Gerd R. Sonnemann

Seasonal variation of middle latitudes wind fields of the mesopause region – A comparison between observation and model calculation

Mean MF-radar prevailing mesospheric wind data over Juliusruh (54.6°N, 13.4°E) for 8 years (1990–1997) have been compared with daytime averaged winds obtained from calculations using the Cologne global three-dimensional dynamic model of the middle atmosphere COMMA. The altitude-seasonal cross section shows that the model reflects the seasonal zonal wind pattern fairly well. The mesospheric summer and especially the winter winds are somewhat weaker in the observations. The model results are better reflected by measurements over Saskatoon (52°N, 107°W). The height of the observed maximum of the summer jet occurs more than 5 km above that one inferred from the model calculations. The long-term averages of the zonal winds will not be significantly changed if the periods with stratospheric warming events, which can lead to short term wind reversals at mesospheric heights during the winter months, are eliminated from the observational basis. The daytime averaged meridional wind component matches the observations over Juliusruh partly and agrees also with the published results obtained from measurements over Saskatoon.

Hydroxyl layer: Mean state and trends at midlatitudes

Based on an advanced model of excited hydroxyl relaxation we calculate trends of number densities and altitudes of the OH*-layer during the period 1961–2009. The OH*-model takes into account all major chemical processes such as the production by H + O3, deactivation by O, O2, and N2, spontaneous emission, and removal by chemical reactions. The OH*-model is coupled with a chemistry-transport model (CTM). The dynamical part (Leibniz Institute Model of the Atmosphere, LIMA) adapts ECMWF/ERA-40 data in the troposphere-stratosphere. The change of greenhouse gases (GHGs) such as CH4, CO2, O3, and N2O is parameterized in LIMA/CTM. The downward shift of the OH*-layer in geometrical altitudes occurs entirely due to shrinking (mainly in the mesosphere) as a result of cooling by increasing CO2 concentrations. In order to identify the direct chemical effect of GHG changes on OH*-trends under variable solar cycle conditions, we consider three cases: (a) variable GHG and Lyman-α fluxes, (b) variable GHG and constant Lyman-α fluxes, and (c) constant GHG and Lyman-α. At midlatitudes, shrinking of the middle atmosphere descends the OH*-layer by ~ −300 m/decade in all seasons. The direct chemical impact of GHG emission lifts up the OH*-layer by ~15–25 m/decade depending on season. Trends of the thermal and dynamical state within the layer lead to a trend of OH* height by ~ ±100 m/decade, depending on latitude and season. Trends in layer altitudes lead to differences between temperature trends within the layer, at constant pressure, and at constant altitude, respectively, of typically 0.5 to 1 K/decade.