A Modeling Study of the Effects of Inhomogeneous Surface Fluxes on Boundary-Layer Properties

Abstract

A numerical modeling study was conducted to examine the response of the atmospheric boundary layer to inhomogeneous surface fluxes. The study was used to extend the results obtained from a field experiment carried out in spring 1992 in north-central Oregon over a region characterized by warm, dry sagebrush and grassland steppe and cooler, irrigated farmland. Characteristic scales of the two prominent land-use types were on the order of 10 km or more. A series of numerical experiments were carried out to analyze boundary-layer behavior on three days selected for detailed study, to perform a set of sensitivity tests to identify the principal mechanisms responsible for secondary circulations in the region, and, with selected two-dimensional simulations, to verify the role of advection in maintaining well-mixed layers over cool surfaces. Although contrasts in land use produce measurable secondary circulations over the study area, terrain effects and ambient winds can mask much of the response to the differential heating over the warm and cool regions. The depth of the mixed layer is poorly correlated with the local underlying surface heat fluxes but is governed instead by a combination of local fluxes, horizontal advection, convergence or divergence, and shear production of turbulence. An analysis of mesoscale heat fluxes, that is, the fluxes associated with secondary circulations that would not be resolved by a coarse resolution model, shows that the contribution, arising from land-use differences, to the domain-averaged atmospheric heating rate is small for this area. The authors suggest that modeling studies based on idealized terrain and land-use configurations may tend to overestimate the effect of mesoscale fluxes on the temperature structure predicted by coarse-resolution models applied to real world conditions. Even so, secondary circulations may be significant for other boundary-layer properties, such as mixed-layer depth and cloud formation.