Variability of tropical upper tropospheric humidity 1979–1998

Abstract

In this investigation we examine the observed interannual variability for the past 20 years using satellite-derived upper tropospheric humidity observations. Satellite remote sensing observations do not provide direct information on the water vapor content (i.e., the water vapor mixing ratio). Rather, the satellite radiance observations, from filtered radiometers operating near the center of the water vapor absorption band at 6.7/am, observe the water vapor and temperature over a relatively thick layer in the upper troposphere. As detailed by Soden and Bretherton [1996] and by Stephens et al. [1996], the satellite observations are most closely related to the relative humidity of a layer and the radiance-to-humidity relationship (for a more complete discussion, see Jackson and Bates [this issue]) results in a quantity called the upper tropospheric humidity (UTH).In our analysis we consider whether some of the observed variability in UTH may be explained by the modulation of transient eddy activity in the tropics by changes in the seasonal to interannual basic state circulation. Basic state is a generic term for the time-mean circulation, in this case the monthly mean zonal wind speed at 200 hPa. We are primarily interested in examining the relationship between UTH and dynamical states in the subtropical regions. We do not examine the role of clouds and precipitation in the water and energy balance.Since accurate long-term satellite data sets require the accurate intercalibration of a number of satellites, we first reexamine our intersatellite calibration methodology (detailed in the work of Bates et al. [1996], hereinafter referred to as BWJ) and compare it to an intersatellite calibration method based on 32,271

A new METEOSAT “water vapor” archive for climate studies

“…The first phase spans the July 1983 to January 1988 period and is characterized by a slow cooling of about 1.5 K. From January 1988 to January 1991, the regional mean BT warms up by 2 K. Finally, from 1991 onward, the regional mean BT evolution does not reveal any trends and the time series seems to evolve over a plateau. Preliminary comparisons of these regional trends with the NOAA-TOVS Upper Tropospheric Humidity archive [Bates et al, 2001] confirm such a smooth evolution of the tropospheric humidity in this region suggesting that this multiannual signal would be a real characteristic of the short-term history of the African and tropical Atlantic Ocean climate.…”

“…Hence thanks to these radiometric observations, some information about the long-term variability of the UTH distribution is available. Recently, Bates et al [2001] reprocessed the whole TOVS archive from 1979 to 2000 in terms of UTH offering a first estimate of the multidecadal variability of the moisture content of the free troposphere over the globe.…”

Diurnal variation of upper tropospheric humidity and its relations to convective activities over tropical Africa

“…Bates et al, 2001;Sohn and Schmetz, 2004;Zveryaev and Allan, 2005). However, our understanding of the distribution, maintenance, and variation in water vapour is still far from being complete.…”

Evaluation of moisture in the Hadley Centre climate model using simulations of HIRS water‐vapour channel radiances

“…Bates and Jackson (1997) developed the HIRS-derived UTH data for 1979-94, along with satellite column water-vapour data to evaluate the performance of a number of atmospheric climate models forced by observed sea surface temperatures. More recently, Bates et al (2001) extended the HIRS 6.7 m UTH record from 1979-98, accounting for the temperature dependence of 6 7 by using HIRS channel-6 (15 m) brightness temperature. Using this method, attempts were made to diagnose trends from the HIRS UTH data (Bates and Jackson 2001).…”