Spatial distributions of upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder

We characterize measurements of upper tropospheric (similar to 300-150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on b oard the Upper Atmosphere Research Satellite (UARS) in terms of their spati al and temporal variability. We present the climatology of water vapor mixi ng ratio for October 1991-June 1997, which includes the seasonal means, the root-mean-square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upp er tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the W alker circulation. We quantify the frequency of dry tropical observations i n seasonal maps. The most prominent dry regions are located in the subtropi cs. On the 316 hPa surface there is some occurrence of low values of humidi ty throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern He misphere winter. The seasonal cycle of MLS water vapor is compared with Str atospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agree ment is found between these two data sets for measurements near the extratr opical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or t ropospheric air. Frequency distributions display distinct characteristics a nd seasonal dependence for these two regions.