Hurricane feedback research may improve intensity forecasts
Forecasts of a hurricane's intensity are generally much less accurate than forecasts of its most likely path. Large-scale atmospheric patterns dictate where a hurricane will go and how quickly it will get there. The storm's intensity, however, depends on small-scale shifts in atmospheric stratification, upwelling rates, and other transient dynamics that are difficult to predict. Properly understanding the risk posed by an impending storm depends on having a firm grasp on all three properties: the translational speed, intensity, and path. Drawing on 40 years of hurricane records representing 3090 different storms, Mei et al. (2012) propose that a hurricane's translational speed and intensity may be closely linked. To maintain its intensity, a hurricane depends on a persistent source of warm water from which it can draw energy. Strong winds, however, drive turbulence in the surface ocean, bringing cold water up from below that can disrupt this flow of energy. The amount of surface mixing increases with the storm's intensity but decreases with the storm's translational velocity because of the shorter duration spent over a given location. On the basis of this cold water feedback system, the authors suggest that a fast-moving hurricane has a high potential of becoming very intense. The authors further suggest that for a hurricane of a given intensity, there is a minimum translational speed above which it can sustain or increase in intensity and below which it will begin to wane. For instance, the authors found that if a category 4 hurricane (surface winds from 209 to 251 km h-1) were to intensify into a category 5 hurricane (winds above 252 km h-1), it must be moving at least 3.4 m s-1. This realization could improve the predictability of hurricane intensity, as it would allow already well-estimated features to be used as the basis for intensity projections.