretherton

Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA


James J. Hack

National Center for Atmospheric Research, Boulder, Colorado, USA


Phillip J. Rasch

National Center for Atmospheric Research, Boulder, Colorado, USA


Abstract

This paper describes a modified formulation of stratiform condensation rate associated with fractional cloudiness in the Community Atmospheric Model Version 2 (CAM2). It introduces an equation to link cloudiness change with the variation of total condensate. Together with a diagnostic cloud relationship that represents subgrid-scale variability of relative humidity, a closed system is formed to calculate the fractional condensation rate. As a result, the new formulation eliminates the two closure assumptions in the Rasch and Kristjànsson [1998] prognostic cloud scheme. It also extends the Sundqvist [1978] scheme by including the influence of convective detrainment and advection of condensates on the fractional cloudiness. Comparison is made between the present formulation and the Rasch and Kristjànsson scheme by using data from the Atmospheric Radiation Measurement Program and through global model simulations with CAM2. It is shown that relative to the Rasch and Kristjànsson scheme, the new formulation produces less clouds and a slightly warmer troposphere, thus reducing the original cold bias in the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM). Even though the overall impact of the new formulation on the model climate is small, the modifications lead to more consistent treatments of fractional cloudiness change, condensation rate, and cloud water change in the model.

Published 15 January 2003.

Index Terms: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 3319 Meteorology and Atmospheric Dynamics: General circulation; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation.

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Citation: Zhang, M., W. Lin, C. S. Bretherton, J. J. Hack, and P. J. Rasch (2003), A modified formulation of fractional stratiform condensation rate in the NCAR Community Atmospheric Model (CAM2), J. Geophys. Res., 108(D1), 4035, doi:10.1029/2002JD002523.