EXAMINATION OF THE DIFFERENCE BETWEEN RADIATIVE AND AERODYNAMIC SURFACE TEMPERATURES OVER SPARSELY VEGETATED SURFACES

A four-layer hydrologic model, coupled to a vegetation growth model, h as been used to investigate the differences between aerodynamic surfac e temperature and radiative surface temperature over sparsely vegetate d surface. The rationale for the coupling of the two models was to ass ess the dependency of these differences on changing surface conditions (i.e., growing vegetation). A simulation was carried out for a 3-mont h period corresponding to a typical growth seasonal cycle of an herbac eous canopy in the Sahel region of West Africa (Goutorbe et al., 1993) . The results showed that the ratio of radiative-aerodynamic temperatu re difference to radiative-air temperature difference was constant for a given day. However, the seasonal trend of this ratio was changing w ith respect to the leaf area index (LAI). A parameterization involving radiative surface temperature, air temperature, and LAI was then deve loped to estimate aerodynamic-air temperature gradient, and thus sensi ble heat flux. This parameterization was validated using data collecte d over herbaceous site during the Hapex-Sahel experiment. This approac h was further advanced by using a radiative transfer model in conjunct ion with the above models to simulate the temporal behavior of surface reflectances in the visible and the near-infrared spectral bands. The result showed that sensible heat flux can be fairly accurately estima ted by combining remotely sensed surface temperature, air temperature, and spectral vegetation index. The result of this study may represent a great opportunity of using remotely sensed data to estimate spatiot emporal variabilities of surface fluxes in arid and semiarid regions. (C) Elsevier Science Inc., 1996.