A mass–flux parameterization for shallow cumulus convection in the NCEP global forecast system

The current shallow cumulus convection scheme in the NCEP Global Forecast System (GFS) uses a simple turbulent eddy diffusion approach with a specified eddy diffusivity profile for the transports of sensible heat and moisture within the cloud layer. While this scheme has been successful for destroying unrealistic moisture accumulation in the layer below the inversion by the additional diffusion of heat and moisture, it has been suffered from the systematic underestimation of the low clouds especially over the eastern Pacific and Atlantic Oceans. To improve the current shallow convection scheme, in this study, we present a bulk mass-flux parameterization for shallow cumulus convection. While the scheme is based on the Simplified Arakawa-Shubert (SAS) convection scheme, which has been developed for deep cumulus convection and is being used in the operational NCEP GFS, some aspects in the SAS scheme such as entrainment and detrainment specification are modified suitable for shallow convection situation. The model predictions with the new parameterization for shallow convection are evaluated against analysis and observation data. For further model validation, in addition, we use the ARM (Atmospheric Radiation Measurement) data that includes cloud properties.