A shallow-convection parameterization designed for use in mesoscale models has been developed at Penn State University (PSU). The scheme is closely associated with boundary layer turbulent processes and can transition to either a deep convection scheme (Kain-Fritsch) in convectively unstable environments or to an explicit moisture scheme in moist stable environments. The scheme’s closure assumption uses a hybrid formulation based on boundary layer turbulent kinetic energy (TKE) and convective available potential energy (CAPE), while its convective trigger is primarily a function of boundary layer TKE. The scheme is also capable of predicting partial cloudiness in a grid cell.

Following earlier evaluations in a 1-D version of the Penn State/NCAR MM5 model, the shallow-convection scheme has been installed and tested against observed data sets in both marine and continental environments. In this research, the scheme is tested in a stratocumulus environment over the Southern Great Plains (SGP) region and the mid-Atlantic convective environment using 36-km horizontal resolution. Comparison between simulations with and without the shallow-convection scheme is performed. It is found that the MM5 model with the new shallow-convection scheme is able to reproduce the observed characteristics of the stratocumulus environment over land and the trade cumulus environment over ocean. The simulated results including the cloud fraction, cloud water/ice content are consistent with the observations from the field experiments (including radar and satellite imageries).