87th AMS Annual Meeting

Wednesday, 17 January 2007
The Morphology of Eyewall Lightning Outbreaks in two Category Five Hurricanes
212B (Henry B. Gonzalez Convention Center)
Kirt A. Squires, University of Hawaii, Honolulu, HI; and S. Businger
The wave guide between the Ionosphere and the Earth's surface allows very long frequency (VLF) noise generated by lightning between 5 and 15 kHz to propagate over very long distances. Data collected by sensors that are part of the National Lightning Data Network (NLDN) can be processed to obtain strike locations at long range. The resulting long-range lightning data set has been used here, along with TRMM and reconnaissance aircraft data, to analyze the morphology of lightning outbreaks in the eyewalls of hurricanes Rita and Katrina. The eyewalls of both hurricanes are shown to have unusually high lightning flash densities when compared to historical storms, with the greatest hourly flash density occurring during periods of rapid intensification.

The lightning flash density displays a consistent relationship with the TRMM Precipitation Ice Concentration (PIC) product. Large lightning flash rates are reliably associated with large PIC. Both storms exhibit secondary eyewall lightning outbreaks surrounding the time of maximum intensity. The period of elevated flash density for each storm lasted for 8 to 9 h centered on the time of minimum sea- level pressure. These prolonged outbreaks occurred at a time when each hurricane eye had contracted to a relatively small size and the eyewall was steep sided.

Maxima in eyewall flash density were collocated with maxima in flight-level radar reflectivity. However, flash density maxima and radar reflectivity maxima are not always collocated with minimums in brightness temperatures and high PIC. Observed spatial displacement between the location of lightning and PIC maxima can be attributed to vertical shear of the horizontal winds over the area of convection. It was observed that PIC maxima are located up-flow of convective cores that rotate cyclonically around the storm center, and down flow of convective cores that are stationary relative to the storm center. The greatest hourly flash density was not produced by the deepest convection during each storm.

The relationship of the eyewall lightning outbreaks to the evolution of the eyewall convection and the morphology of the electro-magnetic dipole will be discussed. Implications of the results for better understanding hurricane evolution through the application of long- range lightning data will be presented.

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