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

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

Vorticity Vorticity = measure of rotation For cyclonic rotation, it’s positive for Northern Hemisphere negative for Southern Hemisphere Horizontal view Vertical view

Temperature Anomaly Vertical view Horizontal view Core dT = T core - T environment sum(dT 300mb + dT 500mb + dT 700mb ) > 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 h = +1 for Northern Hemisphere h = -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 -VTL & -VTU > 0 for tropical cyclones