Comparison of upper tropospheric humidity retrievals from TOVS and Meteosat

Abstract

Two different methods for retrieving upper tropospheric humidity (UTH) from the TIROS Operational Vertical Sounder (TOVS) instruments aboard NOAA polar orbiting satellites are presented and compared. The first one, from the Environmental Technology Laboratory, computed by J. J. Bates and D. L. Jackson, estimates UTH from a simplified radiative transfer analysis of the upper tropospheric infrared water vapor channel at 6.7 μm wavelength measured by High‐resolution Infrared Radiation Sounder (HIRS). The second one results from a neural network analysis of the TOVS (HIRS and Microwave Sounding Unit (MSU)) data developed at the Laboratoire de Météorologie Dynamique. Although the two methods give very similar retrievals in temperate regions (30°–60°N and S), the latter is larger by up to 16% in the tropics. The two data sets have also been compared with the UTH retrievals from infrared radiance measurements at 6.3 μm wavelength from the geostationary satellite Meteosat. These products are taken from the archive without any reprocessing that would take care of known biases. Since the Meteosat UTH in 1989 was confined to clear‐sky areas, it has a dry bias. The differences observed among the three data sets can be explained. UTH computation is sensitive to assumed air temperature and humidity profiles. Despite the biases the spatial and temporal correlations are very good. Overall, the comparison of the two TOVS retrievals provides an assessment of the UTH uncertainties, about 15–25% (relative). With regard to the Meteosat UTH it is concluded that the archived product performs well in depicting spatial and temporal changes. For future quantitative analyses, a reprocessing of the Meteosat UTH is suggested.

Upper tropospheric humidity algorithm assessment

“…Random errors are a measure of the scatter in the radiance to humidity relationship seen in Figure 12. The RMS error from that fit is given in Table 2 Large biases can occur between different methods and different satellite estimates of UTH [Escoffier, 2001]. These differences, however, do not represent a fundamental limitation on the use of UTH for climate studies but reflect the need for satellite intercalibration, UTH validation, and UTH methodology comparison.…”

A simple method to relate microwave radiances to upper tropospheric humidity

“…Quite a number of studies have used the BT transformation method to transform infrared radiances into UTH [ Escoffier et al , 2001; Tian et al , 2004; Bates and Jackson , 2001; Soden et al , 2004], including a recent study on humidity supersaturation with respect to ice as seen by the high‐resolution infrared sounder (HIRS) [ Gierens et al , 2004]. For microwave sensors, on the other hand, the method has not been much used.…”

Evaluation of upper tropospheric water vapor in the NCAR Community Climate Model (CCM3) using modeled and observed HIRS radiances

“…Trends in UTH were shown by Bates and Jackson [2001] to be due largely to changes in UTWV rather than temperature and to be consistent with known changes in large-scale circulation over the past 20 years, which they suggest indicates an intensifying hydrological cycle in the tropics. HIRS radiances have also been applied to water vapor retrieval and the assessment of UTWV observations [Soden and Lanzante, 1996;Engelen and Stephens, 1999;Escoffier et al, 2001].…”