Monday, 29 April 2002: 4:15 PM
Intensity change and eyewall replacement in Hurricane Floyd (1999)
Over the last a few decades hurricane track forecasts have improved
significantly, whereas relatively little progress made in hurricane
intensity forecasts. The lack of skill in the intensity forecasting
may be attributed to insufficient spatial resolution to resolve the
hurricane inner-core structure and inadequate representation of
physical processes in hurricane prediction models. Though
axisymmetric models can resolve the inner-core of the hurricane
vortex, they cannot explicitly treat the effects of asymmetries on
hurricane intensity. It is observed that hurricane intensity changes
are often associated with eyewall contraction and
replacement. However, the mechanisms responsible for the development
of secondary wind maxima and eyewall replacement processes are not
well understood. Here we examine simulations of Hurricane Floyd
(1999) using a nonhydrostatic full physics model that explicitly
resolves the dynamics and microphysics near the vortex center through
use of a 1.67 km mesh that follows the vortex. We show that the model
is able to reproduce observed storm structure and evolution, including
track, intensity, and the spatial pattern of precipitation.
Additionally, the model produces a contracting ring of high
reflectivity and corresponding maximum in the swirling wind profile
outside of the innermost eyewall, similar to what is observed in the
aircraft flight-level data. As observed, the innermost eyewall in the
model dissipates over the period of several hours and is replaced by a
single ring of high wind speed and rain rate at a radius larger than
that of the original eyewall. Focusing on this concentric eyewall
structure, we examine the evolution of the horizontal structure of the
storm's wind and precipitation fields in relation to the evolution of
its intensity, and we perform several sensitivity experiments in order
to isolate physical mechanisms responsible for the secondary maxima in
rain rate and swirling wind speed.
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