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, has been used to investigate the differences between aerodynamic surface temperature and radiative surface temperature over sparsely vegetated surface. The rationale for the coupling of the two models was to assess the dependency of these differences on changing surface conditions (i.e., growing vegetation). A simulation was carried out for a 3-month period corresponding to a typical growth seasonal cycle of an herbaceous canopy in the Sahel region of West Africa (Goutorbe et al., 1993). The results showed that the ratio of radiative-aerodynamic temperature difference to radiative-air temperature difference was constant for a given day. However, the seasonal trend of this ratio was changing with respect to the leaf area index (LAI). A parameterization involving radiative surface temperature, air temperature, and LAI was then developed to estimate aerodynamic-air temperature gradient, and thus sensible heat flux. This parameterization was validated using data collected over herbaceous site during the Hapex-Sahel experiment. This approach was further advanced by using a radiative transfer model in conjunction 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 estimated 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 spatiotemporal variabilities of surface fluxes in arid and semiarid regions.