Hurricane anti-fuel

NCAR scientists chart role of ventilation in weakening tropical cyclones

Image showing the impact of ventilation on hurricane intensity

A healthy, intense hurricane resists intrusions of dry, cool air (top image, brown shading) from the environment into the moist core of deep convection (green, denoting showers and thunderstorms). However, under the influence of wind shear (top right, gray arrows), where the winds steering the hurricane change with height, the deep convection is displaced from the center of the storm and dry, cool air can find a path into the inner core. A vertical cross section through the storm (bottom image, A to B) illustrates how dry, cool air undercuts the convection, gets rained into, and produces downdrafts, which serve as anti-fuel for the hurricane. Click here to enlarge. (Image courtesy Brian Tang.)

With another tropical storm season well under way, scientists continue to investigate the ways in which nature nourishes or mitigates hurricanes. NCAR scientists are among those analyzing the peculiarities of storm intensity.

Meteorologists have hypothesized that ventilation, the injection of cooler and drier air into the core of a tropical cyclone, can significantly lessen the cyclone's intensity. On a quest to further understand this phenomenon, NCAR scientist Brian Tang and colleague Kerry Emanuel (Massachusetts Institute of Technology) used a hurricane model to determine how much ventilation is needed to weaken a tropical cyclone and where this is most likely to happen within it.

Previous research found that strong ventilation cools the upper-level warm core of a tropical cyclone, subsequently leading to a decrease in intensity. Tang and Emanuel’s research reveals a much greater decrease in intensity when the ventilation occurs at the middle to lower levels of a cyclone.

Ventilation needs to be strong in order to noticeably decrease the cyclone's intensity. Once it reaches a certain strength, the effects become quite debilitating, reducing the cyclone's wind speed and disorganizing its structure. The scientists’ research also supports the existence of a threshold amount of ventilation beyond which a tropical cyclone cannot exist.

"I was unsure if I could detect the fingerprints of ventilation in the actual hurricane intensity data. It turns out the signal is quite strong," says Tang. 

Building on the results of his modeling, Tang and colleagues are now working on the creation of a ventilation index, which puts the findings into practice. Developed from statistics of storm intensity, the index serves as a simple and useful metric for operational forecasts of tropical cyclones. The team is also developing ventilation diagnostics for use in real-time forecasts.

B. Tang, K. Emanuel, "Sensitivity of Tropical Cyclone Intensity to Ventilation in an Axisymmetric Model," Journal of Atmospheric Science, 2012: in press DOI:10.1175/JAS-D-11-0232.1