Gravity Waves in the Middle Atmosphere: An Introductory Overview of Recent Progress and Remaining Challenges

Internal atmospheric gravity waves are now well recognized as one of the more important dynamical drivers of the climate and meteorology of the middle atmosphere (altitudes ~10-100 km). Quasi-continuous breaking of gravity waves around the globe drives and maintains the observed middle atmospheric circulation and climate. Yet gravity waves continue to present modeling and observational challenges, especially on global scales. In recent years, high-resolution models have resolved aspects of wave generation, propagation and breakdown that have provided new three-dimensional insights into these important processes. Similarly, new generations of satellite remote sounders can resolve long wavelength gravity waves in the middle atmosphere, and these data are providing our first tentative glimpses into how middle atmospheric gravity waves evolve globally from day to day, seasonally, and interannually. Despite this progress, there is no imminent prospect of computing advances that will allow either global climate or operational numerical weather prediction models to run at resolutions fine enough to resolve the full spatiotemporal spectrum of wave motions in the middle atmosphere, and the progressive breakdown with altitude of this wave spectrum into turbulence that yields mean-flow forcing. Thus, gravity wave parameterizations will remain important components of state-of-the-art climate and weather models for the foreseeable future. In addition to the important subgrid-scale gravity wave drag parameterizations, which have developed significantly in recent years, the gravity wave parameterization approach is now being extended to other areas, such as gravity wave influences on polar stratospheric, polar mesospheric and high cirrus clouds. Yet despite progress on many fronts, important open questions about middle atmospheric gravity waves persist that merit further research. These uncertainties find expression in the broad variety of middle atmospheric gravity wave drag parameterization schemes currently proposed by various researchers for use in models, and the uncertain values of key dependent variables in each scheme. These uncertainties translate into extensive and painstaking tuning of a given gravity wave drag parameterization in a global model to reproduce acceptable middle atmospheric wind patterns and temperature distributions.