Distinguishing Tropical Cyclone Extreme Rapid Intensification from Ordinary Rapid Intensification

Rapid intensification (RI) is widely defined as a 30 kt or greater increase in the maximum 1-minute 10-meter wind speed in 24 hours. This 30 kt threshold was chosen because it is approximately the 95th percentile of over-water 24-h intensity change for tropical cyclones (TCs) in the Atlantic basin. Although this is certainly a useful metric for both forecasting and emergency preparedness, RI is not actually rare on a per storm basis, as about a third of all TCs undergo RI at some point in their lifetime, and when excluding short-lived (2 days or less) TCs, nearly half undergo RI. It is the goal of this study to gain insight into the factors which determine whether or not a TC is able to undergo “extreme RI”, which we define as at least 60 kt per 24 h. Most commonly, the peak intensification rate for an RI storm is exactly 30 kt per 24 h, and 30-35 kt per 24 h make up half of all cases, whereas only 10% of RI storms (4% of all TCs) experience extreme RI. Here, we explore whether there are meaningful differences that distinguish this small set of TCs that experience extreme RI from the much more common cases of “ordinary RI”.

Extreme RI storms occur about twice as often in the East Pacific compared to the Atlantic basin, and in the Atlantic are largely confined to the Main Development Region and the Caribbean Sea. The duration of RI events is related to the peak rate of intensification, as ordinary RI events most commonly last for only 24 h, whereas extreme RI events typically are 36-54 h in duration. Although the peak 24-h intensification rate occurs at a greater intensity (50-75 kt) for extreme RI events, the onset of RI is actually the same (30-35 kt) on average for both extreme and ordinary RI storms. The typical RI storm intensifies from 30 to 60 kt in 24 h, and then the extreme cases continue RI at an amplifying rate of intensification, while the ordinary cases often stop intensifying at this point (or intensify more slowly). It appears that environmental vertical wind shear and potential intensity (and the length of time that these remain favorable) are the most important factors that distinguish extreme RI from ordinary RI, as the shear is about twice as large on average for the ordinary RI storms, and many ordinary RI storms experience sharply declining potential intensity at the end of their intensification. There is also some evidence that the size of the RMW plays a role, although the quality of existing observations may not be sufficient to separate any effect from that of intensity itself. In addition to our observational analysis, we examine a set of idealized simulations to further explore the influence of environmental and internal factors on intensification rate during RI.