A Ventilation Index for Tropical Cyclones: Applications for Genesis and Intensification

An important environmental control of tropical cyclone intensity and genesis is vertical wind shear. One hypothesized pathway by which vertical shear affects tropical cyclones is midlevel ventilation – or the flux of low-entropy air into the center of the system. Based on a theoretical framework, a ventilation index is derived that is equal to the environmental vertical wind shear times the midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on tropical cyclone climatology. Tropical cyclogenesis preferentially occurs when and where the ventilation index is anomalously low. The distribution of tropical cyclone intensification is also strongly constrained by both the ventilation index and the tropical cyclone's normalized intensity, or the intensity divided by the potential intensity. The most rapidly intensifying storms are characterized by extremely low values of the ventilation index and intermediate normalized intensities, while weakening storms tend to be characterized by high values of the ventilation index and high normalized intensities. Since the ventilation index can be derived from large-scale fields, it can serve as a simple and useful metric for operational forecasts of tropical cyclones and a diagnostic in tropical cyclone modeling and observational studies.