Observations of Low level wind maxima using GPS dropsondes and their link to 200 Millibar Clues Identifying the Onset of Rapid Intensification

Observations made by GPS dropsondes of low level wind maxima (jets) in tropical cyclones, have provided forecasters and researchers a valuable new tool in mapping the evolution of rapidly intensifying storms. Atlantic Hurricanes (1999) Bret and Floyd both underwent rapid deepening episodes.

Bret had been exhibiting a broad pressure/wind field typical of a depression or the early stages of a tropical storm. The formation of a much more tightly structured field (where the RMW contracted from 60 NM to less than 5 NM in 12 hours, and an unusually small and well-defined eyewall formed while Bret was still not yet a hurricane) accompanied the onset of this deepening. Winds reached 61 knots at 340 meters with a central SLP of only 999 mb. During a subsequent burst of unusually intense deep convection (with over 60 dbZ of reflectivity measured by the Brownsville, TX NEXRAD in the eyewall where the reconnaissance crew was reporting significant lightning) the central SLP dropped to 952 mb. It was at this point that while 700 mb flight level winds had increased to only 117 knots, an eyewall sonde measured 155 knots at 487 meters above the surface. These were the strongest winds measured by a WC-130 in Bret at any time or altitude, even though the vortex would continue to deepen over the next 12 hours.

Floyd intensified rapidly as well, with the most dramatic phase of deepening also associated with a burst of deep convection and a sudden surge of winds in and just above the boundary layer. GPS eyewall sondes from WC-130 and NOAA WP-3 aircraft measured (at approximately 500 meters) 166 kts and 174 kts respectively. These strongest winds occurred while the convection was at its peak, but well prior to maximum eyewall contraction, and 12 hours prior to the lowest SLP being reached.

A preliminary examination of the data from these two hurricanes yields the following observations: The maximum amplitude of these low level jets near 500 meters occur prior to the lowest SLP being reached in the eye, their sudden appearance precedes more significant pressure falls, and their intensity is most clearly correlated to bursts of deep convection. The strongest winds tend to work their way down to the surface with time and reach the surface at about the same time as the lowest SLP occurs in the eye at the time of maximum eye contraction (when 700 mb flight level winds and surface winds are nearly identical, and the amplitude of the wind "spike" at 500 meters is diminished when compared to the time of maximum core convection). On another topic for further study: Changes in storm track, and the response of the cyclone to deep layer steering currents, appears to be tied to a reduction in the intensity of these convective bursts and their accompanying low level jet winds.