Critical Layers, Breaking, and Secondary Gravity Wave Generation in the Mesosphere and Lower Thermosphere Observed During the DEEPWAVE Campaign

Abstract:

Various conditions within the mesosphere and lower thermosphere (MLT) can lead to critical layers and instabilities for propagating gravity waves (GWs). These critical layers cause gravity wave breaking, resulting in momentum deposition and the generation of secondary gravity waves, which can trigger far reaching effects on the MLT region including induced drag on mean winds. GWs play an important role in the vertical transport of momentum from the lower to the upper and middle atmosphere. A better understanding of their generation, propagation, and breaking is needed to improve predictive capabilities for momentum deposition and coupling within the atmosphere. Understanding the conditions under which GWs break and generate secondary GWs is especially of importance, as secondary GWs allow for momentum to be transported to even higher altitudes within the atmosphere.

Recent measurements from the DEEPWAVE campaign exhibiting critical layers within or near the MLT that lead to GW breaking are discussed. Furthermore, examples are given of observed secondary GW generation. Shown in the attached figure is one such example of observed secondary GWs. The sodium mixing ratios from a flight over the south island of New Zealand on 13 July 2014 demonstrated the presence of strong mountain waves (MWs). Figure A shows strong MW perturbations below 86km. However, the MWs start to dissipate quickly in altitude as they approach a critical level near 90km where the wind was estimated to be near 0 m/s. Figure B shows smaller scale GW perturbations beginning to form near 90km where the MWs have largely dissipated. Figure C shows that these smaller-scale secondary GWs are visible up to higher altitudes, and there are multiple scales of these secondary GWs. These data demonstrate the important role that critical layers play in GW breaking, secondary GW generation, and the transport of momentum.