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  • Author or Editor: W. Wiscombe x
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Bo-Cai Gao and W. J. Wiscombe

Abstract

A method for detecting cirrus clouds in terms of brightness temperature differences between narrowbands at 8, 11, and 12 µm has been proposed by Ackerman et al. In this method, the variation of emissivity with wavelength for different surface targets was not taken into consideration. Based on state-of-the-art laboratory measurements of reflectance spectra of terrestrial materials by Salisbury and D'Aria, it is found that the brightness temperature differences between the 8- and 11-µm bands for soils, rocks and minerals, and dry vegetation can vary between approximately −8 and +8 K due solely to surface emissivity variations. The large brightness temperature differences are sufficient to cause false detection of cirrus clouds from remote sensing data acquired over certain surface targets using the 8-11-12-µm method directly. It is suggested that the 8-11-12-µm method should be improved to include the surface emissivity effects. In addition, it is recommended that in the future the variation of surface emissivity with wavelength should be taken into account in algorithms for retrieving surface temperatures and low-level atmospheric temperature and water vapor profiles.

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C. Prabhakara, D. A. Short, W. Wiscombe, R. S. Fraser, and B. E. Vollmer

Abstract

Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) measurements at five frequencies in the region 6.6 to 37 GHz, at a resolution of 155 km, are analyzed to infer precipitation over the global oceans. The microwave data show, on this spatial scale, that the combined liquid water in the clouds and rain increases the brightness temperature almost linearly with frequency in the 6.6 to 18 GHz region, while at 37 GHz such a simple relationship is not noticed. Further, as the atmospheric water vapor absorption and the effects of scattering by precipitation particles are relatively weak at 6.6 and 10.7 GHz, a technique to remotely sense the liquid water content in the atmosphere is developed based on the brightness measurements at these two frequencies. Seasonal mean patterns of liquid water content in the atmosphere derived from SMMR over global oceans relate closely to climatological patterns of precipitation. Based on this, an empirical relationship is derived to estimate precipitation over the global oceans, with an accuracy of about ±30 percent, on a seasonal basis from satellite measurements made during the three years (1979–81) before the recent El Niño event. The deviations from these three-year means in the precipitation, produced by the 1982–83 El Niño event are then deduced from the SMMR measurements. In the Pacific one notices from these deviations that the precipitation over the ITCZ in the north, the South Pacific Convergence Zone, and the oceans around Indonesia is drastically reduced. At the same time a substantial increase in precipitation is observed over the normally dry central and eastern equatorial Pacific Ocean.

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