MM5 simulation of eyewall replacement in Hurricane Rita

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MM5 simulation of eyewall replacement in
Hurricane Rita

• Marshall Stoner

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MotivationWhy study eyewall replacement cycles?

• Past 30 years
• Have seen much improvement in forecasting
  hurricane tracks
• Meager improvement in forecasting hurricane
  intensity change
• Why have track forecasts improved
• Improved model physics, computational technology,
  and data availability for forecasting large-scale
  steering flow surrounding hurricanes.
• Why havent intensity forecasts improved
• Small scale structures in hurricanes are not as
  well resolved by models.
• Although there has been improvement in our
  understanding of hurricane energetics and
  thermodynamics (e.g. WISHE mechanism, see Emanuel
  1986 JAS) , this theory is not adequate to
  explain changes in intensity.

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MotivationWhy study eyewall replacement cycles?

• What causes intensity change in hurricanes?
• There are many possibilities (not limited to)
• Passage over regions of differing ocean heat
  content
• Changes in the amount of vertical wind shear
• Interaction with synoptic scale systems
• Eyewall replacement cycles
• How do eyewall replacement cycles influence
  intensity?

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Previous Research

• Stages of eyewall replacement (documented in
  Willoughby et. al. 1982 JAS)
• Formation of outer concentric eyewall (cessation
  of intensification)
• Collapse of inner (old) eyewall (weakening)
• Contraction of outer (new) eyewall
  (restrengthening)

Figure adapted from Houze et. al. 2005 BAMS
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Previous Research
What causes outer eyewall to form in the first
place?

• Heat source aloft near the radius of maximum wind
  (RMW) in a balanced baroclinic vortex leads to a
  radial circulation that transports angular
  momentum inward past the RMW.
• This process causes the greatest positive wind
  tendency to be located inside the RMW, hence
  causing contraction of the RMW.
• This theory can also be used to describe the
  contraction of concentric eyewalls.

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What causes outer eyewall to form in the first
place?
Previous Research

• Many theories but no conclusive answers.
• One promising theory is the accumulation of
  momentum at a stagnation radius due to outward
  propogating vortex Rossby waves. (see Montgomery
  and Kallenbach 1997 QJRMS)

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The RAINEX Experiment (2005) Documentation of
eyewall replacement inHurricane Rita

• The most extensive and highest resolution
  airborne Doppler radar observation of concentric
  eyewalls in history.
• Real time high resolution (1.67 km) numerical
  simulations documenting the entire process of
  eyewall replacement were created at University of
  Miami (Shuyi Chen) during the project.
• At the current time I have primarily been
  focusing on analyzing this model output.

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Overview
Adapted from Houze et. al. 2005 BAMS
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Vortex Rossby waves
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2
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Vortex Rossby waves
1
2
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Vortex Rossby waves
1
2
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Vortex Rossby waves
1
2
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Rain band structure
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Rain band structure
Inner
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Rain band structure
Middle
Inner
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Rain band structure
Middle
Inner
Outer
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Rain band structure
From Willoughby 1988 (Aust. Met. Mag 36)
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Wind bursts
Correlated with bursts of convection
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Wind bursts
Correlated with bursts of convection
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Wind bursts
Correlated with bursts of convection
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Axisymmetrization
Breakup of principal band
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Axisymmetrization
Breakup of principal band
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Axisymmetrization
Breakup of principal band
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Axisymmetrization
Breakup of principal band
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Axisymmetrization
Breakup of principal band

• Occurs twice
• 1st time Sep 21, 18 UTC 22 UTC
• 2nd time Sep 22, 06 UTC 10 UTC
• After breakup of principal band an annulus of
  short interleaved bands surrounds a low
  reflectivity moat
• Outer eyewall forms after second breakup period

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Downdrafts within moat
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Downdrafts within moat
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Frontogenesis (Emanuel 1996 JAS)
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Frontogenesis (Emanuel 1996 JAS)
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Frontogenesis (Emanuel 1996 JAS)
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Frontogenesis (Emanuel 1996 JAS)
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Preliminary Conclusions
Before formation of outer eyewall

• Collections of small scale bands (possibly vortex
  Rossby waves) spiral out from the eyewall.
• Bands directly adjascent to the eyewall tend to
  have lighter precipitation.
• Similar to the secondary and connecting rain
  bands described in radar observations by
  Willoughby et. al 1984 JAS and Willoughby 1988
  AMM.
• Heavier, more cellular precipitation breaks out
  once bands enter a radius range (approx 50-100
  km).
• Similar to primary band also discussed by
  Willoughby.
• The band then merges into the large stratiform
  precipitation shield on the front right quadrant
  of the storm.
• Process repeats

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Preliminary Conclusions
Before formation of outer eyewall (cont.)

• Away from eyewall transient regions of strong
  wind (wind bursts) are associated with
  increased convection within rain bands.
• High correlation between reflectivity maxima and
  wind maxima.
• These wind and precipitation bursts generally
  form on the right rear quadrant of the storm as
  convection intensifies within a spiral band.

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Preliminary Conclusions
Formation of outer eyewall

• Principal band becomes more tightly wrapped and
  parallel to inner eyewall.
• Breaks into multiple interleaved bands spaced
  evenly around the vortex.
• Significant downdrafts occur in the moat.
• This matches well with the downdrafts observed in
  the ELDORA Dopplar derived winds in Hurricane
  Rita during the concentric eyewall phase.
• ?e and angular momentum front occurs at low
  levels and builds upward with time.
• Frontogenetic properties of idealized eyewalls
  described in Emanuel 1996 JAS
• Once this front extends all the way to the
  tropopause, inner eyewall dissipates.
• Contraction of the new eyewall continues while
  remnants of old eyewall are completely mixed
  throughout new closed eye.

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Future Research Plans

• Interpolate model output data into cylindrical
  coordinates with winds relative to storm center.
• Break wind into tangential and radial components
  w.r.t. this coordinate system.
• Need ability to analyze sector averages (radius
  height plots) rather than simple cross-sections.
• Analyze ELDORA Dopplar derived fields to compare
  with model output.