Modeling of middle atmosphere dynamics with LIMA

This paper describes a new circulation model of the middle atmosphere called LIMA (Leibniz-Institute middle atmosphere model) which especially aims to model the thermal structure around mesopause altitudes. LIMA is a fully nonlinear, global, and 3-d Eulerian grid-point model which extends from the ground to the lower thermosphere (0-150 km) taking into account major processes of radiation, chemistry, and transport. The major improvements of LIMA compared to its predecessor COMMA/IAP are the implementation of a triangular horizontal grid structure with 41 804 grid points in every horizontal layer (Delta x similar to Delta y similar to 110km), and the assimilation of tropospheric and lower stratospheric data from ECMWF/ERA-40. Applying LIMA we are able to simulate in situ wind reversals of the mesospheric jets and cold summer mesopause states without applying any gravity wave parametrization. In the summer season the mesospheric wind reversal of the westward zonal wind is in the order of +5-10m/s which implies that the resolved eddy fields and hence EP-flux divergences, determining the momentum budget of the mesopause region, are underestimated by approximately a factor of two. On the other hand the thermal structure of the summer mesopause, and hence at least the balance of the energy budget in the summer mesopause region, is reproduced. E.g. during early July the observed mesopause temperature from rocket (falling spheres) experiments over ALOMAR (69 degrees N, 16 degrees E) is 131 K at 88 km compared to corresponding model temperature of 133 K at 88 km. The results of our numerical simulations suggest (1) that the intrinsic variability of the mesosphere is indeed determined by the lower atmosphere variability, and (2) that the simulated eddy fields may be interpreted as some fraction of inertia gravity waves resolved by the model (lambda(xy) >= 500 km, 2h <= T <= 12 h). (c) 2008 Elsevier Ltd. All rights reserved.