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

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)

Tropical vs Extratropical Cyclones

Pop Quiz: TC or not?

Yes: Hurricane Isabel (2003)

Pop Quiz: TC or not?

No: a subtropical system

Pop Quiz: TC or not?

Yes: TC Erika (2009)

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!

Identifying TCs and estimating their intensity can be subjective both in the

real world and in the model

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

VorticityVorticity = measure of rotationFor cyclonic rotation, it’s positive for Northern Hemisphere

negative for Southern Hemisphere

Horizontal viewVertical view

Temperature Anomaly

Vertical view

Horizontal view

Core

dT = Tcore - Tenvironment

sum(dT300mb + dT500mb + dT700mb) > 2K

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

Cyclone Phase technique

• Developed by Bob Hart (Florida State Univ.)

• Comprehensive measure of thermal structure of a storm– Symmetricity– Vertical variation of

thermal structure

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

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

Extra

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)

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