Sensitivity of Tropical Cyclone Intensity to Ventilation in an Axisymmetric Model

Abstract

The sensitivity of tropical cyclone intensity to ventilation of cooler, drier air into the inner core is examined using an axisymmetric tropical cyclone model with parameterized ventilation. Sufficiently strong ventilation induces cooling of the upper-level warm core, a shift in the secondary circulation radially outward, and a decrease in the simulated intensity. Increasing the strength of the ventilation and placing the ventilation at middle to lower levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer.The sensitivity of tropical cyclone intensity to ventilation can be viewed in the context of the mechanical efficiency of the inner core or a modified thermal wind relation. In the former, ventilation decreases the mechanical efficiency, as the generation of available potential energy is wasted by entropy mixing above the boundary layer. In the latter, ventilation weakens the eyewall entropy front, resulting in a decrease in the intensity by thermal wind arguments.The experiments also support the existence of a threshold ventilation beyond which a tropical cyclone cannot be maintained. Downdrafts overwhelm surface fluxes, leading to a precipitous drop in intensity and a severe degradation of structure in such a scenario. For a given amount of ventilation below the threshold, there exists a minimum initial intensity necessary for intensification to the quasi-steady intensity.

Dynamical system properties of an axisymmetric convective tropical cyclone model

“…a bifurcation. Further evidence for the existence of such a threshold is given in their follow-up study (Tang and Emanuel, 2012), where they show that the bifurcation is also visible in a non-hydrostatic axisymmetric convective model. Emanuel (1989) demonstrated with a simplified axisymmetric model that the import of low-entropy air into the boundary layer by shallow clouds or precipitation-induced downdrafts can provide reasons for the finite amplitude nature of tropical cyclogenesis.…”

“…Similar models have been developed and introduced in several previous studies [e.g. Willoughby et al (1984), Rotunno and Emanuel (1987), Bryan and Fritsch (2002), Tang and Emanuel (2012)]. All quantities are computed on a staggered Arakawa-C-type grid as in Rotunno and Emanuel (1987).…”

Extratropical Impacts on Atlantic Tropical Cyclone Activity

“…The combination of strong VWS and dry air can effectively hinder TC formation and intensification (e.g., Tang and Emanuel 2012;Fritz and Wang 2013;Ge et al 2013).…”

Evaluation of a Heuristic Model for Tropical Cyclone Resilience

“…The impact of environmental vertical wind shear on tropical cyclone (TC) structure and intensity depends on the TC's dynamic resilience (Jones 1995;Reasor et al 2004, hereafter RMG04) and interaction with environmental dry air (Simpson and Riehl 1958;Kimball 2006;Riemer and Montgomery 2011), both of which influence the organization of convection and, ultimately, operation of the TC power machine (Emanuel et al 2004;Riemer et al 2010Riemer et al , 2013Tang andEmanuel 2010, 2012).…”