On the Dynamical Importance of Gravity Wave Sources Distributed Over Different Heights in the Atmosphere

Gravity waves (GWs) are generated at all altitudes in the atmosphere, but sources above the lower stratosphere are rarely considered by parameterizations employed in general circulation models. This study assesses the potential impact on 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 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.

Plain Language Summary: Multiple observations demonstrate that gravity waves (GWs) are generated at all atmospheric levels, however numerical general circulation models employing parameterizations that account for wave sources only in the troposphere are able to reproduce the state and dynamics of the middle and upper atmosphere reasonably well. Assessing the role of GWs generated above the troposphere is extremely challenging, because such waves are difficult to separate from those of tropospheric origin in observations. The mechanisms of wave generation in the middle atmosphere are very complex and not fully understood. We developed a numerical framework, in which the strength of the extra‐tropospheric sources is represented by multiples of those in the troposphere. In the series of sensitivity tests, we demonstrate that the contribution of sources to the total wave momentum drops with height following the density decrease, and that the tropospheric sources capture the major part of the total momentum and of the associated GW drag in the thermosphere. One of the conclusions of this study is that the impact in the thermosphere of secondary waves, which are believed to be excited near the mesopause, is negligible compared to that of primary waves propagating from the troposphere.

Key Points: A framework for assessing impacts of gravity waves generated by sources distributed over all heights in the middle atmosphere is developed. The thermospheric response to sources above the tropopause is primarily produced by waves generated in the lower. stratosphere Localized sources produce negligible thermospheric drag unless the forcing is orders of magnitude stronger than in the troposphere.

Earth Sciences Division http://dx.doi.org/10.13039/100014573

https://kauai.ccmc.gsfc.nasa.gov/instantrun/hwm

https://ccmc.gsfc.nasa.gov/modelweb/models/nrlmsise00.php