Spatial Variability of Surface Radiation Fluxes in Mountainous Terrain

Abstract

This paper investigates the magnitude and causes of spatial variability of surface radiative fluxes in a complex alpine landscape in the Southern Alps of New Zealand. Radiative flux components are simulated for the Tekapo watershed at 100-m resolution for clear-sky summer days, using a surface radiation budget model in conjunction with satellite imagery and topographic modeling to derive surface parameters. Overall, the model results agree well with observations made at a range of sites, with shortwave fluxes simulated more accurately than longwave fluxes. Sensitivity studies were conducted to isolate the role of spatial variability of surface characteristics in generating variance in the radiation budget. In order of most to least important, these characteristics were found to be slope aspect, slope angle, elevation, albedo, shading, sky view factor, and leaf area index. Spatial variability was greatest in midmorning and midafternoon, as a function of optical depth. The role of landscape complexity in the spatial distribution of fluxes was investigated by considering three subareas of the watershed that contain strongly contrasting scales of autocorrelation of topography and surface cover. Increase in topographical complexity yielded a small decrease in spatial average net radiation and a large increase in spatial standard deviation, driven most significantly by incident shortwave radiation. The regional averages scaled more or less linearly, whereas subregional-scale spatial variability differed dramatically.