ATMOSPHERES

Tracing Water Vapor

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Science  23 Jun 2000:
Vol. 288, Issue 5474, pp. 2099
DOI: 10.1126/science.288.5474.2099c

Water vapor participates in key reactions in the stratosphere and acts as a tracer for atmospheric motions. Balloons and aircraft measurements have been useful, but global coverage of the stratosphere now has become possible through satellite measurements.

Pumphrey et al. have analyzed data from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS), which performed daily measurements from September 1991 to April 1993. Their analysis extends the usable range of data into the lower stratosphere, significantly lower than in previous analyses. The annual cycle in lower stratospheric water vapor is shown to be initiated in the northern hemisphere tropics, and to spread from there into the southern hemisphere tropics and then into the mid-latitudes. Seasonal trends in stratospheric water vapor also are reported by Smith et al. The trends are derived from Halogen Occultation Experiment (HALOE) data for January 1992 to April 1999. Stratospheric increases in water vapor during this period are attributed to large autumnal increases in upper tropospheric water vapor. Increased convection in the second half of the Asian monsoon season may explain the positive trends in water vapor levels.—JU

Geophys. Res. Lett.27, 1691 (2000); Geophys. Res. Lett.27, 1687 (2000).

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