PICTURE OF THE MONTH Observed Inner-Core Structural Variability in Hurricane

Hurricane Dolly (2008) exhibited dramatic inner-core structural variability during a 6-h rapid intensification and deepening event just prior to making landfall in southern Texas at 1800 UTC 23 July. In particular, the eyewall was highly asymmetric from 0634–1243 UTC, with azimuthal wavenumber m 5 4–7 patterns in the eyewall radar reflectivity and prominent mesovortex and polygonal eyewall signatures. Evidence is presented that the most likely cause of the high-wavenumber asymmetries is a convectively modified form of barotropic instability of the thin eyewall potential vorticity ring. The rapid intensification and deepening event occurred while Dolly was in a favorable environment with weak deep-layer vertical wind shear and warm sea surface temperatures; however, the environmental conditions were becoming less favorable during the period of rapid intensification. Therefore, it is plausible that the internal vortex dynamics were dominant contributors to the rapid intensification and deepening.