HR: 14:40h
AN: SA42A-04    [PDF]
TI: Can Water Vapor Increases Explain Long-term Variability in the Brightness and Occurrence Frequency of Mesospheric Clouds?
AU: * Thomas, G E
EM: thomas@lasp.colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado,Campus Box 392, Boulder, CO 80309 United States
AU: Rapp, M
EM: rapp@iap-kborn.de
AF: Leibniz Institute of Atmospheric Physics, Schloss-Strasse 6, Kuhlungsborn, D-18225 Germany
AU: Olivero, J J
EM: oliveroj@erau.edu
AF: Embry-Riddle Aeronautical University, Department of Physical Sciences, Daytona Beach, FL 32114 United States
AU: Shettle, E P
EM: shettle@nrl.navy.mil
AF: Naval Research Laboratory, Remote Sensing Division,Code 7227, Washington, DC 220375 United States
AU: Deland, M
EM: matt_deland@sesda.com
AF: Science Systems and Applications, Inc, 10210 Greenbelt Road, Suite 600, Lanham, MD 20706 United States
AB: 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.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0325 Evolution of the atmosphere
DE: 1610 Atmosphere (0315, 0325)
DE: 1640 Remote sensing
DE: 1650 Solar variability
SC: SPA - Aeronomy [SA]
MN: 2003 Fall Meeting