Observations, modeling and dynamics of mesoscale gravity wave generated from midlatitude baroclinic jet-front systems

Mesoscale gravity waves originated from baroclinic jet-front systems have important implications to stratospheric dynamics and ozone chemistry. Compared to commonly observed orographic gravity waves, these waves are less studies in literature, especially in the upper troposphere and lower stratosphere. In this study, we will first present the generation of gravity waves during the life cycle of an idealized baroclinic wave through multiply-nested mesoscale numerical simulations with horizontal resolution up to 3.3 km. Long-lived vertically-propagating mesoscale gravity waves with horizontal (vertical) wavelengths about 150 (2.5) km and intrinsic phase speed of about 8 ms-1are simulated originating from the exit region of the upper-tropospheric jet streak. The imbalance diagnosis and model simulations suggest that balance adjustment, as a generalization of geostrophic adjustment, is likely responsible for generating these mesoscale gravity waves. It is hypothesized that, through balance adjustment, the continuous generation of flow imbalance from the developing baroclinic wave will lead to the continuous radiation of gravity waves.

The significance of the baroclinic jet-front systems to the variability of the middle-atmospheric gravity waves in the real atmosphere is analyzed with observations from the NOAA AMSU-A (Advanced Microwave Sounding Unit-A) radiance measurements over North America and the Atlantic Ocean during January of 2003. Analyses of these observations provide detailed information on the horizontal variations of middle-atmospheric gravity waves: besides the geographical modulation especially above regions of significant topography, these middle-atmospheric gravity waves are strongly correlated with the intensity and location of the tropospheric baroclinic jet-fronts systems.

A state-of-the-art mesoscale model has been used to explicitly simulate gravity wave activities for the entire month of January of 2003 which includes several episodes of enhanced gravity wave activities identified from the above observations (e.g., January 7-10 and January 18-20, 2003). The simulated middle-atmospheric gravity waves compared qualitatively well with the satellite observations. Moreover, high-resolution explicit simulations allow us to identify four likely source mechanisms of middle-atmospheric gravity waves associated with extratropical tropospheric baroclinic jet-front systems: (1) topographically-forced waves due to jet streaks incepted by large terrains; (2) adjustment-forced waves due to strong flow imbalance associated with the upper-tropospheric jet streaks; (3) diabatically-forced gravity waves due to moist convection induced by baroclinic waves; and (4) frontally-forced gravity waves due to frontal collapse near the surface. The last three mechanisms are transient in nature and often inseparable from each other. Explicit simulations also enable us to assess qualitatively the momentum fluxes from different sources and their relative importance.