Shuguang Wang and Fuqing Zhang (Texas A&M), Chris Snyder (NCAR) and Dave Muraki (Simon Fraser Univ.)
Vortex dipoles near the surface and at the tropopause level on an f plane are simulated in a barotropic atmosphere using a mesoscale model with balanced initialization. Upward propagating gravity waves with near inertial frequency are continuously generated at the exit region of the surface dipole jet, which are later dissipated by the background flow when approaching the critical levels. Propagating gravity waves are hardly discernible in the exit region of the tropopause dipole jet but can be very pronounced after introducing unbalanced momentum perturbations.
The variations of gravity wave characteristics along the propagation paths are calculated with a four-dimensional ray-tracing model, GROGRAT. For the surface dipoles, gravity wave rays are initialized near the surface with different phase speed, horizontal wavelengths and wave vector azimuth at different locations. Horizontal and vertical wavelengths and intrinsic frequency are all found to decrease as the waves propagate towards the jet exit region. The ray tracing results are compared against theatrical prediction using local horizontal deformation and vertical wind shear, as proposed by the wave capture theory. The characteristics of the gravity waves estimated from the ray-tracing analysis are sometimes found to be significant different from those predicted by the wave capture theory. For comparison, the ray tracing analysis is also performed for the tropopause dipoles as well as in the idealized baroclinic waves.