Estimation Of Radiation Balance Components And Soil Heat Flux Over The Northern Tibetan Plateau Based On Satellite Remote Sensing

The heat exchange between land surface and atmosphere is of great importance to land-atmosphere interactions over the Tibetan Plateau (TP) and to the heating mechanisms of the TP. Most researches focus onestimation of radiation balance components (downward shortwave radiation, upward shortwave radiation, downward longwave radiation, upward longwave radiation and net radiation) and soil heat flux by empirical formula or low to medium resolution satellite imageries. No paramerization scheme has been set up for estimation of radiation balance components and soil heat flux over the Northern Tibetan Plateau from high resolution satellite data. Therefore,based on the Coordinated Enhanced Observing Period (CEOP) Asia-Australia Monsoon Project on the Tibetan Plateau (CAMP/Tibet) and high-resolution satellite data (Landsat-7 enhanced thematic mapper plus (ETM+)), a set of parameterization scheme is optimized to estimate radiation balance components and soil heat flux in this region.Firstly, in-situ observations (ANNI, BJ, D105, NPAM) are used to analyze the diurnal and seasonal variation of radiation balance components at different underlaying surfaces and elevation. The comparison between in-situ soil heat flux and estimated values from a temperature prediction-correction(TDEC) method shows that they have the same heat flux flow direction and phase, but the in-situ values measured byheat flux plate is generally smaller than the estimation. Thus, based on in-situ soil temperature and moisture data, the study adopts TDEC method to estimate and analyze the soil heat flux instead of in-situ values measured by heat flux plate. Moreover, the ground-measured variables will be used to validate the derived values from satellite imageries directly.Secondly, several Landsat-7 ETM+satellite images (taken on June 13 in 2001, November 4 in 2001, February 8 in 2002 and May 15 in 2002) are chosen as the representation cases of summer, autumn winter andspring, respectively. For the estimation of land surface albedo, a so-called Teillet-regression method is coupled into the estimation model to reduce the influence of topography. Based on reflectance data of Landsat-7 ETM+ and in-situ observations of land surface albedo, we build a new model instead of orginal empirical formula. As for the retrieval of land surface temperature, a generalized single-channel method is adopted. Theresult shows that the spatial distributionof surface parameters (land surface albedo, land surface emissivity, Normalized Difference Vegetation Index and land surface temperature) from remote sensing datacorresponds well with the underlying surfaces. Moreover, all surface parameters show reasonable seasonal variations.Finally, radiation balance components and soil heat flux are estimated over the Northern Tibetan Plateau area by using visable-thermal bands (Landsat-7 ETM+) and in-situ measurements(relative humidity and air temperature). The parameterization schemes for solar radiation and soil heat flux are improved by introducing topographic factors (aspect and slope) and vegetation index (NDVI). To validate the proposed methodology, the ground-measured surface variables (radiation balance components and soil heat flux) are compared to the estimation from Landsat-7 ETM+. The resultsshow that the derived variablesand in-situ values agree well with each other. The seasonal variations of the estimated radiation balance components arein good accordance with ground measurements.The maximum-values of seasonly downward longwave radiation, net radiation and soil heat flux are detected in spring-summer while the minimum values appeared in autumn-winter. Moreover, all the peak values appear in summer, which are 256.52 W·m-2,633.54 W·m-2,57.00 W·m-2 and 576.50 W·m-2, respectively.At satellite overpassing time, the soil heat flux over the Northern Tibetan Plateau alwaysshow a positive value. In addition, the surface heating field data demonsrate that the surface heating field intensity is also much stronger in summer(576.50 W·m-2)than that in winter(353.51 W·m-2).