Implementation in the NCEP GFS of a Hybrid Eddy Diffusivity Mass Flux (EDMF) Boundary Layer Parameterization with Dissipative Heating and Modified Stable Boundary Layer Mixing
AbstractThe current operational eddy diffusivity countergradient (EDCG) planetary boundary layer (PBL) scheme in the NCEP Global Forecast System (GFS) tends to underestimate the PBL growth in the convective boundary layer (CBL) To improve CBL growth, an eddy diffusivity mass flux (EDMF) PBL scheme is developed, where the nonlocal transport by large turbulent eddies is represented by a mass flux (MF) scheme and the local transport by small eddies is represented by an eddy diffusivity (ED) scheme. For the vertical momentum mixing, the MF scheme is modified to include the effect of the updraft induced pressure gradient force. While the EDMF scheme displays better CBL growth than the EDCG scheme, it tends to overproduce the amount of low clouds and degrades wind vector forecasts over the tropical ocean where strongly unstable PBLs are rarely found. In order not to degrade the forecast skill in the tropics, a hybrid scheme is developed, where the EDMF scheme is applied only for the strongly unstable PBL, whil.