An Evaluation of the Importance of Surface Flux Variability on GCM-Scale Boundary-Layer Characteristics Using Realistic Meteorological and Surface Forcing

Abstract

Modifications to the properties of the boundary layer arising from surface flux variations over an area of 105 km2 are simulated with a mesoscale model using realistic meteorology and distributions of surface fluxes. The surface fluxes are calculated with the SiB2 land parameterization scheme, which is driven by vegetation, soil, and meteorological data from the U.S. Department of Energy?s Cloud and Radiation Testbed in Oklahoma and Kansas. Simulations were carried out for 15 case study days in July of 1995, during which synoptic conditions were relatively settled and significant flux contrasts existed over the site. Despite the prevalence of conditions apparently favorable to the development of thermally induced secondary circulations, the domain-averaged profiles of potential temperature and mixing ratios and the mean mixed-layer heights showed little differences when the spatially varying fluxes were replaced with uniform fluxes equal to the averages of the varying ones. The effects on simulated cloud fractions were also small except on one day. Sensitivity tests were made with a checkerboard distribution of contrasting surface fluxes and very light ambient wind speeds. These tests show that the use of such idealized conditions in numerical experiments can lead to serious overestimates of the likely strength of secondary circulations, the magnitudes of vertical velocities generated by convergence and divergence, and the importance of mesoscale fluxes of heat, moisture, and momentum.