Coherent Turbulence in the Boundary Layer of Hurricane Rita (2005) During an Eyewall Replacement Cycle

The structure of coherent turbulence in an eyewall replacement cycle in Hurricane Rita (2005) is presented from novel airborne Doppler radar observations using the Imaging Wind and Rain Airborne Profiler (IWRAP). The IWRAP measurements and three-dimensional (3D) wind vector calculations at a grid spacing of 250 m in the horizontal and 30 m in the vertical reveal the ubiquitous presence of organized turbulent eddies in the lower levels (~ 200 ­– 1500 m) of the storm. The data presented here, and the larger collection of IWRAP measurements, currently are the highest resolution Doppler radar, 3D wind vector calculations ever obtained in a hurricane over the open ocean. Coincident data from the NOAA WP-3D tail Doppler and lower fuselage radars, the stepped frequency microwave radiometer and flight level data help to place the IWRAP observations into context and provide independent validation.

The typical characteristics of the turbulent eddies are the following: radial wavelengths of ~ 1 – 3 km (average value of ~ 2 km), depths from the ocean surface up to flight level (~ 1.5 km), aspect ratio of ~ 1.3 and horizontal wind speed perturbations of 10 ­– 20 m s^-1. The tangential and vertical velocity perturbations are in-phase, indicating the presence of a convectively coupled wave feature. These characteristics have several similarities to boundary layer roll vortices, although the larger vertical extent and smaller aspect ratio described here are unique aspects.

The most intense eddy activity is located on the inner edge of the outer eyewall during the concentric eyewall stage with a shift to the inner eyewall during the merging stage. Analysis of tangential and radial wind vertical profiles shows that this shift is connected to the strongest vertical wind shear, for which the radial component has the largest values.