A Study of the Effects of Spatially Varying Fluxes on Cloud Formation and Boundary Layer Properties Using Data from the Southern Great Plains Cloud and Radiation Testbed

Abstract

Measurements from the Southern Great Plains Cloud and Radiation Testbed site, which is situated in Oklahoma and Kansas and extends over an area approximately 300 km ? 350 km in extent, are combined with results from a three-dimensional mesoscale model to study the sensitivity of boundary layer properties to spatially varying surface sensible and latent heat fluxes. Four cloud parameterization schemes are used to estimate the fractional cloudiness expected over the site on three case study days with settled weather conditions during the summer of 1994. Comparisons between observations and model predictions show good qualitative agreement. Although local responses to varying surface fluxes can be found, the replacement of the spatially varying surface conditions with uniform ones makes little difference in the simulated cloud cover or the vertical profiles of potential temperature and water vapor mixing ratio when these are averaged over the full site. Spatial variations in the ambient meteorology were found to be more important than variations in surface fluxes in determining cloud amount and areas of preferred cloud formation. This conclusion is supported by additional simulations in which both the ambient meteorology and surface conditions are averaged over scales ranging from 6.25 km to 300 km. The results call into question the importance of mesoscale fluxes (i.e., fluxes arising from secondary circulations induced by heating contrasts over different surfaces) in coarse-resolution models such as general circulation models, at least for settled weather conditions similar to those considered in this study.