A Real-time System for Evaluating the Ventilation of Tropical Cyclones

Ventilation is an environmental factor that controls both tropical cyclone intensity and tropical cyclogenesis. This paper presents a real-time system for evaluating the ventilation index of tropical cyclones and their precursors. The ventilation index is a nonlinear combination of the deep-layer vertical wind shear, the midlevel entropy deficit, and the potential intensity.

Examples from the past year are given in order to demonstrate how one may use the ventilation index in an operational setting. Analyses and forecasts of the ventilation are used to identify air masses associated with unfavorable and favorable areas for genesis. For a given tropical disturbance, a probability of genesis can be obtained given the climatological formation rates of disturbances in similar magnitudes of ventilation.

For tropical cyclones, analyses and forecasts of the ventilation index and normalized intensity provide guidance on the expected intensity changes associated with ventilation. This can be done in both a deterministic and probabilistic sense depending on the model input. The real-time system also gives probabilities of rapid intensification and rapid weakening in the joint ventilation index and normalized intensity parameter space.