Parameterization of moist convection in the National Center for Atmospheric Research community climate model (CCM2)


  • James J. Hack


The National Center for Atmospheric Research (NCAR) community climate model (CCM) has historically made use of a moist adiabatic adjustment procedure for parameterizing the effects of moist convection. The most recent version of the NCAR CCM, CCM2, has abandoned this approach in favor of a stability-dependent mass-flux representation of moist convective processes. This scheme physically constrains the process of moist convection with the use of a simple bulk cloud model, which provides a basis for estimating convective-scale transports of heat, moisture, and other atmospheric constituents as well as the diabatic heating associated with condensation and the fallout of precipitation. This paper presents the formalism associated with this simple mass-flux approach and contrasts its behavior with the moist adiabatic adjustment procedure used in earlier models. The inclusion of this scheme significantly moistens and warms the model troposphere at all latitudes but particularly in the tropics. Additionally, the simulated magnitude, structure, and location of the large-scale mean circulations are generally improved. The sensitivity of the simulated climate to the formulation of the cloud model is also presented.