Upper tropospheric water vapor and cirrus: Comparison of DC‐8 observations, preliminary UARS microwave limb sounder measurements and meteorological analyses

Abstract

Upper tropospheric water vapor in the tropical West Pacific has been examined by three approaches that were operational during the NASA Pacific Exploratory Mission‐West A (PEM‐West A) in September–October 1991: direct measurement with a Lyman‐α fluorescence instrument on the NASA DC‐8; remote sensing by the microwave limb sounder (MLS) on the Upper Atmosphere Research Satellite (UARS); and deduction of cirrus clouds by differential absorption lidar (DIAL) flown on the DC‐8 together with environmental conditions derived from meteorological analyses made by the European Centre for Medium‐Range Weather Forecasts (ECMWF). Comparison between the DC‐8 direct measurement and the preliminary MLS (12 cases) estimates showed fair agreement with a tendency for MLS to show an overestimate at low mixing ratio values (≤∼200 parts per million by volume). The comparison between the DIAL and the ECMWF analyses (13 cases) showed that cirrus occurred in regions where there was rising motion and relative humidity near 100%. The DIAL observations are from much smaller spatial regions than can be resolved from the ECMWF analyses. The cirrus tops occurred below the tropopause as measured by the vertical ozone gradient in the DIAL data; no cases of tropopause penetration were observed during this season. In most cases the cirrus formed in air of low potential voracity and relatively low ozone. At 150 and 200 hPa the center of the rising motion, as shown by the maximum in the velocity potential pattern based on the ECMWF analysis, is at about 18°N, 145°E in close correspondence to the MLS water vapor mixing ratio maximum deduced for 147 and 215 hPa.