December 2009 PMC Trend Meeting Report (submitted to EOS)


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Mesospheric ice clouds as indicators of upper atmosphere climate change

Workshop on Modeling Polar Mesospheric Cloud Trends

Boulder, Colorado, Dec. 10-11, 2009

The 20-yr old speculation that high-altitude summertime ice clouds (polar mesospheric clouds or noctilucent clouds, here denoted MC) are affected by anthropogenic activities has recently received support from a 30-year timeseries of SBUV (Solar Backscatter Ultraviolet) satellite measurements. SBUV data reveal a significant trend in bright MC properties. However, the robustness of the trend, extracted from interannual, local-time and solar-cycle variability, and its underlying causes remains debatable.

General circulation models (GCM's) that simulate MC have finally reached the sophistication to address this question. Two modeling groups have reported simulating trends in MC optical properties, which closely match the SBUV data. The models are (1) LIMA/ICE model (Leibniz Institute Middle Atmosphere model/Ice) and (2) WACCM-PMC (Whole Atmosphere Community Climate Model-Polar Mesospheric Clouds). Both model simulations agree well with the satellite data in the northern hemisphere (NH). Trends of southern MC are apparently masked by large interannual variability.

To understand how two different simulations could produce such remarkable agreement, MC scientists recently attended an informal workshop at the Laboratory for Atmospheric Physics in Boulder.

Changes of mesospheric temperature and H2O from 1953 to 2003 result from a host of long-term forcings, including changes in greenhouse gases (GHG). Simulations covering the range 0-140 km indicate a negligibly small (<0.4K/decade) cooling in the MC domain (the high-latitude summertime mesopause region, 80-90 km). A simulated 10-15%/decade increase of H2O results in part from oxidation of rising concentrations of methane. This study (with no ice modeling) points toward H2O as the possible driving force for MC trends. Use of the same long-term forcing in WACCM-PMC (with an ice parameterization) showed excellent agreement with SBUV trends in the NH polar region.

To investigate the stratospheric influences on the NH mesosphere, LIMA/ICE, with a Lagrangian formulation of MC, was 'nudged' from below ~40 km by observed winds over period 1961 to 2008. GHG concentrations were held constant. A small mesospheric cooling rate (~1K/decade) was found to be partly due to atmospheric contraction owing to stratospheric cooling. Yet even this model simulated the observed trend in MC, despite the lack of any explicit methane or CO2 trends in the mesosphere!

Subsequent discussion considered unmodeled influences (e.g space shuttle injections of water and detailed nucleation schemes). None were considered to be of major importance for MC trends. Interhemispheric coupling is implicit in both models but its influence has not been separately isolated.

Although the WACCM model predicts realistic upper stratospheric cooling (~1K/decade), it is difficult to segregate its influence in a free-running model. Future work will include sensitivity calculations in which the various forcings are held constant. The LIMA/ICE modeling group plans realistic optical calculations, coupling between chemistry and dynamics, and the addition of GHG increases. Progress should be forthcoming before the next meeting of the IAGA/ICMA/CAWSES Workshop on Long-term Changes and Trends in the Atmosphere, to be held in Boulder, Colorado on June 15-18, 2010.

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