Meteorological Interpretations of the Images from the Nimbus 5 Electrically Scanned Microwave Radiometer

Abstract

The Electrically Scanned Microwave Radiometer (ESMR) on the Nimbus 5 satellite measures the microwave radiation emitted by the earth and the atmosphere in a wavelength band centered at 1.55 cm. The ESMR scans perpendicularly to the spacecraft suborbital track from 50° left to 50° right in 78 steps every 4 s, producing an image which has a spatial resolution of 25 km at nadir. At these wavelengths, the emissivity of the earth and atmosphere varies considerably more than at infrared wavelengths. Thus the contrast in radiance between land surfaces, which have high emissivities, and ocean surfaces, which have low emissivities, makes continents and islands readily distinguishable. There is a minimum of interference from clouds since most non-raining clouds are virtually transparent at these wavelengths. However, atmospheric moisture does modify the radiation emitted by the surface and when cloud droplets reach precipitable size, they enhance the radiation considerably over surfaces of low emissivity (e.g., over oceans), making it possible to map areas of rainfall as well as regions of heavy cloudiness. In this application the ESMR images are meteorologically useful in determining the extent, structure and, qualitatively, the intensity of rainfall. It is then possible, over oceans, to determine the location of frontal rain, rain/snow boundaries, and the structure of tropical storms. Because of the generally high emissivities of land surfaces and the wide range of values they assume, interpretation of atmospheric parameters over land is not possible at present.