Influence of soil moisture and surface roughness heterogeneity on modeled climate

Abstract

Numerical and observational studies have documented the climatic importance of land surface variability, especially in soil moisture and surface roughness. Land surface representations within global c h a t e models, however, have been based on global vegetation data sets that characterize the surface as invariant at scales smaller than about (100 km12. Within climate model grid areas, the vegetation data are spatially aggregated once more to areas on the order of 105 km2 and are assigned to a single, spatially homogeneous vegetation type. In this study, we examine land surface variability within a (100 km)2 grid cell (about 1" of latitude by 1' of longitude) using a 3-dimensional mesoscale atmosphere-land surface model. Our investigation focuses on changes in the coverage and geographic pattern of irrigated maize (corn) and non-irrigated bare soil. Maize and bare soil are among the most disparate land covers that occur together in the present-day landscape and they exemplify highly heterogeneous spatial mosaics. Our simulations indicate that increasing the area of bare soil downwind of irrigated maize produces a nearly linear increase in daily average surface temperatures, along with a linear decrease in the average latent heat flux. Bare soil upwind of irrigated maize, however, forces a more nonlinear response. The largest effects occur when small areas of bare soil are introduced into the domain. Simulations with several mosaics containing 50 % irrigated maize and 50 % bare soil also suggest that changes in the spatial arrangement of the land surface alone can result in differences in areaaveraged surface temperatures and near-surface air temperatures of up to 1 "C. Our results additionally suggest that subgrid-scale area weighting schemes should yield better surface representations than the assignment of a single homogeneous surface type. It also appears that subgrid-scale area weighting may need to account for the spatial distribution of the vegetation within a grid cell.