The distribution of stratospheric water vapour as measured by the microwave limb sounder

• Main Author: Carr, Ewan S.
• Published: University of Edinburgh 1996
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.642611

On September 12, 1991 the Upper Atmosphere Research Satellite (UARS) was launched. It carried a number of instruments one of which was the Microwave Limb Sounder (MLS) (Barath et al., 1993). MLS obtains near-global simultaneous measurements of several atmospheric trace species at heights throughout the middle atmosphere. One of the species measured is water vapour. This thesis describes the zonal-mean climatology of the MLS water vapour measurements in the stratosphere (˜16 to 50 km) and compares it to previous satellite measurements of <I>H<SUB>2</SUB>O </I>from the Limb Infrared Monitor of the Stratosphere (LIMS) and the Stratospheric and Aerosol Gas Experiment II (SAGE II) (e.g. Russell et al., 1984, Chiou et al., 1993)). Some of main features of the distribution in MLS data, the increase in mixing ratio with height, the presence of a tropical minimum in the lower stratosphere, are consistent with these previous results. The extensive dehydration that occurs in the Antarctic polar vortex during winter is comprehensively observed for the first time in UARS data. Water vapour measurements in the arctic winter vortex do not show any significant dehydration. Temporal and spatial variations are also discussed and it is shown that the zonal-mean variability in the lower tropical stratosphere is primarily annual and is modulated by the quasi-biennial oscillation (QBO) in zonal-mean winds. This modulation means that the time between maxima and minima is either shorter or longer than one year according to the phase of the QBO. At higher latitudes, the variability in the lower stratosphere is marked by periods of descent (and dehydration) in the winter polar vortices. In the upper stratosphere, there is a clear semi-annual oscillation in tropical mixing ratios which compares well to the variability in zonal-mean winds in these regions. The location of the maximum amplitude of the H<SUB>2</SUB>O oscillation moves seasonally between hemispheres. At higher latitudes the variability is annual and is marked by periods of stronger winter descent than in the lower stratosphere, consistent with calculations of high-latitude descent rates (e.g. Schoeberl et al., 1992). The annual variations are complicated by the fact that air is drier in the mesosphere than in the stratosphere so that descent from aloft first brings wet air (and thus maxima) and then dry air (and minima).