Gravity waves are important atmospheric phenomena that affect atmospheric circulation, and energy and momentum transfers. A three-year program of modeling, observational analysis, and parameterization development will be conducted, focusing on the local generation and global effects of gravity waves in the atmosphere. The goals of the project are to develop a detailed understanding of the processes causing wave generation, and to study the conditions that determine whether the waves propagate upward into the upper atmosphere or remain trapped in the low atmosphere. Trapped gravity waves can affect weather, while vertically propagating waves influence the general circulation and climate on the global scale via momentum transfer and forcing of the circulation. The project uses a set of modeling tools spanning a hierarchy of complexity and includes strict observational validation.
Intellectual Merit. The project seeks a deeper understanding of the processes leading to gravity wave generation and the translation of this knowledge into more realistic and better constrained parameterizations for global models. Parameterization development means capturing fundamental physical dependences and identifying key tunable parameters that scale with nonlinear or other neglected effects. The research efforts include parameterization applications in a global model to study short-term climate change effects (such as the El Nino-Southern Oscillation) on gravity wave sources and their subsequent effects on atmospheric circulation. The fundamental understanding gained from this work on the processes of gravity wave generation can provide a basis for evaluating the treatment of these processes in future global models.
Broader Impacts. This research will improve the treatment of gravity wave mean-flow forcing processes in global models used for climate prediction, ozone recovery assessments, and weather forecasting. Model improvements can have obvious impacts on human society and the environment in general. In the tropics, additional broader impacts of gravity waves are related to their effect on cirrus cloud occurrence frequencies and ice particle sizes. The process-level studies of gravity waves generated by convection will allow quantification of the roles of these waves in cirrus and upper atmospheric water vapor changes. The study of gravity waves in the vicinity of convection may also impact future work on local weather forecasts and turbulence associated with breaking waves that impacts aviation. This NSF-funded research also contributes to education and research training for a postdoctoral scientist and a number of graduate students.
In this project, we focused our work on atmospheric gravity waves that influence winds and climate. Gravity waves are so called because the Earth’s gravitational force acting on varying air density within the wave causes the oscillation that is the wave. The horizontal length of gravity wave oscillations ranges from about 10 to 1000 km. These gravity waves are known to drive larger-scale patterns in the winds. Global models that are used for climate prediction currently must represent the effects of these waves in an approximate way because the waves are too small to be represented exactly. The broad goals of the project are to improve the realism of these approximations in climate models, since the current state-of-the-art is based on a limited set of theoretical studies that need validation with observations. The approximate methods are called gravity wave parameterizations. Gravity waves in the atmosphere are generated in a variety of ways, but we mainly focused in this project on wave generation by storms and rain. This source of gravity waves is complex due to the wide variety of storm weather conditions and due to the difficulty in predicting precipitation. Previous research had shown that gravity waves generated in storms can depend on many variables, including the rain rate and duration, the depth of the rain cloud, the horizontal dimension of the storm cloud, the speed that the storm travels, and the environmental wind and wind shear. In our three year project, we first focused on studying tropical gravity waves generated by storm clusters and their interactions with tropical winds in the stratosphere. We discovered that a class of gravity waves with large horizontal scales, but small vertical scales, are quite important for driving changes in tropical winds. Using a unique set of observations from Northern Australia we identified the source of a dominant wave event, and the mechanism of wave generation. We also discovered that although the best global weather forecasting models should have no trouble describing the horizontal structure of this large-scale wave event, the number of atmospheric layers in these models is too small, so the waves are poorly represented. Our work showed how many additional layers would be needed to represent this important class of wave. We also developed a specialized weather forecasting global model with more vertical layers and studied this class of gravity waves in this specialized model in a more general way. We discovered that these waves do a large fraction of the work of driving the tropical stratospheric winds. We next turned our attention to summertime Midwestern US thunderstorms, since previous global observations had indicated storms in this region are a particularly prominent source of gravity waves. After developing an accurate weather model of one storm case, we utilized the dense network of weather radar in the region to validate key variables in the model that are known to be important for gravity wave generation. We identified one variable that seems to control the gravity wave energy observed above the storm, and that variable is the depth of the rain cloud. Further, we find that certain parameter choices that have to be made in the weather model, have a dramatic effect on the depth of the simulated rain cloud and on the wave energy. Finally, we have collaborated with international partners to bring these results to the climate modeling community to have a broader impact. We organized a workshop bringing together experts in climate modeling, gravity wave parameterization, and gravity wave observations to complete direct comparisons. The comparisons revealed problems in the climate models near the poles north of 60 degrees, pointing to excessive effects on the winds due to gravity waves in some of the models at these high latitudes. Differences in the tropical and subtropical atmosphere were also discovered that are related to waves generated by storms. These discoveries are leading to changes in the methods that the participating climate models use to approximate the effects of gravity waves on the circulation.
