Identifying Tropical Cyclones in Model Simulations
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Identifying Tropical Cyclones in Model Simulations
Asuka Suzuki-ParkerNCAR 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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A cyclone of tropical origin with maximum sustained wind of >17m/s near the center
What is a Tropical Cyclone?
• 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 that’s 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 > 17m/s Maximum vorticity > 5x10-5/s Temperature anomaly (dT) > 2K Low level winds > high level winds High level dT > low level dT 2 days lifetime
Thresholds differ by model resolution and by research groups
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VorticityVorticity = measure of rotationFor cyclonic rotation, it’s positive for Northern Hemisphere
negative for Southern Hemisphere
Horizontal viewVertical view
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Temperature Anomaly
Vertical view
Horizontal view
Core
dT = Tcore - Tenvironment
sum(dT300mb + dT500mb + dT700mb) > 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
20~30% 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
We’ll 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
Bh(Z600mb Z900mb |R Z600mb Z900mb |L )
h = +1 for Northern Hemisphereh = -1 for Southern Hemisphere
B < 10 for tropical cyclones(Evans and Hart 2003)
(Hart 2003)
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Cyclone Phase technique(cont.)
(Hart 2003)
Thermal wind structure measure
(Z) ln p
900mb
600mb
VTL
(Z) ln p
600mb
300mb
VTU
-VTL & -VTU > 0 for tropical cyclones