A Ventilation Index for Tropical Cyclones

A number of environmental factors control both tropical cyclogenesis and tropical cyclone (TC) intensity, contributing to the challenge of TC prediction. Among these environmental controls is the interaction of TCs with environmental vertical wind shear. Since environmental vertical wind shear is always present in varying amounts during the life of any TC, it is important to understand how wind shear affects TCs to improve genesis and intensity forecasts. Environmental vertical wind shear is observed to be generally detrimental to tropical cyclogenesis (McBride and Zehr 1981; Zehr 1992), making wind shear an important component of empirical genesis indices (Gray 1979; DeMaria et al. 2001; Emanuel and Nolan 2004). In addition to observations, numerical modeling studies find that sufficiently strong vertical wind shear impedes the development of incipient vortices (Tory et al. 2007; Nolan and Rappin 2008). However, weak shear may aid genesis by forcing synoptic-scale ascent, especially in baroclinic environments (Bracken and Bosart 2000; Davis and Bosart 2006; Nolan and McGauley 2012). Vertical wind shear may discourage genesis by ventilating the incipient disturbance with low-entropy (low-equivalent potential temperature) air (Simpson and Riehl 1958). Gray (1968) hypothesized that advection by the environmental flow removes the “condensation heat” from a vortex, preventing it from deepening. The environmental flow also advects dry air into the disturbance, disrupting the formation of a deep, moist column that is postulated to be imperative for genesis (Emanuel 1989; Bister and Emanuel 1997; Nolan 2007). In addition to tropical cyclogenesis, environmental vertical wind shear generally impedes TC intensification. Vertical wind shear is strongly and negatively correlated with intensity changes at various time intervals (DeMaria 1996; Gallina and Velden 2002) and reduces the maximum intensity that a TC can achieve (Rappaport et al. 2010). Thus, vertical