Coupled Dynamic–Thermodynamic Forcings during Tropical Cyclogenesis. Part I: Diagnostic Framework

A diagnostic framework to investigate the role of processes around and during tropical cyclogenesis is presented. The key framework metric is the ratio of bulk differences of moist entropy over differences of angular momentum between an inner and outer region of a tropical disturbance or cyclone. This ratio is hypothesized to decrease and become negative as both the high-entropy core and low-level vortex in the inner region amplify during tropical cyclogenesis. The time tendency of this ratio can be split into two forcings: a moist entropy forcing and an angular momentum forcing. Each forcing can be further divided into components comprising differences in net advective fluxes and nonadvective boundary fluxes of moist entropy or angular momentum between each region. The framework provides a comprehensive way to compare the relative importance of processes leading to tropical cyclogenesis in a tractable, consistent manner. Suggestions on how to apply the framework to numerical model output are given.