Objectives 03-51-5380

During recent years it has become more and more evident that the middle atmosphere reacts to changes in greenhouse gas composition, and a long-term cooling is found concomitant with lower atmosphere warming. Since, however, a major role of the middle atmosphere in the entire climate system lies in its dynamics - through wave propagation and momentum deposition, meridional/vertical circulation with resulting cooling or warming effects in the stratosphere and potential influence on catalytic ozone decrease, or vertical transport of minor species from the thermosphere to lower layers - so that long-term trends of middle atmosphere dynamical parameters are of particular interest for climate and environmental research.

Therefore, the project is focused on the detection of mesospheric and lower thermospheric long-term trends and their change between the 1980s and 1990s, using available radar wind time series, with special focus on the particularly long time series at Collm and Obninsk. Parameters measured are monthly mean (prevailing) winds, semidiurnal and diurnal tidal amplitudes and phases, and parameters of long-period oscillations connected with planetary waves. Since much work on this topic - the long-term mean linear trend - has already been performed, special attention will be laid on the detection of structural changes in trends and types of these changes; we shall analyse the times of trend breaks in different parameters from statistical analyses. Another goal is the detection of trends and trends breaks in other than MLT wind time series to detect possible sources of middle atmosphere trend changes and to correlate the obtained changes for a selection of proper trend models of the MLT winds. Above all, we shall use tropospheric and stratospheric fields from reanalysis data, and calculate mean winds, temperatures, parameters of planetary waves and integral characteristics of the wave activity (e.g., divergence of the EP flux, heat flux) from these as data to be analysed. Planetary waves are of particularly great importance in this connection, because they are especially able to transport signals from the troposphere to the upper atmosphere. In addition, key climate parameters (circulation indices, surface temperatures, climate gas concentrations) will be analysed in the same manner. Further analysis of trends in ozone laminae will support investigations of trends in dynamical parameters in the lower stratosphere. Examination of trends in the MLT region winds will be supported by analysis of ionospheric information from the same heights, reflection heights from Collm wind measurements, further information from absorption data mining (some measurements are available until 2000-2001, although with questionable reliability), and trend information and data on E-region parameters (foE, h’E, typical heights 110-115 km) whenever useful. Since the separation of possible sources on the climate change of extraterrestrial from anthropogenic genesis must be checked, additional potential sources from upper atmosphere variability will be analysed also, in particular solar and magnetospheric parameters will be used.

The MLT dynamics is mainly forced through the interaction of the mean flow with gravity waves. Generally gravity wave trends may occur through (1) changes of tropospheric gravity wave sources or (2) changes of gravity wave filtering in the middle atmosphere. Only few long-term analyses of gravity waves have been performed so far, without conclusive results on long-term trends.

As a result of this work we will be able to demonstrate, which atmospheric or extraterrestrial parameters qualitatively correspond to MLT wind trends, i.e., which parameters show potential trend breaks at the same time as the MLT winds do. This first result will not show any quantitative information, and will, apart from some conceptual considerations to chose parameters that should be taken into account at all, not yet include detailed theoretical analyses on the potential of coupling between the respective parameters, but will show, which parameters can play a role in coupling processes of atmospheric layers from the long-term trend point of view.

In a second step, these possible sources for MLT wind variability will be analysed in more detail. To this end, the analysed changes of tropospheric and stratospheric fields will be used to force a numerical model of the middle atmosphere. Additionally, observed long-term solar variability changes will be considered in the radiation routine of the model. Analysis of MLT wind changes from the model results will show, whether the observed trends and trend variations in the MLT physically and quantitatively are coupled to the variations of the potential source parameters.

The results of the numerical modelling will, taking into account the state-of-the-art of both MLT numerical modelling and experiment, necessarily differ from the experimental results of MLT trend analysis. Through a careful analysis of gaps in the representation of physical processes in numerical models, and error sources in observations we shall be able to provide upper and lower limits of potential sources of variability in the MLT region and of the potential of the MLT as a tracer from global circulation monitoring. The concept of utilising trend breaks and their times in this connection represents a key point in this analysis, because this will allow us to detect similarities and coupling processes through modelling, even if the absolute values of variability differ between model and observation.

To summarise, the following work packages are part of the project that should lead to the respective results:

1. Analysis of middle atmosphere winds:	Detection of breakpoint time and direction of trend changes
2. Analysis of atmospheric and other time series:	Detection of potential trend changes and analysis of qualitative agreement with variations of other middle atmosphere parameters – creation of a multiparameter pattern of observational trend changes in the middle atmosphere,
3. Numerical modelling of the middle atmosphere and its changes:	Estimating effect of observed atmospheric and extraterrestrial variations on the MLT, and comparison with observed changes in MLT winds and other (middle and upper atmosphere) parameters.

The upper mesosphere/lower thermosphere defines a crucial region of the atmosphere, as it is dynamically the boundary between that part of the atmosphere that is essentially driven by lower atmosphere forcing (the middle atmosphere) that is subject to the influence of human activity, and the upper atmosphere that is predominantly forced by extraterrestrial influences. Precise knowledge about the long-term behaviour of this region will give insight into the effect of human activity upon the atmosphere as dynamical system as a whole.