A Dynamically Computed Convective Time Scale for the Kain–Fritsch Convective Parameterization Scheme

Many convective parameterization schemes define a convective adjustment time scale tau as the time allowed for dissipation of convective available potential energy (CAPE). The Kain-Fritsch scheme defines tau based on an estimate of the advective time period for deep convective clouds within a grid cell, with limits of 1800 and 3600 s, based on practical cloud-lifetime considerations. In simulations from the Weather Research and Forecasting (WRF) Model using 12-km grid spacing, the value of tau often defaults to the lower limit, resulting in relatively rapid thermodynamics adjustments and high precipitation rates. Herein, a new computation for tau in the Kain-Fritsch scheme is implemented based on the depth of the buoyant layer and the convective velocity scale. This new tau formulation is applied using 12- and 36-km model grid spacing in conjunction with a previous modification that takes into account the radiation effects of parameterized convective clouds. The dynamically computed convective adjustment time scale is shown to reduce the precipitation bias by approximately 15% while also providing improved simulations of inland rainfall from tropical storms.