JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D4, PAGES 9001-9015, APRIL 20, 1996 Space-time variations in water vapor as observed by the UARS Microwave Limb Sounder Lee S. Elson, William G. Read, and Joe W. Waters Jet PropulsionLaboratory, California Institute of Technology, Pasadena Philip W. Mote, JonathanS. Kinnersley, The University of Edinburgh, Edinburgh, Scotland and Robert S. Harwood Abstract. W•ter v•por in the upper troposphere has • significant impact on the climate system. Difficulties in m•king •ccur•te global measurements have led to uncertainty in understanding w•ter v•por's coupling to the hydrologic cyclein the lower troposphere •nd its role in r•di•tive energy b•l•nce. The Microwave Limb Sounder (MLS) on the Upper Atmosphere Research S•tellite is able to retrieve w•ter v•por concentration in the upper troposphere with good sensitivity •nd nearly global coverage. An analysisof these preliminary retrievals basedon 3 years of observations showsthe w•ter vapor distribution to be similar to that measured by other techniques •nd to model results. The primary MLS w•ter v•por measurements were made in the stratosphere, where this species •cts as • conserved tracer under certain conditions. As is the case for the upper troposphere, most of the stmtosphericdiscussion focuses on the time evolution of the zonal mean •nd zon•lly v•rying w•ter v•por. Stmtosphericresults sp•n • 19-month period •nd tropospheric results• 36-month period, both beginning in October of 1991. Comparisons with str•tosphericmodel c•lcul•tions showgeneral •greement, with somedifferences in the •mplitude •nd phaseof long-term v•ri•tions. At certain times and places, the evolutionof water vapor distributions in the lower stratosphere suggests the preseace of meridion•l transport. 1. Introduction Water vapor (HaO) plays several importantroles in the atmosphere. In the troposphere, phase changes and radiative propertiescause it to dominate processes cru- cial to energy balance. Therefore determination and prediction of climate change will dependheavilyon ac- curate measurements of water vapor, especiallyin the upper troposphere. The significance of upper tropo- spheric HaO stems from Lindzen's [1990] controversial suggestion that a future increase in surfacetempera- tures wouldprobablyresult in a decrease in upper tro- pospheric HaO. Gutzler [1993] reported that current uncertainty in uppertropospheric humidity (UTH) re- sultsin an uncertainty in the upwardlydirected infrared radiance at the tropopause, that is of the same magni- tudeasthe radiative change dueto a doubling of carbon X Now at Department of AppliedMathematics, University of Washington, Seattle. Copyright 1996 by theAmerican Geophysical Union. Paper number 95JD03653. 0148-0227/96/95JD-03653505.00 dioxide. Clearly, the extent to which upper tropospheric water vapor is coupled to boundary layer convectionis important in studies of climate and climate change. The upper troposphere acts as a crucial link be- tween the surface, where many wave variations have their origins, and the stratospherewhere changes are driven mostly by wave activity. Good examplesof this arethe semiannual andquasi-biennial oscillations (SAO andQBO, respectively) which areinfluenced by upward propagating planetary waves. The detectionand quan- tiffcation of such waves help to complete our under- standingof how changes are forced in the stratosphere. The upper tropospherealso exhibits shorter-period os- cillations,suchas the 30-to 60-day or Madden and Ju- lian oscillation [Madden and Julian, 1994],which are driven by and indicative of interactions between con- vection and the circulation. Water vapor, like related quantities such as rainfall, varieson a variety of spatial and temporal scales.This can make observationsdifficult to interpret. In the upper troposphere, in situ measurements provide high vertical resolution, but their usefulness is limited, due in part to measurement inaccuracies of relative humid- ity sensors on conventional radiosondes [Wade, 1994]. From other measurements, such as lidar and Loran tracked soundings [$oden et al., 1994],we know that 9OO1