Climate Parameters from Satellite Spectral Measurements. Part 1: Collocated AVHRR and HIRS/2 Observations of Spectral Greenhouse Parameter

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Abstract
An automated method of monitoring various climate parameters using collocated Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Sounder-2 (HIRS/2) observations has been developed. The method, referred to as CHAPS (collocated HIRS/2 and AVHRR products) was implemented during the months of July 1993 and January and July 1994. This paper presents the oceanic cloud screening method and analysis of the spectral greenhouse parameter (gλ) for July 1993 and January 1994. In addition, the CHAPS derived clear-sky parameters are compared to the NESDIS historical dataset. There is agreement between NESDIS and CHAPS for the g6.7 and g7.3. The NESDIS 8.2-µm data appears to be cloud contaminated. Through comparison with CHAPS, it is suggested that the mode, rather than the mean, provides the better estimate of the central tendency of the NESDIS clear-sky 8.2-µm radiance distribution, particularly for regions with extensive low-level cloud cover. It is shown that the spectral greenhouse parameter at wavelengths sensitive to middle and upper atmospheric water vapor content is dependent on SST via its connection to large-scale atmospheric circulation patterns. It is also shown that the variability of the spectral greenhouse parameter is strongly a function of latitude at these wavelengths, as well as in spectral regions sensitive to lower-level water vapor. Standard deviations are largest in the Tropics and generally decrease poleward. In contrast, variability in the spectral regions sensitive to upper-tropospheric temperature peaks in the middle latitudes and has its minimum in tropical latitudes. Variability in the relationship between gλ and SST shows only a weak dependence on season for channels sensitive to water vapor content. A strong seasonal dependence is found in the g14.2 for the middle-latitude regions associated with changes in the temperature structure of the upper troposphere. Thee relationship between the spectral greenhouse parameter and the broadband greenhouse parameter is presented and discussed. It is found that the range in broadband g for warm tropical SSTs is driven by spectral changes at wavelengths sensitive to upper-troposheric water vapor. For cooler SSTs associated with the middle latitudes, the range in g is a function of the spectral greenhouse parameter sensitive to the temperature structure of the upper troposphere.

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