Global water vapor distributions in the stratosphere and upper troposphere derived from 5.5 years of SAGE II observations (1986–1991)

Abstract

Global distributions of water vapor in the stratosphere and upper troposphere are presented on the basis of ∼5.5 years (January 1986 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 mixing ratios (in parts per million by volume) with 1‐km vertical resolution and an altitude range from 6 to 40 km. Several climatological features identified in a previous study [McCormick et al., 1993], based on 3 years of observations, have been confirmed by this study: (1) the existence of a region of minimum water vapor (the hygropause) at all latitude bands; (2) the increase in the distance between the tropopause and the hygropause from 1 km at low latitudes to 4 km at high latitudes; and (3) the appearance of a positive poleward gradient throughout all seasons for fixed altitudes between 20 km and 40 km. The latitudinal 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 minimum toward the summer hemisphere. For the latitude zones 0°–20° and 20°–40° 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 unchanged from December, January, and February to March, April, and May. During September, October, and November the weakening of the hygropause and the spreading of the region of minimum water vapor to a wider altitude range are identified throughout these low‐latitude and midlatitude zones. For the upper troposphere the clear‐sky relative humidities at 300 mbar show a typical range of 5–60%, which is consistent with previous findings based on Meteosat 6.3 μm measurements. In addition, the unique capability of SAGE II observations has provided us with unprecedented vertically resolved moisture information for the upper troposphere. For example, the integrated column water vapor content for the 300‐ to 100‐mbar layer ranges from 0.002 to 0.01 g/cm2 with larger longitudinal variability in the tropics. The integrated column water vapor content from 500 to 100 mbar is found to be significantly larger in the eastern hemisphere than in the western hemisphere. The corresponding integrated water vapor content at high latitudes increases by a factor of 6 from winter to summer (0.02 g/cm2 compared with 0.13 g/cm2).