A Parameterization for the Triggering of Landscape-Generated Moist Convection. Part I: Analysis of High-Resolution Model Results

Abstract

To develop a parameterization for the triggering of moist convection by landscape-generated mesoscale circulations, a set of relatively high-resolution three-dimensional (3D) simulations was produced. These simulations modeled the development of landscape generated mesoscale circulations that triggered moist convection over west-to-east dry patches. No clear relationship existed between average patch size and average rainfall. Rather, rainfall averaged over the area of individual patches varied linearly with the size of these patches. Thus, cumulus parameterization schemes need to account for a population of clouds (over individual patches) within each domain of a large-scale atmospheric model (i.e., numerical weather prediction and global circulation models). It is demonstrated that mesoscale perturbations in velocity, temperature, and moisture need to be included in triggering functions when evaluating whether moist convection will occur. Yet, the largest patches did not always produce the largest mesoscale perturbations. Instead, the size of the perturbations depended upon the ratio of the local radius of deformation to patch size, the gradient of soil moisture between patches, as well as large-scale environmental conditions such as wind, stability, and specific humidity. These perturbations can be used to improve the representation of triggering functions associated with moist convection over landscape patches. Appropriate dimensionless numbers that can be used in a parameterization for the mesoscale perturbations are identified.