Multi-step vertical coupling in the winter hemisphere

Abstract:

We present a new version of the high-resolution Kuehlungsborn Mechanistic general Circulation Model (KMCM) that is extended vertically to about 450 km. This model is called HIAMCM (HI Altitude Mechanistic general Circulation Model) and simulates gravity waves (GWs) explicitly down to horizontal wavelengths of about 156 km (corresponding to a horizontal grid-point resolution of 52 km). We found that the explicit simulation of GWs in the thermosphere requires a correction for non-hydrostatic dynamics, as well as the incorporation of molecular viscosity in the horizontal diffusion scheme. No artificial sponge layer is required in his case. Recently the HIAMCM was extended by nudging to reanalysis forecast data in the troposphere and lower stratosphere. This nudging is performed in spectral space and specifies only horizontal wavelengths larger than about 2000 km. As a result, the GWs are simulated like in the free-running model without artifacts from the nudging. Case studies for the middle and upper atmosphere during wintertime show that the GWs around the winter mesopause and at higher altitudes are mainly secondary and tertiary GWs that result from multi-step vertical coupling. This coupling means that the intermittency and spatial localization of the momentum deposition from the dissipation of primary GWs creates imbalances that generate secondary GWs; in turn, the dissipation of secondary GWs leads to the generation of tertiary GWs. The thermospheric GWs resulting from this process show up as concentric ring structures, are subject to strong interaction with the tidal background winds, and are consistent with the propagation of quietime TIDs. The tertiary GWs that have the shortest horizontal wavelengths and largest vertical group velocities create a GW hotspot at high altitudes in the thermosphere over the Southern Andes/Antarctic Pensinsula. These results imply that around the winter mesopause and at higher altitudes, the resolved GWs cannot be described by any usual GW parameterization.