Bulk models of the planetary boundary layer (PBL) conventionally rely on diagnostic forms of the turbulence kinetic energy (TKE) equation to determine the entrainment rate. During episodes of rapid deepening or shallowing, the local time rate of change of the vertically integrated TKE becomes significant compared to the difference between gross production and dissipation. A method is presented to determine the entrainment rate in a bulk PBL model that includes a prognostic TKE.
