What selects the scales of mesoscale gravity waves?

Gravity waves are ubiquitous in the atmosphere and play a fundamental role in a wide variety of atmospheric processes, especially for those at the mesoscales. They can initiate and organize convection, transfer significant amounts of energy and momentum, and generate and modulate atmospheric turbulence. There are significant advances in recent decades in understanding the initiation, maintenance and propagation of weather significant gravity waves with enhanced mesonet and radar observations, idealized and real-data high-resolution numerical simulations, along with emergence of new theories and hypotheses. However, except for very few special circumstances, it remains largely unanswered on what determines the spatial and temporal scales of the gravity waves, especially under complex background flows, and/or two-way interactions with moist convection.

In this overview talk, we will first synthesize our recent studies on various scale selection mechanisms under different flow patterns which include but not limited to (1) ducted wave-CISK as in Zhang et al. (2001 QJ) from convection-permitting simulation of an observed gravity wave event embedded within a winter cyclone, (2) direct linkage to the growth rate of background baroclinic waves for jet-front gravity waves (Zhang 2004 JAS; Wang and Zhang 2007 MWR; Lin and Zhang 2008 JAS), (3) selection by the propagating media such as in Wang et al. (2010 QJ) for vortex dipoles consistent with the wave capture theory of Buhler and McIntyre (2005, JFM), and (4) the forcing scales of moist convection modulated by partial ducting between the tropopause and the ground (Lane and Zhang 2011 JAS).

We will then present our ongoing research in the scale selection of gravity waves within moist baroclinic waves. Our preliminary findings show that with weak to moderate convective instability, the dry gravity wave modes as identified in Lin and Zhang (2008 JAS) dominate but at least some of the dry modes become hardly identifiable in the presence of strong moist convection. Ongoing and future studies are to further elucidate the relative roles of the original forcing scales, the characteristics of the propagating media, and the interaction/oscillation with moist convection in the wave scale selection under various flow regimes.