The thermodynamic evolution of extratropically transitioning tropical cyclones

One of the defining characteristics of the extratropical transition of tropical cyclones is the transition of the warm core thermal structure associated with the tropical cyclone into an initially cold core thermal structure associated with the extratropical cyclone. Despite this being a defining characteristic of the extratropical transition process, the literature expresses no consensus or agreement upon or a quantification and physical description of the factors that explicitly cause this transition to occur. Understanding this evolution is important in order to better forecast and describe the evolution of physical features within the cyclone such as its four-dimensional wind field structure and to begin to quantify the contributors to the poleward transport of heat energy associated with the transitioning cyclone and its impacts upon hemisphere weather patterns and model predictability.

This work employs a suite of high-resolution numerical simulations in order to quantify and physically describe the evolution of the thermodynamic structure associated with a typical extratropical transition case, North Atlantic Tropical Cyclone Bonnie of 1998. Thermodynamic budgets native to the numerical model's primitive equation set and physical parameterizations are computed during the transition phase of the cyclone within a four-dimensional analysis framework. An array of analyses detailing the spatial, temporal, and azimuthally-averaged thermodynamic evolution are developed and analyzed for insight into the total thermodynamic evolution. The observed warm-to-cold thermal profile evolution is found to arise out of an imbalance between dynamical cooling and parameterized diabatic warming contributions. This dynamical cooling, as influenced by horizontal advection and adiabatic cooling processes, is of increasingly greater magnitude than warming associated with latent heat release due to condensation and deposition processes within the transitioning cyclone's delta rain region.

Connections are drawn between the extratropical transition of Bonnie as well as four additional simulated cases: a purely tropical cyclone, a purely extratropical cyclone, and two other extratropical transition events, one that acquires a cold thermal structure and weakens (similar to Bonnie) and one that acquires a warm seclusion thermal structure and becomes a powerful extratropical cyclone (unlike Bonnie). In addition, partitioning of the dynamical and diabatic components to the simulated thermodynamic evolution with respect to the effects of vertical wind shear upon the transitioning cyclone is performed and detailed within this work. Extensions to the larger issues of numerical predictability after the transition event as well as to the role(s) of tropical cyclones in climate and global energy balance are discussed in conjunction with the findings.