Simulation of the Diurnal Cycle in Convective Rainfall over Land with a new Convective Parameterization

We report on results of the implementation of a new convective parameterization into version 4 of the Community Atmosphere Model (CAM). The parameterization simulates the effects of convective clouds on the grid scale temperature, humidity, and rainfall tendencies using air parcels moving vertically under the influence of buoyancy. There are several unique aspects of the parameterization. The sub gridscale variance of moist enthalpy in the boundary layer is assumed to increase with rain rate. This allows the parameterization to support convection in the absence of grid scale Convective Available Potential Energy (CAPE), and helps simulate the near surface cold anomalies that develop in association with high rain rates. We also assume that the convective mass flux increases with the local rain rate, and the mixing with the background atmosphere decreases with the local rain rate. These features help contribute to a continuous transition from shallow to deep convection, and generate the observed late afternoon peak in convective precipitation.