Sensitivity of a moist PBL turbulence parameterization on the NCEP GFS forecast

The nonlocal planetary boundary layer (PBL) scheme in the NCEP Global Forecast System (GFS) has been widely used because it provides not only a realistic development of a well-mixed layer despite its simplicity, but has also revealed a consistent improvement in the skill of precipitation forecasts over the continental United States. However, this scheme is optimized for simulation of dry convective and nocturnal boundary layers over land. In this study, a stratocumulus cloud driven vertical diffusion scheme is incorporated into the model. For the cloudy region above lower stratocumulus layers, an enhanced local diffusion is applied. A nonlocal mixing for momentum is also included. The model predictions with the new moist PBL turbulence parameterization are evaluated against analysis and observation data. The effect of the cloud top entrainment instability parameterization is also investigated.