Gross Moist Stability Evolution during Tropical Cyclogenesis

The gross moist stability (GMS), a measure of the net moist entropy export divided by a measure of the convective strength, is a useful metric for understanding the thermodynamic aspects of tropical cyclogenesis. An ensemble of axisymmetric model experiments with simplified physics is used to investigate the evolution of the radial structure of the GMS around tropical cyclogenesis. Before genesis, there is a broad region of negative GMS collocated with a mesoscale region of increasing column relative humidity. As genesis approaches, there exists an increasing positive radial gradient of the GMS. Around the location of the maximum in the radial gradient of the GMS, the radius of maximum wind of the meso-beta-scale protovortex emerges and amplifies. Thereafter, both the region of negative GMS and the positive radial gradient of the GMS contract inward.

A different ensemble of axisymmetric model experiments is run with a prescribed drying rate, which can be thought as a crude way of representing ventilation of dry air into the tropical cyclone. This added ventilation prevents genesis from occurring. While there is still a broad region of negative GMS, a sustained positive radial gradient of the GMS never develops. Hence, a sustained mesoscale region of negative GMS and a positive radial gradient of GMS that contracts inward appear to be necessary for tropical cyclogenesis.