Parameterization of Surface Heat Fluxes above Forest with Satellite Thermal Sensing and Boundary-Layer Soundings

Abstract

Satellite-derived surface temperature measurements can be used in conjunction with temperature and wind soundings in the boundary layer to determine the surface sensible heat flux from forests at the regional scale. The underlying formulation is derived on the basis of similarity principles applied to the unstable turbulent boundary layer in which the scalar roughness for sensible heat is determined by calibration. This is illustrated herein with data acquired during the HAPEX?MOBILHY (Hydrologic Atmospheric Pilot Experiment-Modélisation du Bilan Hydrique), which was conducted in the spring and summer of 1986 over a 100-km ? 100-km square in southwestern France. The northwest third of the area is covered by the Landes forest. The surface temperatures for a 25-km ? 25-km area in this forested region were determined from the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) channels 4 and 5. Atmospheric collections were applied with LOWTRAN-7 and measured atmospheric profiles, as well as with split-window techniques. The atmospheric profiles were obtained by interpolation of data from radiosondes launched from a clearing in the forest. The calculated fluxes were compared with those measured above the forest at the Institute of Hydrology (Wallingford, England) tower, some 4,5 km from the radiosonde launch site. The comparisons were favorable for 6 clear days but not for a day with partial cloud cover. The sensitivity of the results shows that the accuracy of the surface temperature measurements should be better than 0.5 K. The effect of the spatial scale of the surface temperature measurement on the scalar roughness is also considered.