Group seminar on 21. June, 14:15 CET

Dynamics of mixed Rossby - gravity waves in the tropical atmosphere: TIGAR simulations with realistic background flows

Sándor Maho

 This seminar discusses the generation mechanism of the mixed Rossby – gravity waves (MRGWs) also known as Yanai waves. The MRGWs are defined using the linear theory: they are asymmertic special solutions of the rotating shallow water equations (RSWs) on the equatorial beta plane or on the sphere. In real atmosphere, MRGWs are considered to play a prominent role in inter alia forcing the QBO, modulation of convection over the Pacific, etc. Visual inspection of the results of wavenumber filtering of MRGWs in the ECMWF operational data by the MODES software suggests that tropospheric and upper stratospheric-mesospheric MRGWs may be less coupled than commonly thought. The maximal upper stratosphere-mesospheric signal appears at large scales (k = 2-3) whereas the maximal MRGW signal in the troposphere is found at synoptic scales (k = 5-7). This suggests a possibility of an independent MRGW generation mechanism in the two regions. To study this, we perform numerical simulations using TIGAR, which has the MRG mode as a prognostic variable. This allows us to follow its time evolution and to quantify processes contributing to its tendencies. Earlier work showed that the MRGWs can be excited in TIGAR by asymmetric heat sources and by non-linear wave – mean flow interactions. In this seminar, I will present the two mechanisms and discuss their potential roles in the troposphere and in the upper stratosphere-mesosphere by showing results of TIGAR simulations with realistic background flows derived from ERA5.