Extreme Updrafts and Wind Speeds Measured by Dropsondes in Tropical Cyclones

Abstract:

A few previous studies have found surprisingly strong vertical motions in low levels of some tropical cyclones. In this study, all available dropsondes (~12,000) within tropical cyclones from 1997-2013 are examined, in order to create a dataset of the most extreme updrafts (≥ 10 m/s; 169 sondes) and wind speeds (≥ 90 m/s; 64 sondes). It is shown that extreme low-level (0-3 km) updrafts are ubiquitous within intense (Category 4- and -5) tropical cyclones, and that few such updrafts have been observed within weaker storms. These extreme updrafts, which are almost exclusively found within the eyewall just inwards of the radius of maximum winds, sometimes occur in close association with extreme horizontal wind speeds. Consistent with previous studies, it is suggested that both the extremes in vertical velocity and wind speed are associated with small-scale (~1 km) vortices that exist along the eye/eyewall interface. As a substantial number of updrafts are found within a kilometer of the surface, it can be shown that it is implausible for buoyancy to be the primary mechanism for vertical acceleration. The azimuthal distribution of both the extreme updrafts and wind speeds is strongly controlled by the orientation of the vertical wind shear. Some previous studies suggest that extreme near-surface windspeeds should tend to occur within downdrafts. In contrast, the dropsondes indicate that peak windspeeds are found to occur in the presence of an updraft or downdraft with roughly equal frequency.