A moist vertical turbulence mixing parameterization in the NCEP GFS

The vertical turbulence mixing scheme in the NCEP Global Forecast System (GFS) was developed for simulation of dry boundary layers. Recently, a stratocumulus cloud top driven vertical diffusion scheme has been incorporated into the model to enhance turbulence mixing in the cloudy region of the lower troposphere. However, the model is still based on variables conserved in dry adiabatic process and thus, may not be appropriate for treating cloudy layer mixing that needs consideration of the latent heating associated with changes of state of water. In this study, we present a moist vertical turbulence mixing parameterization in terms of variables conserved during changes of state of water, such as total liquid water and liquid water potential temperature, allowing for more realistic calculation of atmospheric stability and moist turbulence mixing in cloudy regions. In global models, partial cloudiness is necessary so the determination of the model temperature, water vapor, and liquid water from the moist conserved variables is not trivial. A method to diagnose the changes of the moisture and liquid water from that of the total water before and after the turbulence mixing will be examined. The new moist turbulence mixing model is tested with the single column model version of the GFS.