An Observational Case for the Prevalence of Roll Vortices in the Hurricane Boundary Layer*

Abstract

Doppler velocity data from Weather Surveillance Radar-1988 Doppler (WSR-88D) radars during four hurricane landfalls are analyzed to investigate the presence of organized vortices in the hurricane boundary layer (HBL). The wavelength, depth, magnitude, and track of velocity anomalies were compiled through analysis of Doppler velocity data. The analysis reveals alternating bands of enhanced and reduced azimuthal winds closely aligned with the mean wind direction. Resulting statistics provide compelling evidence for the presence of organized secondary circulations or boundary layer rolls across significant areas during four hurricane landfalls. The results confirm previous observations of the presence of rolls in the HBL. A potential limitation of the study presented here is the resolution of the WSR-88D data. In particular, analysis of higher-resolution data (e.g., from the Doppler on Wheels) is needed to confirm that data aliasing has not unduly impacted the statistics reported here. Momentum fluxes associated with the secondary circulations are estimated using the covariance between the horizontal and vertical components of the wind fluctuations in rolls, with resulting fluxes 2-3 times greater than estimated by parameterizations in numerical weather prediction models. The observational analysis presented here, showing a prevalence of roll vortices in the HBL, has significant implications for the vertical transport of energy in hurricanes, for the character of wind damage, and for improvements in numerical simulations of hurricanes.

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

“…Spectral analysis of these images over 5 km 2 ocean cells was able to retrieve a direction that was nearly equal to the wind direction, as well as a mean wavelength (Katsaros et al 2002). Figure 11 (bottom) illustrates a histogram averaged over six such 2D spectra (6 km 2 ) that shows a peak near 900 m. This value is close to that determined from groundbased WSR-88D radar observations in landfalling hurricanes (Morrison et al 2005). Their results are supported by more recent higher-resolution portable Doppler radar observations in landfalling hurricanes (Losorlo and Schroeder 2006a,b) and by DOW highresolution portable Doppler radar observations in Hurricane Fran (Wurman and Winslow 1998), and more recently in Hurricane Rita (Wurman et al 2006).…”

“…This peak disappeared when the aircraft turned and flew along wind. The significance of these observations is that these linear features, and their possible major effect on air-sea fluxes [as suggested by Morrison et al (2005) and Foster (2005)], are not currently modeled in any major hurricane-coupled modeling effort, and may be an important factor in the prediction of hurricane intensity change. This leads to the third major CBLAST finding to date, that is, that secondary boundary layer circulations, which while not a major thrust of the original CBLAST plan, are a significant factor in the hurricane boundary layer flow field and are a likewise significant factor in air-sea fluxes.…”

A Simple Method for Simulating Wind Profiles in the Boundary Layer of Tropical Cyclones

“…11a). Such structures have been observed in hurricane boundary layers (e.g., Wurman and Winslow 1998;Morrison et al 2005;Zhang et al 2008b;Kosiba and Wurman 2014) and are thought to be important for strong wind gusts near the…”

“…The primary advantage of LES, of course, is that the statistical properties of turbulent flow can be predicted primarily by the model's governing equations, with only a small role being played by a subgrid turbulence scheme. Furthermore, coherent structures within the tropical cyclone boundary layer such as quasi-two-dimensional roll vortices (e.g., Foster 2005;Morrison et al 2005) can only be resolved using grid spacing of ≈100 m or less, i.e., typical resolution for LES. However, tropical cyclones extend hundreds of km horizontally, and so LES becomes prohibitively expensive unless the domain size is restricted, thereby making it difficult to account for the dynamical processes in rapidly rotating flow mentioned above (e.g., centrifugal acceleration).…”

Influence of geometrical parameters on convective flows in non-uniformly heated cylindrical fluid layers

“…There is remarkable similarity of these secondary flows and rainbands in real tropical cyclones [9,10]. Secondary flows may significantly influence on heat and mass transfer in the boundary layer [11,12]. The nature and characteristics of secondary flows over heating area in a non-rotating cylindrical layer were studied in [13].…”