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 pola r orbiting satellites are presented and compared. The first one, from the E nvironmental Technology Laboratory, computed by J. J. Bates and D. L. Jacks on, estimates UTH from a simplified radiative transfer analysis of the uppe r tropospheric infrared water vapor channel at 6.7 mum wavelength measured by High-resolution Infrared Radiation Sounder (HIRS). The second one result s from a neural network analysis of the TOVS (HIRS and Microwave Sounding U nit (MSU)) data developed at the Laboratoire de Meteorologie Dynamique. Alt hough 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 trop ics. The two data sets have also been compared with the UTH retrievals from infrared radiance measurements at 6.3 mum wavelength from the geostationar y 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 se nsitive to assumed air temperature and humidity profiles. Despite the biase s the spatial and temporal correlations are very good. Overall, the compari son of the two TOVS retrievals provides an assessment of the UTH uncertaint ies, about 15-25% (relative). With regard to the Meteosat UTH it is conclud ed that the archived product performs well in depicting spatial and tempora l changes. For future quantitative analyses, a reprocessing of the Meteosat UTH is suggested.