Generation and Propagation of Gravity Waves from Jets

Jets are a major source of atmospheric gravity waves, but they remain less understood compared to other source mechanisms such as topography and convection. Although many observational studies have demonstrated that the atmospheric jet can support gravity wave generation, numerical studies of jet-generated gravity waves are rather limited because of complicated jet structure with high nonlinearity and also because of the difficulty of finding the appropriate way to represent jet structures. One way to avoid the problem is to simulate a jet in the context of developing baroclinic waves and to identify the gravity waves near the jet exit region (Zhang, 2004). Another way is to create a localized jet associated with vortex dipoles (Snyder etc. 2007), which are quite stable even in the time scale of many inertial periods.

In this study, idealized balanced jets and vortex dipoles in a uniformly stratified atmosphere are produced by inverting prescribed Ertel poetntial vorticity. The balanced wind structure is then used to initialize a mesoscale model. Three scenarios are studied: a surface vortex dipole, a middle level vortex dipole and a vortex dipole centered at the tropopause. In all these cases, inertial gravity waves with frequencies 1-2 times that of the Coriolis parameter are generated in the jet exit region with a Rossby number greater than 0.1. Gravity wave propagation is investigated using a ray tracing technique. The ray tracing demonstrates the variation of wave characteristics along the ray paths and suggests that an arc-shaped wave pattern results from inertial critical levels, and effects of the effective Coriolis parameter.

Similar to the ideas of spontaneous generation of gravity waves, Plougoven and Zhang (2007) cast the equation of vertical velocity in the form of a linear wave equation forced by large scale primary flow. We calculated the wave forcing diagnostics that include contributions from the residual of the nonlinear balance equation, the Lagrangian of vorticity and the Lagrangian of potential temperature. The diagnostics clearly demonstrate that the forcing appears very early near the jet exit region and thus provides a good predictor. We also calculate the response to wave forcing by finding the steady state solution of the linear wave equation.