Identifying Tropical Cyclones in Model Simulations

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Identifying Tropical Cyclones in Model
Simulations

• Asuka Suzuki-Parker

NCAR Earth System Laboratory National Center for
Atmospheric Research
NCAR is Sponsored by NSF and this work is
partially supported by the Willis Research
Network and the Research Program to Secure Energy
for America
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What is a Tropical Cyclone?
A cyclone of tropical origin with maximum
sustained wind of gt17m/s near the center

• Large low pressure
• Circular rotating flow
• Convective (cluster of thunderstorms)
• Warm center (or core)

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Tropical vs Extratropical Cyclones
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Pop Quiz TC or not?
Yes Hurricane Isabel (2003)
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Pop Quiz TC or not?
No a subtropical system
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Pop Quiz TC or not?
Yes TC Erika (2009)
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Identifying TCs in Operational Fields

• Satellite
• Dvorak technique estimate TC intensity from
  cloud pattern
• Aircraft reconnaissance
• More accurate measurements of intensity

Yup thats a hurricane cuz we almost crashed!
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Identifying TCs and estimating their intensity
can be subjective both in the real world and in
the model
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Defining TCs in the model(The general rule of
thumb tracking)

• Have a distinctive pressure minimum
• Maximum wind gt 17m/s
• Maximum vorticity gt 5x10-5/s
• Temperature anomaly (dT) gt 2K
• Low level winds gt high level winds
• High level dT gt low level dT
• 2 days lifetime

Thresholds differ by model resolution and by
research groups
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Vorticity
Vorticity measure of rotation For cyclonic
rotation, its positive for Northern
Hemisphere negative for Southern Hemisphere
Vertical view
Horizontal view
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Temperature Anomaly
Core
Horizontal view
Vertical view
dT Tcore - Tenvironment
sum(dT300mb dT500mb dT700mb) gt 2K
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Example result from the general rule of thumb
tracking
Looks very reasonable but are they all really
TCs? Filter these tracks through Cyclone Phase
technique
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Cyclone Phase technique

• Developed by Bob Hart (Florida State Univ.)
• Comprehensive measure of thermal structure of a
  storm
• Symmetricity
• Vertical variation of thermal structure

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Result from Cyclone Phase filtering
Before filtering
After filtering
2030 of the rule of thumb storms failed to
satisfy cyclone phase
Picks up subtropical and strong extratropical
storms
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Summary

• Identifying TCs can be subjective both in
  operation and in models
• Conventional mean of tracking TCs from models may
  pick up non-TCs
• Cyclone Phase technique is an effective way of
  removing non-TCs

Well be presenting results using simulated TCs
that satisfy Cyclone Phase filtering
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Extra
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Cyclone Phase technique(Hart 2003)
Symmetry measure
h 1 for Northern Hemisphere h -1 for
Southern Hemisphere
B lt 10 for tropical cyclones (Evans and Hart 2003)
(Hart 2003)
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Cyclone Phase technique(cont.)
Thermal wind structure measure
-VTL -VTU gt 0 for tropical cyclones
(Hart 2003)