A Coupled Moist Entropy and Angular Momentum Framework for Tropical Cyclogenesis Investigations

A framework for studying tropical cyclogenesis is introduced. The framework is based off the coupled relationship between the moist entropy and angular momentum fields in describing the vortex development and structure. Fluxes by the time-dependent secondary circulation and boundary sources/sinks of moist entropy and angular momentum are key processes in this framework. The framework provides a comprehensive and powerful diagnostic tool for studying the relative roles of these processes during tropical cyclogenesis.

Using the framework, tropical cyclogenesis processes in an axisymmetric model are investigated. Two experiment ensembles are presented. One experiment is initialized with the Dunion (2011) moist tropical (MT) sounding, and the other experiment is initialized with the Dunion (2011) Saharan Air Layer (SAL) sounding. Ensemble members are created by adding small amplitude white noise to the low-level moisture.

For both experiments and across all ensemble members, the beginning of a sustained increase in the low-level kinetic energy tends to be coincident with a sufficiently negative time tendency of the ratio of Δs and ΔM, where Δs and ΔM are the bulk moist entropy and angular momentum differences between an “inner” and “outer” region. This period effectively marks the formation of a deep warm core structure. Advective fluxes of moist entropy and angular momentum dominate over surface fluxes during this period. The results suggest that tropical cyclogenesis is associated with both a negative gross moist stability in the inner region and a positive gradient of gross moist stability with radius. Additionally, the role of wind induced surface heat exchange appears to be minimal during tropical cyclogenesis.