Surface-Induced Brightness Temperature Variations and Their Effects on Detecting Thin Cirrus Clouds Using IR Emission Channels in the 8–12-µm Region

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  • a University Space Research Association, NASA Goddard Space Flight Center, Greenbelt, Maryland
  • | b Climate and Radiation Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland
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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.

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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