Atmosphere Ocean Science Colloquium

Coupled Dynamic-Thermodynamic Forcings during Tropical Cyclogenesis

Speaker: Brian Tang, University at Albany

Location: Warren Weaver Hall 1302

Date: Wednesday, September 19, 2018, 10:35 a.m.

Synopsis:

A diagnostic framework to investigate the role of processes around and during tropical cyclogenesis is presented. The key framework metric is the ratio of bulk differences of moist entropy over differences of angular momentum between an inner and outer region of a tropical disturbance or cyclone. This ratio is hypothesized to decrease and become negative as both the high-entropy core and low-level vortex in the inner region amplify during tropical cyclogenesis. The time tendency of this ratio can be split into two forcings: a moist entropy forcing and an angular momentum forcing.

An ensemble of axisymmetric model experiments with simplified physics is used to evaluate the diagnostic framework. Within a day before tropical cyclogenesis, the moist entropy forcing results in a decrease of the ratio. Net advective fluxes act to export moist entropy from the outer region and import moist entropy into the inner region, resulting in a positive radial gradient in gross moist stability that is maximized around the time of genesis. While surface moist entropy fluxes are needed for genesis to occur, they act synergistically with the net advective fluxes to decrease the ratio before and during genesis. Within a day after tropical cyclogenesis, surface moist entropy fluxes directly amplify the positive difference in moist entropy between the inner and outer regions, and radial fluxes of angular momentum reduce the magnitude of the negative difference in angular momentum between the inner and outer regions. Both of these processes act to reduce the ratio further. The framework highlights differences in processes occurring before, during, and after genesis as the meso-beta-scale protovortex develops and intensifies.