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Effects of Possible Scan Geometries on the Accuracy of Satellite Measurements of Water Vapor

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  • 1 National Environmental Satellite, Data and Information Service, Office of Research and Applications, Climate Research and Applications Division, Washington, D.C.
  • | 2 Research and Data Systems Corporation, Greenbelt, Maryland
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Abstract

For future satellite instruments, two scan geometries have been proposed: cross-track and conical scanning. With the mixtures of possible instruments, a future sounding suite may consist of all cross track, all conical, or a combination with temperature channels on one and moisture channels on the other. This paper evaluates the effect of scan angle on the accuracy of moisture soundings. It is found that, as the scan angle increases from nadir, the accuracy of the moisture soundings near the surface decreases because less of the surface signal reaches the satellite. At the same time, the accuracy of the upper-level sounding increases because the weighting functions become sharper as the angle increases. If a mixed system is required where the temperature and moisture channels have different scan geometries, the best accuracy is obtained if the temperature channels are on a cross-track scanner. The scan angles also affect the number of measurements that are too cloud contaminated to provide accurate meteorological information. A second study shows that the number of radiances that exceed a given error level increases with scan angle. This results in a decrease in coverage for a conical scanning instrument.

Corresponding author address: Dr. Larry M. McMillin, National Environmental Satellite, Data and Information Service, Office of Research and Applications, Climate Research and Applications Division, 4700 Silver Hill Road, Stop 9910, Washington, DC 20233-9910.

Email: lmcmillin@nesdis.noaa.gov

Abstract

For future satellite instruments, two scan geometries have been proposed: cross-track and conical scanning. With the mixtures of possible instruments, a future sounding suite may consist of all cross track, all conical, or a combination with temperature channels on one and moisture channels on the other. This paper evaluates the effect of scan angle on the accuracy of moisture soundings. It is found that, as the scan angle increases from nadir, the accuracy of the moisture soundings near the surface decreases because less of the surface signal reaches the satellite. At the same time, the accuracy of the upper-level sounding increases because the weighting functions become sharper as the angle increases. If a mixed system is required where the temperature and moisture channels have different scan geometries, the best accuracy is obtained if the temperature channels are on a cross-track scanner. The scan angles also affect the number of measurements that are too cloud contaminated to provide accurate meteorological information. A second study shows that the number of radiances that exceed a given error level increases with scan angle. This results in a decrease in coverage for a conical scanning instrument.

Corresponding author address: Dr. Larry M. McMillin, National Environmental Satellite, Data and Information Service, Office of Research and Applications, Climate Research and Applications Division, 4700 Silver Hill Road, Stop 9910, Washington, DC 20233-9910.

Email: lmcmillin@nesdis.noaa.gov

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