Synergism Of Satellite Observations Of Microwave Emission And Visible Reflectance To Study Land Surface Change

Any realistic study of the Earth's climate system should consider the forcing of the atmosphere by the land surface and important feedback relations operating between land surface and the atmosphere. Spatially and temporally representative data for surface biophysical characteristics are needed for a quantitative understanding of heat, mass and momentum exchange between land surface and the atmosphere. Applicability of visible and near-infrared reflectances to study land surface characteristics has been studied extensively by field, aircraft and satellite observations together with model development and validation. Biophysical consideration and radiative transfer models suggest that there is much to be gained by synergistic use of reflectances and microwave emission. The interaction of microwave and visible/near-infrared radiation with green leaf canopies might be compared as follows: (1) The cellular structure of leaves, which plays the major role in determining the near-infrared reflectance, is of much less importance in the microwave region because the wavelength of radiation is often two-to-three orders of magnitude larger than leaf thickness, and (2) While concentration of pigments (chlorophyll etc.) plays the major role in determining visible reflectance, water content of leaves determine its microwave response. Radiative transfer models have been developed to explore relations between reflectances and microwave emission. Surface reflectivities at 19 and 37 GHz for horizontal and vertical polarizations have been calculated by atmospheric corrections to observations by the special sensor microwave imager (SSM/I), which are compared with visible and near-infrared reflectances calculated by atmospheric corrections to observations by the advanced very high resolution radiometer (AVHRR). This comparison has been made. for more than 200 globally distributed locations representing most major vegetations types for July and August of 1987, and for four locations in Africa for the period July 1987 to December 1989. The results are being interpreted in terms of radiative transfer models.