An Eddy-Diffusivity Mass-Flux (EDMF) Boundary Layer Parameterization Combined with a Higher-Order Turbulence Closure Model in the NCEP GFS

Recently, the National Centers for Environmental Prediction's (NCEP) Global Forecast System (GFS) has implemented an eddy-diffusivity mass-flux (EDMF) planetary boundary layer (PBL) scheme only for strongly unstable conditions, 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. While the GFS EDMF scheme improves the PBL growth in the convective boundary layer, it still suffers from a long-standing problem of large near-surface temperature bias during the PBL transition time (i.e., during sunset and sunrise) when the memory of turbulence may play an important role in the vertical turbulent mixing of heat. Since the current GFS PBL scheme is a first-order turbulence closure model, it does not remember turbulence at the previous time step. To better handle vertical turbulent mixing during the PBL transition time, therefore, in this study we develop an EDMF PBL scheme combined with a higher-order turbulence closure model for the ED part where the memory of turbulence is retained. For the higher-order turbulence closure model, we employ a turbulent kinetic energy (TKE) closure model which requires a prognostic equation for TKE. One problem of the TKE closure scheme is under-development of daytime PBL growth due to lack of the nonlocal turbulent mixing. Combined with the GFS MF scheme, the TKE closure scheme can include the nonlocal mixing and consequently, may display better daytime PBL growth. On the other hand, a MF scheme for the stratocumulus-top-induced turbulence mixing is also included in the new EDMF scheme.