Noctilucent Clouds and Mesospheric Water Vapor: Past and Future

Details Noctilucent Clouds and Mesospheric Water Vapor: Past and Future Noctilucent Clouds and Mesospheric Water Vapor: Past and Future

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsa42a..01v&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SA42A-01

Physics

0300 Atmospheric Composition And Structure, 0305 Aerosols And Particles (0345, 4801), 0320 Cloud Physics And Chemistry, 0340 Middle Atmosphere: Composition And Chemistry, 1610 Atmosphere (0315, 0325)

Scientific paper

Observations of long-term (here: 10 to 25 years) variations of NLC brightness have often been explained by postulating similar, positively correlated long-term variations of the mesospheric H2O mixing ratio. The weak point in these arguments has always been, however, that until today it is not possible to measure the postulated variations of water vapor mixing ratio at NLC altitudes (~83 km) with an accuracy and duration as required to really support the argument. Even more unsettling is the fact, that the scenario develops just into the opposite direction. All available long-term ground-based and satellite-borne observations of the H2O mixing ratio at 70 km altitude show clearly, but unexpectedly, that since the summer of 1995 this ratio decreases near-continuously, while over the same period satellite-borne observations show the mean NLC albedo to increase significantly. We have therefore studied the sensitivity of the volume backscatter coefficient of NLC layers on the ambient H2O mixing ratio by means of our 3-D COMMA/IAP model. Our results show, as expected, that a decrease in the mesospheric H2O mixing ratio should cause a strong non-linear decrease of NLC volume backscatter coefficient. Hence, our and other models fail to explain the observed long-term anticorrelation of NLC brightness and mesospheric H2O mixing ratio. From this we conclude that the lately observed increase of NLC brightness is either primarily driven by an atmospheric parameter other than the H2O mixing ratio or due to an odd latitude dependence of the long-term variations of H2O mixing ratio above 80 km altitude. This situation makes a robust prediction of future changes in NLC brightness very difficult.

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