Mobile Radar Observations of the Kinematics and Microphysics of Hurricanes Isaac (2013) and Hermine (2016)

The role of asymmetric dynamics (e.g., coherent eddy structures and vortex Rossby waves) in tropical cyclone (TC) intensification and weakening is a fundamental issue being addressed in the tropical cyclone community. Many observational studies have focused on asymmetric dynamics in an over-water environment with few studies focusing on the onshore transition of asymmetric convection in the inner core. Even fewer, if any, ground-based mobile radar observations have captured this process. While the overall intensity of the TC may be weakening during this period, deep convection induced by vortex asymmetries in the inner core environment plays a pivotal role in total surface rainfall. The intensity of the TC both during landfall and over the open ocean also relies on the distribution of convection, both symmetric and asymmetric.

Hurricane Isaac (2013) and Hurricane Hermine (2016) were two landfalling TCs in which the Shared Mobile Atmospheric Research and Teaching (SMART) radars gathered observations as both TCs made landfall on the Gulf Coast. Dual-Doppler wind retrievals were possible approximately every 10 minutes in Isaac and approximately every 6-7 minutes in Hermine. A two-hour period of dual-Doppler wind retrievals (0900-1100 UTC on 29 August 2012) will be shown for Isaac, and a two-hour period will be shown (0230-0430 UTC) for Hermine. Both of these periods were characterized by multiple rainbands (asymmetric convection) on the order of 20-60 km in length in the downshear half of the TCs. The kinematic evolution and forcing mechanism of these rainbands will be presented with particular attention paid to the onshore transition of coherent eddies and vortex Rossby waves. Initial results from Isaac indicate that azimuthally elongated vorticity maxima associated with rainbands become degraded by frictional influences of the land surface, but remain somewhat coherent aloft as they move farther inland.

In addition to the kinematics of Isaac and Hermine, microphysical observations will be discussed. In Hermine specifically, three Parsivel² disdrometers were placed approximately 10.5 km from the SR2 SMART radar. Drop size distributions recorded from the disdrometers and retrievals from the dual-polarimetric variables from SR2 will be related to the kinematic structure of the rainbands. Utilizing radar-measured kinematic and microphysical quantities, implications for enhanced rainfall as a result of asymmetric, small-scale, convective rainbands will be discussed.