Effects of Subgrid Spatial Heterogeneity on GCM-Scale Land Surface Energy and Moisture Fluxes

Abstract

The results of simulation experiments are reported in which two 1° lat ? 1° long regions were discretized into pixels of size roughly 180 m ? 120 m and modeled using a hydrologically based, spatially distributed water and energy balance model. Fluxes aggregated from the distributed model (ADM), and computed using a macroscale equivalent model (MSE), which treats the entire region as a point, were compared for 2 years for two regions in Montana: one in the mountainous, semihumid western part of the state, and another in the drier, less mountainous east. The forcings for MSE were the spatial averages of precipitation, downward shortwave and longwave radiation, air temperature, wind, and vapor pressure over the respective regions spatially averaged from the distributed model. In the western region, major differences in predicted snow water equivalent between ADM and MSE were observed during the spring snowmelt period, primarily due to snow at high elevations, which is not represented by MSE. These differences persisted for smaller 0.2° ? 0.2° subregions; however, an alternate probability-based partitioning of the region into 10 elevation bands greatly reduced the differences. In the eastern region, where snow accumulations are episodic, differences in snow water equivalent were due primarily to the failure of MSE to represent topographic variations in solar radiation. Differences in latent and sensible heat fluxes between ADM and MSE were greatest when MSE predicted no snow cover and ADM predicted partial area snow coverage. When both models predicted at least partial snow cover, or both models predicted no snow cover, the diurnal patterns in latent and sensible heat fluxes were similar, although MSE tended to predict larger diurnal extremes. This is attributable to the representation of partial area coverage of snow in winter and spring, and partial areas coverage of convective rainfall in summer and fall, in ADM.