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

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