Cloud clusters and tropical cyclogenesis: Developing and nondeveloping systems and their large-scale environment

Abstract

Tropical cyclone (TC) genesis occurs only when there is persistent, organized convection. The question of why some cloud clusters develop into a TC and others do not remains unresolved. This question cannot be addressed adequately without studying nondeveloping systems in a consistent manner together with developing systems. This study presents a systematic approach in classifying developing and nondeveloping cloud clusters based on their large-scale environments. Eight years of hourly satellite IR data and global model analysis over the western North Pacific are used.A cloud cluster is defined as an area of≤ 208-K cloud-top temperature, generally mesoscale in size. Based on the overlapping area between successive hourly images, they are then tracked in time as time clusters. The initial formations of nearly all TCs during July-October 2003-10 were associated with time clusters lasting at least 8 h (8-h clusters). The occurrence of an 8-h cluster is considered to indicate the minimum degree of convective organization needed for TC genesis. A nondeveloping system is defined as an 8-h cluster that is considered to be a viable candidate for TC genesis, but was not associated with the TC genesis. The large-scale environmental conditions of cyclonic low-level vorticity, low vertical wind shear, low-level convergence, and elevated tropospheric water vapor are statistically more favorable for developing systems. Generally, the environment became more (less) favorable with time for the developing (nondeveloping) systems. Nevertheless, many developing (nondeveloping) systems formed (dissipated) in seemingly unfavorable (favorable) environments within a lead time of < 24 h. © 2013 American Meteorological Society.