6.4 Implementing a new shallow convection scheme into WRF

Deng, Aijun, Brian Gaudet, Penn State University, Jimy Dudhia, National Center for Atmospheric Research, and Kiran Alapaty, United States Environmental Protection Agency

To help advance the understanding of cloud life cycle and improve model skill in representing clouds, especially shallow clouds, Deng et al. (2003a and b) developed a mass-flux-based shallow convection parameterization (SCP) in MM5. In the prototype SCP, convection is triggered by the vertical velocities determined by various factors including the turbulent kinetic energy (TKE) within the planetary boundary layer (PBL). 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 prototype 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 SCP is currently being implemented into WRF, with the goal of improved forecasts of cloud fraction and radiation budgets. In this WRF Workshop paper, we will present some preliminary results from several convective cases selected from both marine and continental environments. We will also discuss some of the modeling issues such as those associated with the use of the TKE values from PBL schemes.