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.
This research grant supported modeling and analysis activities by five scientists and several colleagues at other institutions during its 5-year term. One scientist received his Ph.D. as part of these efforts. The research led to publication of two book chapters, one Ph.D. thesis, and 15 journal papers, of which eight are published, three are under review, and four others will be submitted in the near future. The research has also motivated a number of additional applications in the interpretation of other observational data that we expect to lead to additional publications in the future. The primary focus of our modeling efforts was on the dynamics of relatively small-scale gravity waves (typical vertical and horizontal wavelengths of ~2-20 km and ~10-200 km, respectively, at altitudes of ~50-100 km). Despite their much smaller scales than tides and planetary waves, which are more easily observed globally by satellite instruments, gravity waves have very large impacts throughout the atmosphere because they propagate upward very quickly and they account for large energy and momentum transport to higher altitudes. Our approach has employed two state-of-the-art numerical models that describe the detailed dynamics accompanying gravity wave attainment of large amplitudes and the resultig instabilities and turbulence that account for energy and momentum deposition. These processes have very large effects on the circulation and structure of the atmosphere extending from the surface to over 100 km altitude. Hence they are key contributors to processes affecting weather and climate, yet their influences are not able to be modeled directly by numerical weather prediction (NWP) and climate models. This requires desciptions of these dynamics in such models that are acknowledged to perform poorly at present. As a consequence, models such as ours are required to advance our understanding of these dynamics and enable better parameterizations, and better resulting weather forecasts and climate predictions in the future. Specific research efforts that address these needs have included the following: 1. numerical studies of the fundamental dynamics involved in gravity wave instability (typically referred to as wave "breaking"), turbulence production, and dissipation that account for energy and momentum deposition and accelerations of the background flows, 2. studies of Kelvin-Helmholtz shear instabilities that arise due to gravity wave and mean shear superpositions and account for many incidents of strong turblence at airline altitudes and elsewhere from the surface into the thermosphere, 3. studies of superposed larger- and smaller-scale flows, typically referred to as multi-scale dynamics, that lead to intermittent instabilities and turbulence events that appear ubiquitous throughout the atmosphere and that must also be accounted for in accurate parameterizations of gravity wave influences, 4. studies of gravity wave dynamics and interactions with large-scale flows that lead to "secondary" generation of additional gravity waves that may themselves have large influences at even higher altitudes, and 5. studies of other specific nonlinear phenomena, e.g., mesospheric bores and gravity wave "self acceleration" dynamics, that also impact the evolution of the gravity wave spectrum as a whole throughout the atmosphere. We expect our results will ultimately contribute benefits in the following areas: 1. development of improved parameterizations of small-scale dynamics in stratified flows extending from Earth's surface to very high altitudes, 2. improved weather and climate prediction, as noted above, 3. improved understanding of radar measurements of gravity wave and turbulence dynamics throughout the atmosphere, 4. more quantitative interpretation of small-scale dynamics observed in mesospheric airglow layers and noctilucent cloud displays, and 5. guidance for modeing and parameterization of gravity wave influences in the oceans, other planetary atmospheres, and stellar interiors.
