GLOBAL WATER-VAPOR DISTRIBUTIONS IN THE STRATOSPHERE AND UPPER TROPOSPHERE DERIVED FROM 5.5 YEARS OF SAGE-II OBSERVATIONS (1986-1991)

Global distributions of water vapor in the stratosphere and upper trop osphere are presented on the basis of similar to 5.5 years (January 19 86 to May 1991) of observations from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aboard the Earth Radiation Budget Satellite ( ERBS). Tabulations are included for seasonal zonal mean water vapor mi xing ratios (in parts per million by volume) with l-km vertical resolu tion and an altitude range from 6 to 40 km. Several climatological fea tures identified in a previous study [McCormick et al., 1993], based o n 3 years of observations, have been confirmed by this study: (1) the existence of a region of minimum water vapor (the hygropause) at all l atitude bands; (2) the increase in the distance between the tropopause and the hygropause from 1 km at low latitudes to 4 km at high latitud es; and (3) the appearance of a positive poleward gradient throughout all seasons for fixed altitudes between 20 km and 40 km. The latitudin al variation of water vapor mixing ratio at 20 km is characterized by a symmetric pattern with a minimum occurring at the equator. However, the corresponding variations at 25 and 30 km indicate a shift of the m inimum toward the summer hemisphere. For the latitude zones 0 degrees- 20 degrees and 20 degrees-40 degrees in both hemispheres, the seasonal variations of the hygropause reveal that the altitude as well as the value of the minimum water vapor mixing ratio remain essentially uncha nged from December, January, and February to March, April, and May. Du ring September, October, and November the weakening of the hygropause and the spreading of the region of minimum water vapor to a wider alti tude range are identified throughout these low-latitude and midlatitud e zones. For the upper troposphere the clear-sky relative humidities a t 300 mbar show atypical range of 5-60%, which is consistent with prev ious findings based on Meteosat 6.3 mu m measurements. In addition, th e unique capability of SAGE II observations has provided us with unpre cedented vertically resolved moisture information for the upper tropos phere. For example, the integrated column water vapor content for the 300- to 100-mbar layer ranges from 0.002 to 0.01 g/cm(2) with larger l ongitudinal variability in the tropics. The integrated column water va por content from 500 to 100 mbar is found to be significantly larger i n the eastern hemisphere than in the western hemisphere. The correspon ding integrated water vapor content at high latitudes increases by a f actor of 6 from winter to summer (0.02 g/cm(2) compared with 0.13 g/cm (2)).