Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa21a1763b&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA21A-1763
Physics
[0305] Atmospheric Composition And Structure / Aerosols And Particles, [0320] Atmospheric Composition And Structure / Cloud Physics And Chemistry, [0340] Atmospheric Composition And Structure / Middle Atmosphere: Composition And Chemistry
Scientific paper
Ice clouds in the summer mesopause region are very sensitive to the back-ground status of the upper mesosphere/lower thermosphere and are considered to be indicators of long term trends caused by anthropogenic increase of greenhouse gases (GHG). The longest record of PMC observations (28 years) comes from SBUV instruments on various satellites. The dataset has been intensively analyzed for trends and solar cycle variations. A solar cycle modulation and an increase of PMC albedo and occurrence rates were identified. The magnitude of the effects observed by SBUV increases with latitude which asks for trend studies at polar latitudes. Inter-hemispheric differences in ice layer morphology have also been detected by satellites. Whether or not ice layers show trends is disputed in the literature. Some analysis of the same SBUV data set indeed does not show a clear trend. A better understanding of the physical mechanisms causing trends in NLC is needed to solve the open issue emerging from these studies. The new circulation model called LIMA (Leibniz-Institute Middle Atmosphere) is used to study the dynamics of the middle atmosphere. LIMA takes advantage of global ECMWF-ERA-40 data sets from troposphere/lower stratosphere regions which are processed through data assimilation techniques. One example of LIMA studies is the simulation of the thermal state at the summer upper mesosphere since 1961 until 2009, and its impact on the morphology of ice particle related phenomena such as NLC and PMC. In this paper we will show with our LIMA model that trends in the stratosphere alone (e.g. with no GHG trends in the mesosphere) can result in significant trends and solar cycle variations of mesospheric ice layers. The main mechanism involved is shrinking of the stratosphere and mesosphere which induces a small cooling at NLC altitudes which is sufficient to cause substantial trends in NLC parameters such as backscatter coefficient and occurrence rates. We concentrate on latitudinal variations of trends in NLC both for the Northern and Southern Hemisphere. In our present study we assume that there are no GHG trends in the mesosphere thereby concentrating on stratospheric effects on mesospheric ice layers. We note that the effect of increasing trace gas concentrations in the troposphere and lower part of the stratosphere (up to approximately 35 km) is indirectly present in LIMA since the model adapts to data from ECMWF at these altitudes. As a result we will show that, firstly, LIMA nicely reproduces the mean characteristics of observed ice layers, for example their NH/SH differences in variation with season, altitude, and latitude. Secondly, not only seasonal means, but also 11-y solar cycle effects and decadal long term behavior by LIMA nicely agree with SBUV data.
Baumgarten Gerd
Berger Ulrich
Luebken F.
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