A Case Study of Hurricane Formation in Strong Shear: Claudette (2003) Kay Shelton University at Albany, SUNY

Data USAF reconnaissance data –Flight level observations –Dropsondes IR, visible and microwave satellite imagery Gridded ECMWF (1.125 o x1.125 o ) analyses –Shear calculated using 500km radius NHC Best Track (modified using reconnaissance centre fixes)

Track SSTs in central/western Caribbean: o C  Case Study period H1 (12 UTC/10 th July)  Pressure and Windspeed  Very rapid pressure change at H1 time

hPa 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 Observations 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 10 th July –5 hours prior to H1 time 400km

Fl. 6 Pass 1 – Centre Cross-section SWSENE     SWSENE 50m ee Windspeed T, T d D-value

Fl. 7 Pass 1 – Centre Cross-section SW E EWSWSESSWS-N WSW    SSW S-N SE 145m 9oC9oC ee Windspeed T, T d D-value

Surface Observation At time of H1 No eye seen in IR image Dropsonde (cyan) –1203 UTC 850hPa winds (white) – UTC 700hPa winds (black) – UTC No vortex tilt from surface to 850hPa

Fl. 7 Pass 2 – Centre Cross-section NE SW 100m 7oC7oC ee Windspeed T, T d D-value

1456UTC/10 th – TRMM Partial eyewall on western side surrounding the eye Possible signature of dry air spiraling into eye 85GHz H Sensitive to ice phase Shows upper levels (5-9km) 16 o N 18 o N 20 o N 84 o W82 o W

Fl. 7 Pass 4 – Centre Cross-section SW SSE S S   NN N N NNE 12 o C ee Windspeed T, T d D-value

Summary 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 10 th July? Why did the storm weaken so quickly?

Hypothesis 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 Shear Z 700hPa Surface ABC