Organized Kinetic Energy Backscatter in the Hurricane Boundary Layer
from Radar Measurements
Abstract
The fluid mechanics of hurricanes strongly depends on boundary layer
energetics due to the warm-core nature of the system with peak
velocities located at lower levels. One barrier that has inhibited a
more complete characterization of energy transfer in the boundary layer
is a lack of observations that resolve large, turbulent eddies. In
particular, the occurrence and structure of upscale energy transfer
(backscatter) in the hurricane boundary layer as well as the effects of
backscatter on the vortex intensity are unknown. The analysis presented
here of very high-resolution, three-dimensional wind observations from
Hurricane Rita (2005) at peak intensity reveals large regions of
organized backscatter in the boundary layer associated with coherent,
turbulent eddies. Strong forwardscatter is also found next to the
backscatter regions due to the interaction between adjacent eddies. Two
components of the stress tensor are primarily responsible for this
alternating scatter structure, as shown by large correlation
coefficients between the fields: the radial–vertical component (τ13)
and the azimuthal–vertical component (τ23) with average correlations of
79% and 49 %, respectively. The Leonard, Reynolds and cross-term
stress components are also provided. The impact of the sub-filter-scale
energy transfer is estimated by computing the kinetic energy budget for
the resolved-scale and eddy-scale motions. The results show that the
sub-filter-scale energy transfer term is of the same order as the other
terms in the eddy-scale budgets, contributing between 16% and 40% to
the local time tendency with an average contribution of approximately
30%. These results indicate that the coherent turbulent eddies can
affect the vortex dynamics through wave–wave nonlinear interactions,
which can subsequently influence the wave–mean flow interactions. This
is the first study to examine the full sub-filter-scale energy transfer
and its impact on the kinetic energy budget in the hurricane boundary
layer. These findings emphasize the importance of coherent turbulence in
the energy cascade and have the potential to improve turbulence closure
schemes used in numerical simulations. Sroka, S. and S.R. Guimond, 2021.
Organized kinetic energy backscatter in the hurricane boundary layer
from radar measurements. Journal of Fluid Mechanics, 924, A21.
doi:10.1017/jfm.2021.632