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MM5 simulation of eyewall replacement in Hurricane Rita

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Title: MM5 simulation of eyewall replacement in Hurricane Rita


1
MM5 simulation of eyewall replacement in
Hurricane Rita
  • Marshall Stoner

2
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.

3
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?

4
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
5
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.

6
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)

7
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.

8
Overview
Adapted from Houze et. al. 2005 BAMS
9
Vortex Rossby waves
1
2
10
Vortex Rossby waves
1
2
11
Vortex Rossby waves
1
2
12
Vortex Rossby waves
1
2
13
Rain band structure
14
Rain band structure
Inner
15
Rain band structure
Middle
Inner
16
Rain band structure
Middle
Inner
Outer
17
Rain band structure
From Willoughby 1988 (Aust. Met. Mag 36)
18
Wind bursts
Correlated with bursts of convection
19
Wind bursts
Correlated with bursts of convection
20
Wind bursts
Correlated with bursts of convection
21
Axisymmetrization
Breakup of principal band
22
Axisymmetrization
Breakup of principal band
23
Axisymmetrization
Breakup of principal band
24
Axisymmetrization
Breakup of principal band
25
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

26
Downdrafts within moat
27
Downdrafts within moat
28
Frontogenesis (Emanuel 1996 JAS)
29
Frontogenesis (Emanuel 1996 JAS)
30
Frontogenesis (Emanuel 1996 JAS)
31
Frontogenesis (Emanuel 1996 JAS)
32
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

33
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.

34
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.

35
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.
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