Examining Storm Asymmetries in Hurricane Irma (2017) Using Polarimetric Radar Observations

Dual-polarization radar observations of Hurricane Irma (2017) provide new insight into the microphysical structure of a mature tropical cyclone that can be tied to the cyclone dynamics. The primary eyewall exhibited a radar signature of hydrometeor size sorting, which implied that large drops fell out near persistent upward motion in the front-right quadrant of the storm, while smaller drops were advected downstream. In the outer rainbands, convective initiation was also preferred in the front-right quadrant, whereas stratiform precipitation was predominant downwind. For both the primary eyewall and outer rainbands, the preferred quadrant for convective initiation was consistent with the expected kinematic asymmetry of a tropical cyclone in weak environmental wind shear but with moderate translation speed. The developing secondary eyewall exhibited a different asymmetry that indicated a stratiform-to-convective transition associated with heavy precipitation in the rear quadrants. This transition is consistent with hypothesized dynamical theories for secondary eyewall formation.