干旱区生态环境知识资源中心

Heat fluxes at heterogeneous land surfaces are often modeled using single-source resistance-type transport equations, i.e. assuming horizontal homogeneity of the land surface and of the boundary layer. Large deviations from these conditions occur at partial canopies which are geometrically and thermally heterogeneous. Improved models of heat transfer have been proposed in literature to deal with these conditions. Such models require a measure of thermal heterogeneity of the land surface. Directional measurements of the radiance emitted by the land surface have the potential of providing a measure of thermal heterogeneity and improved parameterizations of sensible heat transfer. The paper proposes a methodology, together with two case studies on the use of directional measurements of spectral radiance to estimate the component temperatures of soil and vegetation and their subsequent use to model sensible heat fluxes at length scales of 10(-1)m and 10(3)m.

The first case study relied on multi-temporal field surface temperature measurements at view angles of 0 degrees, 23 degrees and 52 degrees collected at sparse grass covered surface during the Inner-Mongolia Grassland-Atmosphere Surface Study (IMGRASS) experiment in China. This provided useful insights on the applicability of a simple linear mixture model to the analysis of observed directional radiances. Sensible heat fluxes were estimated both at field and regional scales by using The Along-Track Scanning Radiometer (ATSR)-2 observations. The second was done with directional ATSR-1 observations only and was a contribution to the Hei He International Field Experiment (HEIFE) in China. The HEIFE case study was focused on the large oasis of Zhang-Ye and led to useful estimate of soil and vegetation temperatures. Sensible heat flux is modeled separately for each component heat source, i.e. soil and vegetation. Heat flux densities were compared with field measurements made with an eddy correlation device and values obtained with vertical profiles of air temperature and horizontal wind speed. Agreement was good for the IMGRASS case study based on field measurements. ATSR-based estimates were also in good agreement with values obtained with observed and modeled through vertical profiles, although few data points were available because of the large spatial scale of the ATSR estimates.