A Case Study of Hurricane A Case Study of Hurricane Formation in Strong Shear: Formation in Strong Shear:

Claudette (2003)Claudette (2003)

Kay SheltonUniversity at Albany, SUNY

DataData• USAF reconnaissance data

– Flight level observations– Dropsondes

• IR, visible and microwave satellite imagery

• Gridded ECMWF (1.125ox1.125o) analyses– Shear calculated using 500km radius

• NHC Best Track (modified using reconnaissance centre fixes)

TrackTrackSSTs in central/western Caribbean: 26-28oC

Case Study period

H1 (12 UTC/10th July)

Pressure Pressure and and

WindspeedWindspeed

Very rapid pressure change at H1 time

850-200hPa Shear850-200hPa Shear

Storm reaches hurricane intensity when shear is close to its maximum and still increasing.

A large portion of the storm’s life occurs with shear greater than 12.5ms-1, which is considered a cut-off value, above which storms can no longer sustain themselves (Zehr, 1992)

H1

Flight 6 - Composite Flight 6 - Composite ObservationsObservations

Closed circulation is very small scale and located under deep convection.

Larger-scale wave structure evident.

• 850hPa• ~ 3 hours of

observations• Centred on 0715

UTC on 10th July– 5 hours prior to H1

time

400km

Fl. 6 Pass 1 – Centre Cross-Fl. 6 Pass 1 – Centre Cross-sectionsection

SW S E NE

SW S E NE

50m

eWindspeed

T, Td

D-value

Fl. 7 Pass 1 – Centre Cross-Fl. 7 Pass 1 – Centre Cross-sectionsection

SW SW EEWSW SESSW S-N WSW

SSW S-N SE

145m

9oC

eWindspeed

T, Td

D-value

Surface ObservationSurface Observation• At time of H1• No eye seen in IR

image

• Dropsonde (cyan)– 1203 UTC

• 850hPa winds (white) – 1131-1208 UTC

• 700hPa winds (black)– 1209-1300 UTC

• No vortex tilt from surface to 850hPa

Fl. 7 Pass 2 – Centre Cross-Fl. 7 Pass 2 – Centre Cross-sectionsection

NE NESWSW

100m7oC

eWindspeed

T, Td

D-value

1456UTC/101456UTC/10thth – TRMM – TRMM

• Partial eyewall on western side surrounding the eye

• Possible signature of dry air spiraling into eye

85GHz HSensitive to ice phaseShows upper levels

(5-9km)

16oN

18oN

20oN8

4oW

82

oW

Fl. 7 Pass 4 – Centre Cross-Fl. 7 Pass 4 – Centre Cross-sectionsection

SW SW SSESSES S

NN NNNNE NNE

12oC

eWindspeed

T, Td

D-value

SummarySummary• Deep convection developed over the

storm centre during the time of high shear.

• Dry air persistent upshear and eventually appeared downshear as well.

• How did Claudette manage to become a hurricane in the presence of strong shear?

• How did the convection manage to stay over the storm centre on 10th July?

• Why did the storm weaken so quickly?

HypothesisHypothesis• A: 700hPa circulation located upshear of surface

circulation. Shear advects 700hPa circulation over surface circulation

• B: Circulations are vertically aligned, convection breaks out rapid intensification

• C: Shear advects 700hPa circulation further downshear, circulations decouple system weakens

ShearZ

700hPa

Surface

A B C