Numerical Simulations of Gravity Wave Instabilities, Wave-Wave and Wave Mean Flow Interactions, Momentum Transport, and Spectral Evolution in the Mesosphere and Lower Thermosphere

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This project will perform comprehensive modeling studies intended to quantify gravity wave (GW) instability dynamics and nonlinear wave-wave and wave-mean flow interactions that drive energy and momentum deposition and energy transfers within the GW spectrum in the mesosphere and lower thermosphere (MLT). GW momentum transport is the major driver of the large-scale circulation and thermal structure of the MLT at middle and high latitudes and appears to play an important role at equatorial latitudes. Observations and modeling also suggest that GW momentum fluxes are strongly modulated by tides and planetary waves (PWs) and that variable GW momentum fluxes can in turn modulate these larger-scale motions and map the effects of their GW filtering to much higher altitudes. Despite the critical importance of these processes, the GW instability and interaction dynamics controlling these large-scale responses have been quantified only in very idealized environments to date. The descriptions of these dynamics via GW parameterizations in various large-scale general circulation model, climate, and numerical weather prediction models are widely recognized to be poor approximations of these dynamics in many applications, despite their important influences on larger-scale dynamics throughout the atmosphere. The goal of the project is to define the various instability dynamics for large domains and broad GW spectra sufficiently well to provide a critical understanding of the most important components and guidance that is of value in the design of parameterizations of these GW influences that would substantially improve the performance of the various weather and climate models that depend on them. To quantify the most important GW instability and interaction dynamics and their mean and variable responses as fully as possible, very high resolution direct numerical simulations (DNS) and large-eddy simulations (LES) will be done in order to assess the competition between different instability classes (including wave-wave interactions) and the circumstances where one or the other is clearly dominant for various GWs scales, frequencies, and amplitudes in various environments; the importance of localization of GW instability events in GW dissipation, amplitude (and momentum flux) constraints, and spectral evolution; the consequences of GW (and mean shear) superposition for instability occurrence and type and turbulent mixing and transport. The project aims to provide a more quantitative characterization of these dynamics on GW momentum deposition, spectral evolution, and mean and large-scale forcing of the MLT.