Simulated and observed 37 GHz emission over Africa

Abstract

A radiative transfer model is developed for simulating polarized microwave emission at 37 GHz frequency from vegetated surfaces. The model treats vegetation as a uniform layer of leaves embedded within a woody structure at a constant temperature in equilibrium with the soil temperature. The radiative transfer parameters are related to biophysical parameters under the high frequency approximation of electromagnetic wave scattering by the canopy elements. The radiative transfer equation is solved considering multiple scattering within the canopy by a two-point Gauss quadrature method leading to an analytic expression for horizontally and vertically polarized brightness temperatures. These brightness temperatures are supplemented by atmospheric attenuation and emission of 37 GHz radiation to simulated satellite observations. Illustrative results are given together with the results of numerical sensitivity analysis. The average of vertically and horizontally polarized brightness temperatures is determined most strongly by the surface temperature, while the polarization difference is determined by several biophysical parameters. In particular, for a prescribed (seasonal) range of leaf area index, the corresponding range of polarization difference decreases almost exponentially as the stem area index increases. For a stand of naturally dried grass, the polarization difference decreases as the biomass increases. The model simulations are evaluated against multispectral satellite data from January 1982 to December 1983 along a transect going from rainforest to hot desert over Africa (0°–20°N, 11 °E). The data from the advanced very high resolution radiometer (AVHRR) on board the NOAA-7 satellite have been processed to obtain surface temperature, reflectance and normalized difference, while polarized 37 GHz brightness temperatures are observations by the scanning multichannel microwave radiometer (SMMR) on board the Nimbus-7 satellite. Temporal variation of average brightness temperature matches closely the surface temperature. The seasonal variation of polarization difference was found to be confined within the latitude band 8°–18°N, but the normalized difference continued to show seasonal variation south of 8°N. These observations are generally consistent with the model implications. While polarization difference and normalized difference have a non-linear relationship, polarization difference and reflectance are almost linearly related. Physical mechanisms underlying these relationships, which have to be confirmed, are identified and discussed.