Eric Rappin (U-Miami)
MIT Atmospheric Seminar Series
Two methods have been developed to explore tropical cyclogenesis with climate change. One is to count and track tropical cyclone-like vortices in global climate models. A second is to use quantities thought to be relevant to tropical cyclogenesis, combined through statistical analysis, to provide an index related to the likelihood of genesis for a given environment, termed the genesis index. Both of these methods use large scale fields with insufficient resolution to incorporate processes fundamental to genesis. In order to obtain high resolution details of variations in the genesis process with climate, we resolve the cloud scale while approximating the large scale with a state of radiative convective equilibrium (RCE). RCE provides a useful approximation to the large scale tropical state since the temperature and moisture profiles are largely controlled by the sea surface temperature (SST) and the mean surface wind.

The thermodynamic aspects of tropical cyclogenesis are explored through the relationship between a genesis parameter determined by the RCE state and a genesis proxy. A genesis proxy is taken to be a measure of the likelihood of tropical cyclone formation from a pre-existing mid-level disturbance in RCE environments through the use of high resolution numerical model simulation. Furthermore, the components of the genesis parameter will be directly tied to the success or failure of simulated genesis.

To elucidate the kinematic aspects of genesis, the orientation of the shear with respect to the mean surface wind is varied in an array of numerical simulations. It will be shown that the role of shear orientation is as influential on genesis as the mean surface wind and SST. A detailed diagnosis of the simulated genesis process will be presented in addition to the sensitivity of genesis to the magnitude of the vertical shear.