A Simulation Study of Shallow Moist Convection and Its Impact on the Atmospheric Boundary Layer

Abstract

By comparing regional model simulations with the observations collected at the southern Great Plains (SGP) site and the tropical western Pacific (TWP) Nauru site of the Atmospheric Radiation Measurement (ARM) project, this paper evaluates the overall performance of a recently developed shallow cumulus parameterization scheme under different meteorological conditions. The scheme is incorporated into the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5). The simulations indicate that the shallow cumulus scheme can accurately simulate both marine shallow cumuli and the observed diurnal cycle of continental shallow cumuli. Both subgrid cloud properties and the resolved thermodynamic structures and the surface energy budget are well simulated by the model. Using the simulations performed in this study, the authors also investigate the impact of shallow cumuli on the boundary layer structure. The simulations indicate that maritime shallow convection moistens and cools the cloud layer but dries and slightly heats the subcloud layer. This effect also occurs over land, where it is weaker in the mean but has a pronounced diurnal cycle. Although continental shallow cumuli barely affect the surface energy budget, their maritime counterpart can have a significant impact on the surface evaporation. This study also compares the impacts of continental shallow convection on the boundary layer in winter versus summer at the SGP site, and addresses the effects of shallow cumuli on the middle troposphere and their interaction with stratiform clouds.