Applications of long-range lightning data to hurricane formation and intensification

Lightning locations from the ground-based National Lightning Detection Network have proven to be a useful indicator of convective outbreaks in incipient and mature hurricanes that are within 400-500 km of the U.S. coast. Such outbreaks often signal substantial deepening of the storms (Molinari et al. 1994, 2004; Demetriades and Holle 2005, 2006). Lightning distribution in tropical cyclones also gives insight into their evolving thermodynamic structure. Because major questions still remain about how tropical cyclones form, lightning information is potentially of greatest value, both for prediction and understanding of storms, during the early stages of development.

Most hurricane formation and early intensification occurs outside the range of the NLDN. A long-range detection network is now available (Murphy et al., 2006 (this conference)) that detects ground flashes 2000 km or more from the U.S. coast. The detection efficiency exceeds 60% in the northwest Caribbean, and exceeds 20% even at the far reaches of the eastern Caribbean. This dataset will be used to examine the lightning evolution during the early stages of Hurricane Claudette (2003) as it traversed the Caribbean Sea. Claudette displayed unprecedented behavior during this time. First, it unexpectedly developed from a disorganized tropical storm to a hurricane within six hours. Then it weakened just as quickly, returning to marginal tropical storm intensity. Neither event was predicted. After weakening, Claudette did not achieve hurricane intensity again until several days later, even though it remained over warm water.

The frequency and distribution of lightning flashes with respect to the storm center and to vertical wind shear have been examined in Claudette. More than 90% of flashes lay downshear of the center. On the first day of study (9 July 2001), fewer than 50 flashes per hour occurred within 100 km of the center. The storm did not change intensity during this time. Early on 10 July, a substantial outbreak produced more than 150 flashes in one hour, and the storm began to deepen. Between 0700 and 1000 UTC on the 10th, a second large outbreak occurred. The storm rapidly deepened during this time and reached hurricane intensity at 1200 UTC. Unlike in the case of Hurricane Danny (1997), there was no evidence in aircraft reconnaissance data of multiple centers and reformation within downshear convection. Rather, intensification occurred during a strong lightning outbreak within 100 km of the center of the existing storm. The results suggest that the initiation of deepening events can be detectable even at these great distances from the U.S. mainland using the long-range lightning data.

Claudette traveled a long distance without becoming a hurricane again until near landfall in Texas. Lightning will be examined with the long-range network during the entire life of the storm. Results will be presented at the conference. Possible real-time uses of lightning data will be discussed.