Keynote Talk: Tropospheric Mesoscale Gravity Waves

Gravity waves are ubiquitous in the atmosphere and play a fundamental role in a wide variety of atmospheric processes. They can initiate and organize convection, transfer significant amounts of energy and momentum, and generate and modulate atmospheric turbulence. The momentum transport and deposition by gravity waves have significant impacts on the general circulation of the atmosphere. A better knowledge of these processes demands a complete understanding of the mechanisms by which the gravity waves are generated, together with their characteristics, distribution and variability. This review will cover recent advances in many aspects of tropospheric mesoscale gravity waves including observations, modeling and theories. Special emphases are given to (1) the detection of gravity waves in the troposphere using radar observations; (2) the role of flow imbalance and geostrophic adjustment in the gravity wave generation; and (3) the importance of gravity waves in initiating and modulating mesoscale precipitation bands.

Many studies used conventional surface and sounding observations to detect the presence of mesoscale gravity waves and to retrieve their characteristics based on dispersion and polarization relationships. The exit region of an upper-level jet streak and to the cold-air side of a surface frontal boundary is commonly found to be a preferred area of observed mesoscale gravity wave activities. However, because of their spatial and temporal scales, the three-dimensional structure and propagation of typical mesoscale gravity waves are in general not directly observable with the conventional observing networks. Radar observations, especially in the presence of wave associated mesoscale cloud and precipitation bands, may provide much needed three-dimensional details of gravity waves with high spatial and temporal resolution, as have been demonstrated in past field experiments (e.g., CCOPE of 1981 and STORM-FEST of 1992).

With advances in computational power, numerical models begin to be capable of simulating mesoscale gravity waves realistically. The nearly continuous model output in both time and space and for all variables from both idealized and real-data case studies has become a powerful tool not only to examine the characteristics and structures of the gravity waves but also to examine their generation and maintenance mechanisms as well as their impacts and predictability. For example, most recently, a state-of-the-art mesoscale model with grid increments down to 3 km has been used to successfully simulate mesoscale gravity waves generated during the life cycle of idealized baroclinic waves. The simulated location and characteristics of the gravity waves are very consistent with observational studies. Balance adjustment, as a generation of geostrophic adjustment, is hypothesized to be primarily responsible for these gravity waves generated in the exit region of the upper-level jet streaks. Other studies have shown that the jet-streak-generated gravity waves can propagate to the lower troposphere, initiate and modulate convection. Moist convection, in the meantime, may initiate, maintain and be coupled with the gravity waves. Moreover, many observational and numerical studies have suggested that mesoscale gravity waves may be responsible for the initiation and propagation of many of the “mysterious” mesoscale precipitation bands, especially for those occurred during the summertime