Collapsing precipitation cores in open-eyewall hurricanes at landfall: Are these cores actually downbursts associated with extreme surface wind gusts?

Hurricanes produce strong winds at landfall, particularly within their eyewalls. Over the last several years, GPS dropsonde data and coastal Doppler radars have indicated low level wind maxima, often extending down to elevations at or below 500 m elevation. Observations of surface convective wind gusts in hurricanes are extremely rare because of power outages and anemometer failures. Convective 3-s gusts may approach values twice that of the sustained wind. Earlier studies suggest that many of these extreme convective winds are associated with convective downdrafts. Also some studies indicate that strong horizontal shear along the lateral edge of the downdraft as it spreads along the ground may develop small vortices and extreme winds.

Collapsing cores of heavy precipitation appear to be prevalent in many tropical cyclones. Indeed, preliminary investigation of radar data indicate that “open-eyewall” storms repeatedly display large intense elevated cores of precipitation within their eyewalls which subsequently collapse toward the surface. This type of behavior appears to be less common in closed-eyewall storms. Earlier studies show that collapsing precipitation cores are often associated with downbursts; the maximum outflow velocity in microbursts (i.e., small downbursts) occurs at approximately 250 feet AGL. These downburst winds, when superimposed on the already strong hurricane wind field, can produce localized regions of extreme winds and damage. It is hypothesized that collapsing precipitation cores within landfalling hurricanes, detectable from nearby Doppler radars, may be associated with extreme near-surface wind gusts and possible mini-vortex spin-up (i.e., miniswirls?). These collapsing cores are indicative of downbursts and should theoretically be associated with extreme wind gusts in landfalling hurricanes.

These downburst phenomena are only evident through careful and methodical use of high-resolution Doppler radar data, such as the WSR-88D Doppler radar Level II information, but radar can only detect the wind near the ground close to the radar due to earth curvature limitations, thus radar data must be coupled with surface-based weather station observations during hurricane landfall events or with surface damage patterns to determine the effect of these downburst on the surface wind field and to determine if, in fact, they are associated with extreme surface wind gusts.

Additionally in this study, a numerical model, configured specifically for hurricane research, may be used to provide clues to the source of some of the surface gusts associated with these collapsing cores. For example, dry air intrusion into the hurricane several thousand feet above the surface needs to be investigated for its role in possible generation and enhancement of downbursts and extreme surface wind gusts in hurricanes. Evaporative cooling within dry air could significantly enhance the strength of thunderstorm downdrafts (downbursts), and therefore the resulting surface winds may be stronger than with just precipitation loading only. Numerical modeling will help evaluate the potential for dry air entrainment into some of these landfalling storms and the possible enhancement of downbursts.