Tropical cyclogenesis in wind shear: Climatological relationships and physical processes

David S. Nolan, Michael G. McGauley

Research output: Chapter in Book/Report/Conference proceedingChapter

31 Scopus citations

Abstract

The formation of tropical cyclones remains a topic of great interest in the field oftropical meteorology. A number of influential studies have considered the process oftropical cyclone formation (also known as TC genesis) from a pre-existing, weak tropicaldisturbance in a quiescent atmosphere from theoretical perspectives and using numericalsimulations. However, it is shown that the large majority of TC genesis events occurunder the influence of significant vertical wind shear. The effects of wind shear on TCgenesis is explored from both a climatological perspective and from the statistics of windshear in environments around individual TC genesis events. While earlier studiessuggested that moderate wind shear values, in the range of 5 to 10 ms-1, were the mostfavorable states for genesis, it is shown that small values of wind shear in the range of1.25 to 5 ms-1are the most favorable, and very little shear (less than 1.25 ms-1) is notunfavorable. Statistically, easterly shear appears to be more favorable than westerlyshear.The physical process of TC genesis in wind shear is explored with high-resolutionnumerical simulations using a mesoscale model in an idealized framework. Thetransformation of a weak, mid-level vortex into a warm-cored tropical cyclone issimulated in environments with no flow, with mean flow and no wind shear, and withmean flow and wind shear. The simulations show that in terms of the formation of aclosed, low-level circulation, moderate wind shear is indeed more conducive to genesis,but is also prohibitive to further development. However, in contrast to the statisticalfindings and some previous results, westerly shear is found to be significantly morefavorable 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, arediscussed within the context of the numerical simulations. Further statistical analysissuggests that the greater favorableness for easterly shear in the real atmosphere may bedue to a correlation between easterly shear and more favorable thermodynamicconditions.

Original languageEnglish (US)
Title of host publicationCyclones
Subtitle of host publicationFormation, Triggers and Control
PublisherNova Science Publishers, Inc.
Pages1-36
Number of pages36
ISBN (Print)9781619429765
StatePublished - Oct 1 2012

ASJC Scopus subject areas

  • Earth and Planetary Sciences(all)

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    Nolan, D. S., & McGauley, M. G. (2012). Tropical cyclogenesis in wind shear: Climatological relationships and physical processes. In Cyclones: Formation, Triggers and Control (pp. 1-36). Nova Science Publishers, Inc..