Tropical Cyclogenesis in Wind Shear: Climatological Relationships and Physical Processes

The formation of tropical cyclones remains a topic of great interest in the field of tropical meteorology. A number of influential studies have considered the process of tropical cyclone formation (also known as TC genesis) from a pre-existing, weak tropical disturbance in a quiescent atmosphere from theoretical perspectives and using numerical simulations. However, it is shown that the large majority of TC genesis events occur under the influence of significant vertical wind shear. The effects of wind shear on TC genesis is explored from both a climatological perspective and from the statistics of wind shear in environments around individual TC genesis events. While earlier studies suggested that moderate wind shear values, in the range of 5 to 10 m/s, were the most favorable states for genesis, it is shown that small values of wind shear in the range of 1.25 to 5 m/s are the most favorable, and very little shear (less than 1.25 m/s) is not unfavorable. Statistically, easterly shear appears to be more favorable than westerly shear.

The physical process of TC genesis in wind shear is explored with high-resolution numerical simulations using a mesoscale model in an idealized framework. The transformation of a weak, mid-level vortex into a warm-cored tropical cyclone is simulated in environments with no flow, with mean flow and no wind shear, and with mean flow and wind shear. The simulations show that in terms of the formation of a closed, low-level circulation, moderate wind shear is indeed more conducive to genesis, but is also prohibitive to further development. However, in contrast to the statistical findings and some previous results, westerly shear is found to be significantly more favorable for TC genesis than easterly shear. The reasons for the greater favorableness of wind shear versus no wind shear, and of westerly shear versus easterly shear, are discussed within the context of the numerical simulations. Further statistical analysis suggests that the greater favorableness for easterly shear in the real atmosphere may be due to a correlation between easterly shear and more favorable thermodynamic conditions.