Sensitivity to Turbulent Prandtl Number, Background Diffusivity, and Interaction of TKE with Cumulus Convection in a TKE-Based EDMF Vertical Turbulent Mixing Scheme

A scale-aware TKE (turbulent kinetic energy)-based moist eddy-diffusivity mass-flux (EDMF) vertical turbulent mixing scheme is being tested in the NCEP GFS (Global Forecasting System) for potential operational implementation as it well simulates daytime well-mixed boundary layer (BL) and marine stratocumulus-topped BL in the single column model (SCM) tests. In this study, sensitivity experiments to background diffusivity, turbulent Prandtl number (Pr), and interaction of TKE with cumulus convection in the scheme are presented.

It appears that the stability dependence of Pr in stable condition is somewhat controversial. The GFS experiments with different Pr values in the stable BL and free atmosphere indicate that the medium-range forecast skills are quite sensitive to Pr. On the other hand, a SCM test indicates that the vertical diffusion for the strongly stable BL is mainly conducted by the background diffusivity (K₀). The current K₀ in GFS which has been used for the current 13 km or larger grid sizes appears to be too large to reproduce the large-eddy simulation results and needs to decrease with increasing grid resolution. A parameterization for the interaction of TKE with cumulus convection is also developed, with which the cloudiness is somewhat reduced with enhanced TKE by cumulus convection and the medium-range forecast skill is significantly improved.