Group seminar on 16. January, 14:15 CET
Scale selection of mixed Rossby-gravity waves by wave-mean flow interactions
Sándor Mahó
Observational and reanalysis-based studies agree on that mixed Rossby-gravity (MRG) waves concentrate in the upper troposphere in synoptic scales and in the upper stratosphere in planetary scales. Although there are multiple theories for MRG generation in the atmosphere none of them can explain the distinct scale selection of tropospheric and stratospheric MRG waves. In this talk, I will show how a recently proposed excitation mechanism of MRG waves by wave-mean flow interactions solve the scale selection problem. I will show results from numerical simulations with a spherical shallow water model (TIGAR) that incorporates the MRG wave as a subset of prognostic variables. Such a framework allows to identify wave-mean flow interactions as the main driver of MRG scale selection in comparison with excitation driven by external forcing and wave-wave interactions. Simulations with reanalysis-derived background flows of the upper troposphere and the stratosphere and simulations with idealized background flows show that the central latitude of the jet is the main factor influencing the MRG scale selection. In particular, subtropical jets produce synoptic-scale MRG waves, whereas jets embedded in midlatitudes generate planetary-scale MRG waves.