Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsa53a..03t&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SA53A-03
Physics
0305 Aerosols And Particles (0345, 4801, 4906), 0320 Cloud Physics And Chemistry, 0325 Evolution Of The Atmosphere (1610, 8125), 0340 Middle Atmosphere: Composition And Chemistry
Scientific paper
Polar Mesospheric Clouds (PMC) are sensitive indicators of thermal and humidity conditions in the summertime mesosphere. Long-term changes in annually-averaged brightness and occurrence frequency have been identified in data from the SBUV/2 suite of satellite instruments (1979-present). Both cyclic behavior and secular increases in both PMC properties have been identified by DeLand et al [2003]. The cyclic changes are in apparent response to the 11-year solar cycle, in a nearly out-of-phase relationship to solar activity indicators. The origin of the long-term changes is currently under debate, being ascribed to changes in temperature or water vapor. Long-term changes in dynamics could also be responsible, but up to now, the appropriate data sets have not been available. A new analysis of long-term observations of winds and tides from Scott Base, Antarctica (78oS) now provides such a suitable comparison [Baumgaertner et al, 2005]. In particular, analysis of winds from an MF radar indicates that diurnal and semi-diurnal tidal amplitudes underwent significant 11-year and long-term increases from 1987 to 2004, and possibly since 1982. The origin of the long-term changes in tidal activity could be due to changes in: planetary wave activity, winds through which the tides must propagate, ozone heating, temperature, etc. The radar data show that these 12-hr and 24-hr oscillations maximize near the mesopause during summertime, where mesospheric ice particles originate, causing both intense radar echoes (PMSE) and PMC. To demonstrate how tides can influence PMC properties, we have forced a microphysical model of ice evolution (CARMA) with tidal perturbations (taken from the well-known GSWM tidal model) superimposed on atmospheric background parameters from the mechanistic circulation model of the middle atmosphere COMMA-IAP. The results of these microphysical model calculations were then analyzed applying a suitable scattering theory, with which we have determined the sensitivity of PMC brightness to tidal perturbations in temperature and vertical wind. We find in a time- dependent integration that, during the cold phase of the semi-diurnal tide, ice particle sizes are enhanced over the case of a steady background atmosphere, and depressed during warm phases. This sensitivity supports the notion that tides may play a role in forcing long-term variations of PMC.
Baumgaertner A.
DeLand Matt
Palo S.
Rapp Michael
Thomas Gareth E.
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