LIMA

The global thermal and dynamical structure of the middle atmosphere modelled by LIMA

Scope

The LIMA (Leibniz Institute Middle Atmosphere) model, the successor of COMMA/IAP, uses the same modules of radiation and for mesospheric chemistry-transport. But, firstly, LIMA changes the horizontal grid structure now solving the atmospheric equations on a nearly triangular mesh, a so-called reduced Gaussian grid, in horizontal direction. These fine mesh sizes of approximately 110 km enable LIMA to abandon the use of a gravity wave parameterization. Secondly, LIMA assimilates tropospheric and lower stratospheric data from ECMWF (European Centre for Medium-Range Weather Forecasts, Reading, England) in order to account for realistic lower atmosphere conditions. This 'prescribed' atmospheric variability penetrates into the upper atmosphere resulting in a large internal variability of mesosphere/lower thermosphere (MLT) region.

One of the important application by LIMA is the connecting of the LIMA data archive to noctilucent cloud modeling with LIMA-ICE. Therefore our major goal is to simulate as precise as possible the observed thermal and dynamical state of summer mesopause region (80-95 km) at high latitudes. Besides temperature, winds and air density, water vapour is one additional key parameter which is essentially needed in ice cloud modelling .

Summer climatology of temperature in K (top left) from observations 'falling spheres' rocket experiments (Lübken, 1999) at Andoya, Norway (69N) from May, 23th (month 4.75) to October, 8th (month 10.25). The lowest (highest) contour label is 131 K (283 K). The bottom left panel shows the summer climatology from LIMA simulations. The

Results

Based on in situ measurements with the so called `falling sphere' technique Lübken (1999) published a temperature and density climatology at high latitudes (69N) for the summer season up to an altitude of 95 km. Furthermore, horizontal wind observations are available in the altitude range 35-80 km published by Müllemann and Lübken (2005). The figure shows a comparison of  this climatology with LIMA results. LIMA temperatures nicely matches the climatolgy. For example, the minimum temperature is 132 K in LIMA compared to 131 K in reality, and the summer mesopause altitude on July 1 is basically identical (88 km). There are some minor discrepancies regarding the length and vertical structure of the summer season. For example, LIMA temperatures are slightly too warm in spring. This might be due to the fact that the `falling sphere' observations had been performed during 1991-1998 whereas the LIMA climatology is valid for the years 2001-2005. In general, the overall seasonal structure agrees nicely with observations, which is also true for neutral air densities and zonal winds.

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