MONTHLY MEAN LARGE-SCALE ANALYSES OF UPPER-TROPOSPHERIC HUMIDITY AND WIND-FIELD DIVERGENCE DERIVED FROM 3 GEOSTATIONARY SATELLITES

This paper describes the results from a collaborative study between th e European Space Operations Center, the European Organization for the Exploitation of Meteorological Satellites, the National Oceanic and At mospheric Administration, and the Cooperative Institute for Meteorolog ical Satellite Studies investigating the relationship between satellit e-derived monthly mean fields of wind and humidity in the upper tropos phere for March 1994. Three geostationary meteorological satellites GO ES-7, Meteosat-3, and Meteosat-5 are used to cover an area from roughl y 160 degrees W to 50 degrees E. The wind fields are derived from trac king features in successive images of upper-tropospheric water vapor ( WV) as depicted in the 6.5-mu absorption band. The upper-tropospheric relative humidity (UTH) is inferred from measured water vapor radiance s with a physical retrieval scheme based on radiative forward calculat ions. Quantitative information on large-scale circulation patterns in the upper troposphere is possible with the dense spatial coverage of t he WV wind vectors. The monthly mean wind field is used to estimate th e large-scale divergence; values range between about -5 x10(-6) and 5 x 10(-6) sec-1 when averaged over a scale length of about 1000-2000 km . The spatial patterns of the UTH field and the divergence of the wind field closely resemble one another, suggesting that UTH patterns are principally determined by the large-scale circulation. Since the upper -tropospheric humidity absorbs upwelling radiation from lower-troposph eric levels and therefore contributes significantly to the atmospheric greenhouse effect, this work implies that studies on the climate rele vance of water vapor should include three-dimensional modeling of the atmospheric dynamics. The fields of UTH and WV winds are useful parame ters for a climate-monitoring system based on satellite data. The resu lts from this 1-month analysis suggest the desirability of further GOE S and Meteosat studies to characterize the changes in the upper-tropos pheric moisture sources acid sinks over the past decade.