Environmental Sensitivity of the Structural Evolution of Hurricane Irene (2011)
Jason W. Godwin
Rosenstiel School of Marine and Atmospheric Science
University of Miami, Miami, FL
1. Introduction
Since 1990, there have been substantial
improvements in tropical cyclone (TC) track forecasts.
Official 72-hour track forecast errors from the National
Hurricane Center (NHC) have decreased from 556 km
in 1990 to 185 km in 2012; however, there has been
little decrease in intensity forecast errors. In 1990, the
average intensity error for a 72-hour forecast was 10.3
m s-1 compared with 8.7 m s-1 in 2012 (Cangialosi and
Franklin 2013). While a lot of attention has been given
to accurately predicting the maximum sustained winds
of a TC, this research aims to forecast the overall
structure of the TC. Knowing the extent of the TC wind
field as well as its maximum sustained winds could
provide more advanced information about the need for
coastal evacuations, ship rerouting, and the scope of
coastal and inland impacts.
2. Research Background
Gray (1968) identified warm sea surface
temperatures (SST), convective instability, a moist
lower to middle troposphere, and weak vertical wind
shear as the necessary conditions for TC formation and
intensification. These factors are used by forecasters to
assess whether a TC will weaken or strengthen, but it is
not well known to what degree these factors impact the
structural evolution of the TC. Uhlhorn et al. (2014)
found that there is a dependence on the radius of
maximum winds on the vertical wind shear with the
wind maximum being found on the downshear left side
of the TC. Rogers et al. (2003) found that the heaviest
precipitation is found on the downshear left side as well.
Hill and Lackmann (2009) used the Advanced Weather
Research Weather Research and Forecasting (WRFARW) model to perform idealized TC experiments in
which atmospheric moisture was controlled in order to
examine the effect of moisture variations on the size of
the TC. They found that a moister environment
produced a larger TC. Komaromi et al. (2011) examined
the initial condition sensitivity in WRF-ARW for
Typhoon Sinlaku (2008) and Hurricane Ike (2008) by
including balanced relative vorticity perturbations to the
synoptic environment of the TCs. They found that
perturbations to often distant synoptic-scale features had
a significant influence on the tracks of the TCs. Brennan
and Majumdar (2011) assimilated synthetic temperature
“observations” into the National Center for
Environmental Prediction (NCEP) Global Forecast
System (GFS) model to test the influences of synopticscale features on Hurricane Ike, and found that multiple
sources of error exist in the initial model states and that
correction of those errors would lead to improved TC
track forecasts.
3. Methodology
The storm that will be used for the case study of this
research is Hurricane Irene (2011). The track for Irene
was fairly well forecast with track errors being smaller
than the five-year NHC average; however, the intensity
errors were larger than average due to a consistent highbias in the forecasts, largely due to the unexpected
evolution of Irene from a small, compact storm with
strong winds concentrated in an intense eyewall, to a
large, broad TC without a well-defined eyewall. While
the minimum pressure of Hurricane Irene fell, the
maximum sustained winds also decreased, but the radius
of gale force winds expanded (Avila and Cangialosi
2011).
In order to understand the physical mechanisms
underlying the poor predictability of Irene’s structural
evolution, we perform a series of initial condition
sensitivity experiments. Two types of perturbation
experiments will be performed: (1) relative vorticity
perturbations using the methodology from Komaromi et
al. (2011), and (2) moisture perturbations, which will be
performed using a similar technique to Komaromi et al.,
but will initially adjust the water vapor mixing ratio
before rebalancing the other fields. The perturbation
experiments will be carried out using WRF-ARW, with
the results of each experiment compared against a
control simulation and the NHC best-track data.
4. Relative Vorticity Perturbation Experiment
A relative vorticity perturbation was performed in
which a shortwave trough over northwestern North
Dakota was strengthened (Figure 1). After integrating
the model forward using these new initial conditions,
little model sensitivity was found, with the track,
intensity, and wind radii in the perturbed simulation
being very similar to those in the control simulation.
Future moisture perturbation experiments will
include increasing the moisture within the TC core, and
changing the moisture in the environment ahead of the
TC path. These moisture experiments will also be
performed on a finer convection-permitting grid scale (3
km) in order to examine the impact of moisture on the
convective structure of the TC.
6. References
Avila, Lixion A., and John Cangialosi, 2011: Hurricane
Irene. Tropical Cyclone Report, 45 pp.
Figure 1: relative vorticity perturbation over
northwestern North Dakota
We plan to perform vorticity perturbation
experiments in the future in which synoptic features are
perturbed during the simulation (i.e. perturbations will
be introduced to WRF restart files). Furthermore, we
plan to perturb the TC vortex itself in order to examine
the effects of TC outflow on the structural evolution.
5. Moisture Perturbation Experiment
A moisture perturbation was performed in which the
TC core and its surrounding environment were dried
(Figure 2). The drier initial state results in a weaker TC
(as would be expected based on Gray 1968), but when it
eventually intensified to similar maximum sustained
winds as the control simulation, the perturbed
simulation did in fact exhibit a smaller radius of gale
force winds, a finding consistent with Hill and
Lackmann (2009).
Figure 2: moisture perturbation within the TC core and
its surrounding environment
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Cangialosi, John P. and James L. Franklin, 2013: 2012
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