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

Abstract

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 of the storm. The data presented here, and the larger collection of IWRAP measurements, currently are the highest-resolution Doppler radar 3D wind vectors ever obtained in a hurricane over the open ocean. Coincident data from NOAA airborne 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 (mean value is ~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 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. The evolving structure of the vertical wind shear is connected to this shift and together these characteristics have several similarities to boundary layer roll vortices. However, eddy momentum flux analysis reveals that high-momentum air is being transported upward, in contrast with roll vortices, with large positive values (~150 m2 s−2) found in the turbulent filaments. In the decaying inner eyewall, elevated tangential momentum is also being transported radially outward to the intensifying outer eyewall. These results indicate that the eddies may have connections to potential vorticity waves with possible modifications due to boundary layer shear instabilities.

Hurricane Boundary Layer Height Relative to Storm Motion from GPS Dropsonde Composites

“…The only in situ data of this kind was collected in regions either 100 km away from the hurricane eyewall or in weak tropical storms [25][26][27][28]. Note that a new airborne Doppler radar dataset from the Imaging Wind and Rain Airborne Profiler (IWRAP) is able to provide three-dimensional winds, deep into the hurricane boundary layer, at high resolution (~200 m horizontal and 30 m vertical), which will be useful for characterizing the boundary layer height using dynamical metrics [29,30].…”

Vertical Eddy Diffusivity Parameterization Based on a Large‐Eddy Simulation and Its Impact on Prediction of Hurricane Landfall

“…Guimond et al. (2018) found that RV structure could extend up to several kilometers in radar observations. The introduction of LE effects into the PBL scheme led to better wind profile simulations that are more similar to radar observations.…”

Organized kinetic energy backscatter in the hurricane boundary layer from radar measurements

“…The production term is normalized by , where is the average characteristic eddy velocity of and is the characteristic eddy length scale of 2 km (Guimond et al. 2018 b ). While to the best of the authors’ knowledge no data at a similar Reynolds number are available for comparison, hopefully these scale factors will facilitate comparisons between these results and future studies.…”

“…Our focus in this paper is on these larger-scale turbulent structures with wavelengths of approximately 2 km that span at least the full depth of the HBL (Guimond et al. 2018 b ), which is typically approximately 1 km. To the best of the authors’ knowledge, there are no systematic studies of the full energy transfer on these scales in the HBL.…”

“…These measurements reveal the ubiquitous presence of coherent turbulent eddies with scales of approximately 2 km that extend from the ocean surface, through the boundary layer and into the lower free atmosphere at a height of approximately 1.5 km (Guimond et al. (2018 b ); hereafter G18b). The role of the sub-filter-scale (SFS) energy transfer term in the kinetic energy (KE) budget is also examined.…”