Title: Reexamination of the moisture-vortex and baroclinic instabilities in the South Asian monsoon
Lecturer: Prof. Tianming Li (University of Hawaii)
Time: Wednesday Mar 27, 2024 at 3:00 PM
Venue: Lecture Hall D103, School of Atmospheric Sciences
Abstract: Observational analyses reveal that a dominant mode in the South Asian Monsoon region in boreal summer is westward-propagating synoptic-scale disturbances with a typical wavelength of 4000 km that are coupled with moistening and precipitation processes. The disturbances exhibit an eastward tilt during their development before reaching their maximum activity center. A 2.5-layer model that extends a classic 2-level quasi-geostrophic model by including a prognostic lower-tropospheric moisture tendency equation and an interactive planetary boundary layer was constructed. The eigenvalue analysis of this model shows that the most unstable mode has a preferred zonal wavelength of 4000 km, a westward phase speed of 6 m s-1, an eastward tilt vertical structure and a westward shift of maximum moisture/precipitation centers relative to the lower-tropospheric vorticity center, all of which agree with the observations. Sensitivity experiments show that the moisture-vortex instability determines, to a large extent, the growth rate, while the baroclinic instability helps set up the preferred zonal scale. The current work sheds light on understanding the moisture-vortex and the baroclinic instability in a monsoonal environment with a pronounced easterly vertical shear.