16B.2 A TKE-based Eddy-Diffusivity Mass-Flux (EDMF) Parameterization for Convective Boundary Layer and Stratocumulus-top-driven Turbulence Mixing with Scale-aware Capability

Friday, 24 June 2016: 3:45 PM
Bryce (Sheraton Salt Lake City Hotel)
Jongil Han, SRG at NCEP/EMC, College Park, MD; and C. S. Bretherton

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. However, the current operational GFS PBL scheme is a first-order turbulence closure model. For higher-order accuracy, in this study we develop an EDMF parameterization based on turbulent kinetic energy (TKE) closure model. Similar to the current GFS PBL scheme, in the new TKE-based EDMF scheme the non-local mixing for the daytime convective boundary layer (CBL) is represented by a MF scheme for updrafts. For the stratocumulus-top-driven turbulence mixing, the additional eddy diffusivity profile in the current GFS PBL scheme is replaced by a MF scheme for downdrafts in the new EDMF scheme. In addition, a simple scale-aware parameterization for the non-local MF scheme is presented for very high-resolution runs with grid sizes less than CBL depth where the large turbulent eddies are partially resolved. The new TKE-based EDMF PBL scheme well simulates daytime well-mixed PBL, nighttime deepening SBL, residual layer above the SBL, and marine stratocumulus-topped boundary layer (MSTBL) with a good agreement with the benchmark large-eddy simulation (LES) results. For the CBL, the new scheme predicts a PBL feature very similar to that from the current operational GFS PBL scheme. For the MSTBL, the new scheme shows somewhat better prediction than the operational scheme especially for the wind and cloud condensate compared to the LES results.
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