Comparison of upper tropospheric humidity retrievals from TOVS and Meteosat

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 mum 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 Meteorologie Dynamique. Although the two methods give very similar retrievals in temperate regions (30 degrees -60 degreesN 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 mum 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.