Shallow convection over land: a mesoscale modelling study based on idealized WRF experiments

A shallow cumulus over land redistributes heat and moisture in the boundary layer, but is also important on larger scales, because it can trigger severe convection events. Due to its small (102 - 103 m) spatial scale, this feature is defined as a sub-grid process in mesoscale models. The goal of this research is to examine the representation of shallow cumulus clouds in the mesoscale model WRF by reproducing a shallow cumulus situation observed over land. In particular, we focus on the role of the convection parameterisation in the characteristic vertical energy transport in the boundary layer. The analysis focusses on the thermodynamic structure of the boundary layer and on the cloud properties derived from a simple parcel method theory. This numerical experiment is designed to be as close as possible to the Large-Eddy Simulations (LES) model intercomparison study of Brown et al. (2002). They concentrated on the representation of shallow cumulus clouds over land in LES, using data from the American Southern Great Plains of 21st June 1997. To imitate the dynamic structure of LES, we have designed a Multiple Single Column version of WRF. Using identical surface forcing and initial thermodynamic profiles, WRF boundary layer structure shows good agreement with the LES results. However, the parcel method indicates that a larger inversion and the absence of a conditionally unstable layer suppress shallow cumulus clouds development by WRF. In addition, WRF does not show any cloud development in terms of cloud liquid water. We show also that a convective parameterisation is necessary to represent the enhanced boundary layer vertical transport by shallow cumulus clouds. Different convective parameterisation schemes are analyzed and compared