The Convective Evolution and Rapid Intensification of Hurricane Earl (2010)

The relationship between an inner-core (r < 100 km) lightning outbreak and the subsequent rapid intensification (RI) of Hurricane Earl (2010) is examined using lightning strikes recorded by the World Wide Lightning Location Network (WWLLN) and in situ observations from various aircraft missions. Moderate (8.4 m s(-1)) northeasterly deep-layer (850-200 hPa) vertical wind shear, caused by outflow from Hurricane Danielle, existed over Earl at the beginning of a prolonged period of RI. Over 70% of the lightning strikes within a 500-km radius occurred downshear, with a preference toward downshear right in the outer rainbands, in agreement with previous studies. The location of inner-core strikes in Earl differed markedly from previous studies. The inner-core lightning activity precessed from left of shear to upshear, an extremely rare event, beginning just prior to the onset of RI. Diagnosis of the vortex tilt midway through the lightning precession showed this convection was occurring downtilt in the upshear-left quadrant; however, limited observations could not confirm if the vortex tilt was precessing with the lightning. Elevated values of low-level relative humidity and CAPE were also found upshear and supported the inner-core convection, which was found to occur within the radius of maximum wind (RMW). Previous studies have shown that convection located inside the RMW promotes intensification. It is hypothesized that intensification may have occurred in part because the vertical wind shear acted to reduce the upshear tilt, and the occurrence of convection inside the RMW helped to enhance the warm core.