Investigation of Lightning Structure during the Rapid Intensification of Hurricane Earl (2010)

A clear relationship between lightning activity and tropical cyclone (TC) intensity change has not been established. Some suggest an inner core (< 100 km) lightning outbreak signals a weakening TC, while others argue it could intensify the TC. This study investigates Hurricane Earl (2010), a case with an inner core lightning outbreak that preceded intensification. Earl is an ideal case study because it rapidly intensified, the World Wide Lightning Location Network (WWLLN) recorded a remarkable number of strikes, and the NASA Genesis and Rapid Intensification Processes (GRIP) mission collected valuable additional observations.

Lightning peaked within 500 km of the center of Earl during a period of rapid intensification (RI) from 0600 UTC 29 August to 0000 UTC 31 August. This period of RI was preceded by an inner core lightning burst upshear of the center. Moderate northeasterly deep-layer vertical wind shear existed over Earl at the beginning of RI resulting from the strong outflow of Hurricane Danielle. More than 70% of the lightning strikes within a 500 km radius occurred downshear, with a preference towards downshear right in the outer rainbands. This agrees with previous studies, but the inner core lightning distribution does not: a majority of the inner core strikes occurred upshear.

Observations indicate Earl's vortex was tilted upshear at the time of the inner core lightning outbreak. While this tilt direction is consistent with the observed lightning, it opposes work that found downshear left to be a stable configuration for reducing tilt. Thermodynamic and kinematic data collected during GRIP will be used to investigate the unusual upshear tilt and inner core lightning burst.