Can Water Vapor Increases Explain Long-term Variability in the Brightness and Occurrence Frequency of Mesospheric Clouds?

Details Can Water Vapor Increases Explain Long-term Variability in the Brightness and Occurrence Frequency of Mesospheric Clouds? Can Water Vapor Increases Explain Long-term Variability in the Brightness and Occurrence Frequency of Mesospheric Clouds?

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsa42a..04t&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SA42A-04

Physics

0300 Atmospheric Composition And Structure, 0325 Evolution Of The Atmosphere, 1610 Atmosphere (0315, 0325), 1640 Remote Sensing, 1650 Solar Variability

Scientific paper

It is now well established that stratospheric water vapor (H2O) has undergone significant increases (of the order of 1 percent/year), at least since measurements began in the 1960's, and are likely to have occurred over nearly the entire 20th century. The principle of continuity of hydrogen atoms calls for similar changes in the mesosphere. This is confirmed by recent reports of non-polar trends (0.5-1 percent per year) in H2O at 70 km as measured by the HALOE instrument on board of UARS. These trends apply to the recent period 1992-2002. It has been long suspected that water vapor variability can explain much of the variability of mesospheric clouds (MC). We consider this possibility from two aspects: (1) observationally, a long-term increase in the albedo and occurrence frequency of bright MC has been observed in SBUV and SBUV-2 data series, as well as a 10-11 year periodic component presumably related to solar cycle changes, and (2) theoretically, the CARMA microphysical model predicts an exponential dependence of MC brightness with H2O. In this paper, we compare theoretical expectations with the observed trends in albedo and occurrence frequency. Our results show that long-term increases in H2O at polar latitudes, combined with observed solar modulation in H2O, are fully capable of explaining the variability in MC brightness, although details are still uncertain (e.g. north-south differences). The measured albedo changes observed by SBUV/SBUV-2 may also contain the signal of a recent reversal of the water vapor trend since 1996. Although there is yet no evidence for long-term trends in mesopause region temperature (or temperature variability), we illustrate the model sensitivity to possible long-term cooling/warming near the mesopause. It will be necessary to conduct simultaneous measurements of these controlling factors in the presence of MC in order to sort out their relative importance.

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