Monitoring global land surface using Nimbus-7 37 GHz data Theory and examples

Abstract

Radiative transfer models for 37 GHz brightness temperatures are developed under simplifying assumptions and the implications of the models are illustrated and discussed using the Nimbus-7 scanning multichannel microwave radiometer (SMMR) observations of the Earth's surface from January 1979 to December 1985. The difference of vertically and horizontally polarized brightness temperatures (ΔT) decreases with increasing vegetation density (that is, as the canopy water content per unit ground area increases) and this effect of vegetation density on ΔT is illustrated by the time series of ΔT over areas with varied vegetation density and phenology. Furthermore, the ΔT decreases as the surface roughness increases, and thus mountainous areas within sparsely vegetated areas (like the Sahara) have the appearance of more densely vegetated areas. Also, the ΔT values increase when the soil gets wet or flooded, and this effect is illustrated by correlating the temporal variations of ΔT with the river height for an area within the Amazon Basin. As examples of a more quantitative analysis of the ΔT data, empirical relations are developed between the integrated-average of the 84 months of ΔT data and the annual rainfall, primary productivity and actual evaporation. An analytical relation between primary productivity and ΔT is also established from physically based models under simplifying assumptions. Colour-coded maps of primary productivity and actual evaporation based upon the SMMR data are displayed and discussed in relation to other observations. It is concluded that to realize the full potential of the ΔTdata it would be necessary to calibrate and validate this data against ground observations and develop more realistic radiative transfer models in order to gain a better physical understanding.