Thermodynamic control of tropical cyclogenesis in environments of radiative-convective equilibrium

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 for a given vertical shear magnitude.

Based on our earlier work, a new genesis index has been developed. This index is evaluated by comparison to a genesis index proxy, or genesis proxy for short. The genesis proxy is 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. For a given shear profile, it is shown that there is a strong correlation between the new genesis index and the genesis proxy. Furthermore, both the genesis index and genesis proxy are strongly modulated by the mean surface wind while displaying just a weak dependence on SST.

This presentation will show that the orientation of the shear profile, for a given shear magnitude, may weaken the mean surface wind control of the genesis proxy, suggesting a need for the genesis index to incorporate shear orientation in addition to shear magnitude. Here, shear orientation refers to the relative orientation of the shear direction and the surface flow. Analysis of the high resolution simulations will be used to display the role of vertical shear orientation on idealized precursor disturbances in various climate states.