Kinematics of the Secondary Eyewall Observed in Hurricane Rita (2005)

Abstract

irborne Doppler radar data collected from the concentric eyewalls of Hurricane Rita (2005) provide detailed three-dimensional kinematic observations of the secondary eyewall feature. The secondary eyewall radar echo shows a ring of heavy precipitation containing embedded convective cells, which have no consistent orientation or radial location. The axisymmetric mean structure has a tangential wind maximum within the reflectivity maximum at 2-km altitude and an elevated distribution of its strongest winds on the radially outer edge. The corresponding vertical vorticity field contains a low-level maximum on the inside edge, which is part of a tube of increased vorticity that rises through the center of the reflectivity tower and into the midlevels. The secondary circulation consists of boundary layer inflow that radially overshoots the secondary eyewall. A portion of this inflowing air experiences convergence and supergradient forces that cause the air to rise and flow radially outward back into the center of the reflectivity tower. This mean updraft stretches and tilts the vorticity field to increase vorticity on the radially inner side of the tangential wind maximum. Radially outside this region, perturbation motions decrease the vorticity at a comparable rate. Thus, both mean and perturbation motions actively strengthen the wind maximum of the secondary eyewall. These features combine to give the secondary eyewall a structure different from the primary eyewall as it builds to become the new replacement eyewall.