A multi-scale analysis of tropical cyclogenesis within the critical layer of tropical easterly waves in the Atlantic and western North Pacific sectors

Abstract

A newly proposed tropical cyclogenesis sequence that describes the transition of a tropical wave's critical layer to a tropical cyclone is used to examine two formation cases in the western North Pacific basin. Typhoon Nuri (2008), formed from a precursor easterly wave during the Tropical Cyclone Structure 2008 field experiment, and Typhoon Man-yi (2007), formed within an equatorial Rossby wave as it interacted with a monsoon trough. In each case, i) the critical layer of the parent wave protects a proto-vortex from an external hostile environment and allows it to strengthen until it becomes a self-sustained entity and ii) the intersection between the wave trough and critical latitude, within the Kelvin cat's eye, is the preferred location for tropical cyclogenesis. Numerical simulations suggest that the so-called "bottom-up" pathway to tropical cyclogenesis is favored within Typhoon Man-yi's critical layer. Additionally, Tropical Rainfall Measurement Mission (TRMM) composite analyses of 55 developing easterly waves indicate that as genesis approaches, i) convection is favored in the Kelvin cat's eye circulation, ii) the convective contribution to total rain rate becomes dominant, iii) the radius of maximum convection decreases, and iv) a convective-type heating profile is present. These findings support the "bottom-up" development model within easterly wave critical layers.