Title: Spontaneous balance adjustment and gravity wave excitation in baroclinic lifecycles

Fuqing Zhang

Abstract: This talk presents our recent studies in simulating and understanding mesoscale gravity waves in baroclinic waves with state-of-the-art high-resolution mesoscale models and advanced diagnosis of the flow imbalance. Spontaneous balance adjustment, as a generalization of the geostrophic adjustment hypothesis, is proposed to be the likely mechanism in generating the simulated mesoscale gravity waves in the unbalanced upper-tropospheric jet-front systems. In this hypothesis, the flow can become increasingly unbalanced after the gravity waves are being generated if the production of imbalance by the background flow outweighs the reduction of imbalance through the radiation of gravity waves. This hypothesis is further demonstrated through formulating a wave equation forced by terms from the large-scale flow in which scaling arguments are used to determine the form of the wave equation and the forcing terms. It is shown that the residual of the nonlinear balance equation (as an index of large-scale flow imbalance) plays an important role in this forcing. The second part of the talk will discuss gravity wave generation during different baroclinic lifecycles. Consistent with the balance adjustment hypothesis and the wave forcing equation discussed above, the characteristics of the simulated gravity waves are found to be very sensitive to the baroclinicity (growth rates) of the background jet-front systems. I will also present our current efforts in using mesoscale models and ray tracing method in understanding the sources and variations of the gravity waves for both idealized real-data case studies of various baroclinic lifecycles.