Implementing a New Shallow Convection Scheme into WRF

To help advance the understanding of cloud life cycle and improve model skill in representing clouds, Deng et al. (2003a and b) investigated cloud processes, and developed a mass-flux-based shallow/deep convection parameterization (SCP, hereafter referred as the Penn State SCP) scheme that involves all aspects of the cloud life cycle, including dynamics, microphysics and radiation. In the Penn State SCP, convection is triggered by the vertical velocities determined by various factors including the grid-resolved forcing and the turbulent kinetic energy (TKE) within the planetary boundary layer (PBL). A plume model is used to calculate the parcel properties based on the convective available potential energy (CAPE), buoyancy, entrainment and detrainment. The convective parameterization closure is determined using a hybrid approach combining the boundary layer TKE and CAPE removal, depending on the depth of the convective updraft. In addition, there are two predictive equations for cloud water and cloud fraction of neutrally-buoyant clouds (NBC), with comprehensive cloud production and dissipation processes that take into account the cloud microphysical processes. The Penn State SCP can transition to a deep convection regime when the cloud layer becomes sufficiently deep. On the other hand, the subgrid clouds produced by the shallow convection scheme can smoothly transition to explicit clouds when the grid cell becomes saturated. The subgrid-scale partial clouds in the SCP can interact with the atmospheric radiation calculation to produce more realistic heating profiles and surface radiative fluxes. Preliminary tests in a single-column MM5 simulation had shown success when compared with the observations for several cases in both continental and marine environments (Deng et al. 2003a and 2003b). As an ongoing effort supposed by U.S. EPA, the Penn State SCP is being implemented into the Weather Research and Forecasting (WRF) model. This paper presents an overview of the Penn State SCP and some test results using the single column model (SCM) version of WRF.