The importance of low deformation vorticity in tropical cyclone formation

Studies of tropical cyclone formation from tropical waves have shown that cyclone formation requires a wave-relative closed circulation: the “marsupial pouch” concept. This results in a non-ventilated region of atmosphere in which the modification of moisture, temperature and vorticity profiles by convective and boundary layer processes occurs undisturbed. The pouch concept is further developed in this paper. Near the centre of closed circulations in which TCs form regions of near solid body rotation are observed. A reference-frame independent parameter is introduced to measure the level of solid-body rotation of the lower troposphere. The parameter is referred to as low deformation vorticity, LDV.

Using 20 years of ERA-interim reanalysis data and the IBTrACS global TC database, it is shown 95% of tropical cyclones are associated with enhanced levels of LDV on both the 850 and 500 hPa pressure levels at the time of TC declaration, while 90% show enhanced LDV for at least 24 hours prior to declaration. This result prompts the question of whether the pouch concept extends beyond wave-type formation to all tropical cyclone formations world-wide.

Combining the LDV with a low vertical shear requirement and lower troposphere relative humidity thresholds, an imminent genesis parameter is defined. The parameter includes only relatively large-scale fluid properties that are resolved by coarse grid model data, which gives rise to two useful applications. The first is a cyclogenesis diagnostic for use in real-time global forecast models. The second is a tropical cyclone detector for climate model applications.