The investigators will conduct comprehensive experimental studies aimed at quantifying gravity wave (GW) momentum transport and the GW instability dynamics that drive energy and momentum deposition and energy transfers in the mesosphere and lower thermosphere (MLT). GWs having small horizontal scales and large amplitudes and momentum fluxes provide the majority of the mean and variable forcing in the MLT, and these vary significantly in space and time. Understanding of this forcing and variability is highly uncertain, but it is extremely important for parameterization of such dynamics in large-scale modeling of the MLT, and throughout the atmosphere. The same parameterization needs are also critical for general circulation models (GCMs), climate models, and numerical weather prediction (NWP) models in related fields. To quantify these GW and instability dynamics and their mean and variable forcing as fully as possible, the investigators will employ instrument suites at the only two sites able to define these dynamics in the most complete and quantitative manner. Low-latitude measurements will use the comprehensive instrumentation of the new Cerro Pachon Observatory in Chile (30°S); high-latitude measurements will employ the even more extensive suite of instrumentation at the ALOMAR observatory in northern Norway (69°N).
These instrument suites permit the most complete, and redundant, specification of the GW, instability, and mean parameters needed to quantify GW amplitudes, momentum fluxes, and instability dynamics at any site. The recognized role of GWs in driving the mean and variable structure of the MLT makes an understanding of their various contributions, and an ability to model their effects, a high priority. These dynamics also influence a wide range of other processes ranging from tidal and planetary wave structures and dynamics to minor species transport. The need to describe such effects accurately also has broader implications for modeling climate change, responses to variable solar forcing, and Space Weather.
Research performed under this NSF grant addressed what are considered small-scale dynamical processes in the atmosphere at altitudes of ~80 to 100 km. These dynamics include what are called gravity waves because they cause air parcels to oscillate about their equilibrium altitudes (where they would reside if the atmosphere were at rest), instabilities that accompany large gravity wave amplitudes, and turbulence that resuts from the various instabilities. Gravity waves arise due to airflow over mountains, strong convective storms, and stong wind shears in the lower part of the atmosphere. They are responsible the "cap" clouds often seen over mountains, for strong winds in the lee of mountains, for the Kelvin "cats-eye" features often seen in this cloud layers, and for sometimes severe turbulence at airline flight altitudes. Their vertical and horizontal wavelengths can range up to ~10's and 100's of km or more, respectively. Gravity wave amplitudes grow strongly as they propagate upward because they attempt to conserve energy, and atmospheric density decreases strongly with altitude. So velocities and temperature fluctuations must increase acordingly. As a result, gravity waves yield strong instabilities at ~80 to 100 km altitudes, and these result in gravity wave dissipation and turbulence that mixes the atmosphere at these altitudes. The effects of these processes are important to include in weather prediction and climate models, but the detailed dynamics and effects are poorly understood at present. It is widely believed that a better understanding of these small-scale processes will enable more accurate weather and climate forecasting. Our research has yielded an improved understanding of these dynamics by quantifying gravity wave structures and their evolution, including the generation of instabilities and turbulence, with new and combined instruments that have allowed us to define gravity wave amplitude growth, the occurrence and character of instabilities and turbulence, and their environmental influences more completely than has been possible previously. These new instruments include lasers that measure temperatures and winds and cameras that can map temperatures in a horizontal plane at the same altitudes measured by the lidars. Together, these new instruments provide dramatic improvements over previous measurement capabilities - and a far more quantitative understanding of these small-scale dynamics that arise in many fluids (including the oceans, the atmospheres of other planets, and the interiors of stars) and of their infuences that need to be included in weather and climate models on which society increasingly relies for the mitigation of risks to human safety and property.
