Frequency of collapsing precipitation cores within the eyewalls of U.S. landfalling hurricanes (1994-2007)

An introductory two-dimensional glimpse into collapsing precipitation cores in a couple of recent hurricanes (Ivan and Katrina) was presented by Holmes et al. (2006). The current study continues that investigation, whereby all U.S. landfalling hurricanes between 1994 and 2007 are investigated for the presence of collapsing cores within their eyewalls. The study uses temporal sequences of two- and three-dimensional depictions of archived WSR-88D Doppler Level II digital radar data to identify these cores, document their life-cycle and frequency of occurrence, and determine any potential surface impacts.

Collapsing cores of heavy precipitation appear to be prevalent in many tropical cyclones, but their frequency of occurrence has yet to be documented. Preliminary investigation of radar data (Holmes et al., 2006) indicate that some “open-eyewall” storms repeatedly display large-diameter intense elevated cores of precipitation within their eyewalls which subsequently collapse toward the surface. It is unknown if these cores routinely occur in closed-eyewall storms. A climatology of such cores is being developed from all hurricanes making landfall in the U.S. within fairly close proximity of coastal WSR-88D radars over the past 14 seasons.

Over the last decade, GPS dropsondes (Franklin et al. 2003) and coastal Doppler radars (Blackwell 2000) have observed low-level eyewall wind maxima, often extending down to elevations at or below 500 meters above the surface. In hurricanes, convective 3-s wind gusts may approach values twice that of the sustained wind (Powell et al., 2003). Fujita, Parrish et al., (1982) and Powell et al., (1991) suggest that many of these extreme convective winds in hurricanes are associated with thunderstorm downdrafts. Also, Powell and Houston (1996) 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 in hurricanes.

Willoughby and Black (1996) indicate that heavy convective rain can generate precipitation-induced downdrafts which inject high-velocity air from the free atmosphere into the frictional boundary layer and that the surface wind accelerates even more as the "downburst" spreads along the ground. In Hurricane Andrew, they indicate that the most severe damage lay in streaks along the downwind of the convective cells' trajectories around the eye where the downbursts may have caused 20 m/s surges in wind speed on a >60 m/s basic flow (Wakimoto and Black, 1994).

The atmosphere in a hurricane often contains huge quantities of moisture and convective precipitation in this tropical cyclone may produce some of the heaviest rainfall events on earth. Often, hurricane cumulonimbus convection contains the greatest concentrations of precipitation at much lower altitudes than is the case in mid-latitude convective storms. Also in hurricanes, “thunderstorm-like” convection associated with heavy rain in cumulonimbus clouds may or may not contain lightning (Medlin et al., 2007).

A hurricane contains rapidly-moving cumulonimbus convection. In addition, the hurricane exhibits strong vertical wind shear in its lower near-surface layers which is often nearly unidirectional in nature. The hurricane eyewall contains extremely fast-moving convective storms embedded within the tremendous momentum of the wind aloft. Orf and Anderson (1999) show that traveling microbursts which occur in unidirectionally sheared environments produce dynamic features that are not found in stationary microbursts. As the speed of the microburst-producing storm increases, the magnitude of the surface winds increase in the direction of storm motion. Their results indicate that the magnitude of the damaging surface winds of a microburst can be enhanced significantly when the parent cloud is moving in a unidirectionally sheared environment.

This study investigates the link between collapsing precipitation cores within landfalling hurricanes, detectable from nearby Doppler radars, and extreme near-surface wind gusts and possible mini-vortex spin-up. Because radar cannot see the wind near the ground due to earth curvature limitations, radar data is being coupled with surface-based weather station observations from hurricane landfall events and with surface damage patterns to determine the effect of these downburst-like features on the surface wind field and to determine if, in fact, they are associated with extreme surface wind gusts. Mechanisms for the development of collapsing cores within hurricane eyewalls are currently being explored, along with processes that may initiate and sustain the downdraft's negative acceleration. These downward-directed accelerations through the eyewall's maximum wind zone at the top of the boundary layer may bring significant wind momentum into close proximity with the surface and may pose an additional hazard during hurricane landfall events.