Group seminar on 10. November, 14:15 CET
Impact of Moisture on the Propagation of Inertio-Gravity Waves in the Tropics
In the tropics, moist processes impact the evolution and propagation of inertio-gravity waves, in particular by reducing their phase speeds. Latent heat release partly compensates for the cooling caused by vertical motion, decreases the atmospheric stability and reduces the phase speeds. However, the impact of moisture and stability on the propagation of equatorial waves, in general, is not yet entirely understood.
The thesis focuses on the effect of reduced stability on the propagation properties of inertio-gravity waves. For this purpose, idealised experiments are carried out using a simplified numerical model - the Moist Atmosphere Dynamics model (MAD). It solves the non-linear rotating shallow water equations for the baroclinic vertical mode combined with a moisture conservation equation for the lower troposphere. Thereby, atmospheric stability is either defined as a constant, i.e. without horizontal variations, or as a local value, computed from the local Brunt-Väisälä frequency. This work develops the local stability concept in MAD and investigates differences in the circulation response between the two stability concepts. Additionally, MAD is expanded by a convection relaxation scheme following the Betts-Miller scheme and its impact on the circulation response is analysed.
Comparing the response in a saturated atmosphere with that in an unsaturated one, the phase speed of the excited Kelvin wave is decreased by up to 10 ms-1. The application of local instead of constant stability reduces the phase speed of the Kelvin wave by 2-5 ms-1. Assuming local stability can lead to local instabilities which grow non-linearly and generate new mass-field perturbations. The application of the convection scheme, however, avoids these instabilities and distributes the moisture more evenly in space and time.