Testing a Dynamically Computed Convective Time Scale in the Kain-Fritsch Scheme for Meso-Gamma-Scale Convection Parameterization in WRF
The default method for estimating the time scale over which convection removes available buoyant energy in the Kain-Fritsch scheme is based on the model grid spacing and the horizontal wind speed at the lifted condensation level and the mid-troposphere, essentially forcing the sub-grid convection to dissipate at least 90% of buoyant energy based on a Courant condition. This convective time scale is restricted to a range of 30-60 minutes. Nonetheless, finer grid spacing tends to produce a shorter convective time scale for a given wind speed. This leads to more vigorous convection for our 12-km grid spacing which, one could argue, is where sub-grid parameterizations should be yielding to resolved model processes rather than becoming more vigorous.
This work will compare 12-km WRF simulations using the Kain-Fritsch scheme in its original form, with sub-grid radiation effects, and with both sub-grid radiation and the new convective time scale. Model simulations of precipitation are evaluated against monthly data from the Parameter-elevation Regressions on Independent Slopes Model (PRISM). The results show approximately one-third of the positive bias in precipitation is removed with sub-grid radiation effects and another third is removed by employing the new dynamically computed convective time scale.