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Abstract

Gravity waves (GWs) are generated at all altitudes in the atmosphere-ionosphere system, however their sources above the lower stratosphere are rarely considered by GW parameterizations employed in general circulation models. This study assesses the potential impact in the thermosphere produced by small-scale waves originating at different heights. Within the proposed numerical framework, GW sources are represented by wave momentum forcing, whose values are expressed relative to the forcing required to obtain typical wave spectra around the tropopause. The relative importance of tropospheric and extra-tropospheric sources and the response in the thermosphere are studied in a series of sensitivity experiments. They demonstrate that the accumulation of wave momentum steeply drops with height as a consequence of decreasing density, even when the forcing is maintained at a uniform level throughout the middle atmosphere. When a broad spectrum is forced at twice the tropospheric rate, the thermospheric drag is increased by only a factor of two, and that increase is produced by waves that were forced in the lower stratosphere. With increasing altitude, vertically localized sources contribute progressively less. For example, for GWs excited near the mesopause to produce an impact comparable with that due to waves propagating from below, the forcing must be orders of magnitude stronger than in the troposphere. The estimated forcing of the so-called secondary harmonics by breaking/saturating primary waves is much weaker, such that the systematic dynamical effect of secondary waves in the thermosphere is negligible compared to that of the primary GWs generated in the troposphere.