Interaction and generation of small-scale waves by medium-scale waves in the MLT region.

Interactions between gravity waves of different scales are frequent in the mesosphere and lower thermosphere (MLT) region of Earth's atmosphere. Recent coordinated observational studies [e.g., Fritts et al. 2014, Taylor et al. 2015, Bossert et al. 2015 ] have identified numerous cases where small-scale waves (~10s km wavelength, minute periods) are found to be propagating within strong medium-scale wave fields (~100s km wavelength and intrinsic periods of approximately hours). Despite this, it is unclear to what extent the small-scale waves are generated from the same source, are ambient in the atmosphere as generated by other sources, or are generated in-situ by the medium-scale wave and its interactions. A 2D compressible, nonlinear model is used to simulate cases of (1) the refractive effects of medium scale wave-like wind and temperature fields in addition to (2) the full dynamic interactions between small scale and medium scale waves under realistic atmospheric conditions. We investigate possible mechanisms for the medium scale wave to provide favorable regions of propagation for smaller scale waves from other sources in contrast to scenarios favoring in-situ wave generation or the generation of smaller-scale wavelike features. We also examine the propagation of small-scale waves generated from the same source location as the medium-scale waves, and how the propagation characteristics of waves determine which parts of the gravity wave spectrum may be susceptible to possible interactions. Small scale waves are shown to have a significant impact on the momentum and energy budget of the MLT through dissipation, filtering, and instability mechanisms [e.g. Fritts and Alexander 2003, Fritts et al. 2014]. Understanding and separating the roles of larger scale waves in refracting, interacting with, or generating smaller scale waves will be crucial in predicting the location and distribution of small-scale wave energy and momentum depositions and the resulting local variability of the MLT.