The Role of Upshear Convection in Expediting the Tropical Transition of Atlantic Hurricane Karen (2001)

Andy Hulme and J.E. Martin28th Hurricane/Tropical

Meteorology30 April 2008

Background

• Tropical transition: TCG from a baroclinic precursor – Bracken and Bosart (1991), Montgomery and Farrell (1993)

• Upshear convection organizes latent heating, removes shear, generates and merges small-scale vortex towers– Davis and Bosart (2001, 2003, 2004), Hendricks et al. (2004)

• Diabatic processes significantly alter the bent-back warm fronts of purely extratropical cyclones.– Kuo et al. (1991, 1992), Neiman et al. (1993)

• During occlusion upper-level PV also altered– Martin (1998), Posselt and Martin (2004), Martin and Otkin

(2004)

Karen: Overview• Resulted from interaction

between upper level trough and stalled cold front.

• Developed rapidly on 11-12 October while subtropical

• Eye-like structure develops on the 12th

• Became tropical on the 13th, max. intensity=984 hPa

2315 UTC 09 Oct 2001

2315 UTC 10 Oct 2001

2315 UTC 11 Oct 2001

2315 UTC 12 Oct 2001

SLP, 900 hPa winds

200 hPa height, 330-340 K PV

Global (1ox1o) data from FNL

WRF ModelWRF Model• Initial/ boundary Initial/ boundary

conditions from conditions from FNL.FNL.

• 31 vertical levels, 31 vertical levels, nested grids of 27 nested grids of 27 km and 9 kmkm and 9 km

• Simulation lasts 48 Simulation lasts 48 h beginning 0000 h beginning 0000 UTC 11 October UTC 11 October – Microphysics: WSM6Microphysics: WSM6– Cumulus: KF2Cumulus: KF2– PBL: YSUPBL: YSU– Radiation: Dudhia Radiation: Dudhia

(SW), RRTM (LW).(SW), RRTM (LW).

WRF

HURDAT

WRF HOURL

Y PRECIP

.

9 km grid

IR SAT.

1800 UTC 11 October 1500 UTC 12 October

Model Evaluation

HourlyRain Rate

(mm h-

1)

Precipitation

10 m winds

Absolute Vorticity

10-4 s-1

• Until F15: banded

• F18: negative vorticity, low-level jet

• F21 to F27: cyclonic wrapping of vorticity

• End result: circular vorticity around center

Enagonio and Montgomery (2001), Molinari et al. (2004)

900 hPa

F15

AdvectionTendency (10-4 s-1 h-

1)

F19

StretchingTendency (10-4 s-1 h-

1) 900 hPa

p v v p

Frontogenesis

F12 F15

F18 F21

Cross Section

AVOR NORM(WINDS)

• F12: negative vorticity at midlevels at the top of the frontal updraft

• F18: extends to near the surface, intense/narrow along front jet (cold tongue)

F12

F15

F18

Trajectory Analysis• Forward and

backward trajectories from negative vorticity area at F21

• Start north of warm front

• Ascend rapidly upon reaching end of front

• Descend into boundary layer

• Wrap around warm anomaly

F06

F18

F12

F24

335-345-K PV, PVU 305-315-K PV HVOR vectors

HgtP LH paη /

pH

HY

horiz

zz

/ˆ vkY

Cammas et al. (1994)

Emanuel et al. (1987),

Raymond (1992)

PV flux = LHR x SHEAR

Thus if shear is strong, large horizontal displacement between upper and lower anomilies

(Lackmann 2002)

F18

F24

Advective

Tendency

(PVU h-

1)

Diabatic Tendenc

y (PVU h-

1)

335 K surface

Conclusions

• Upshear of the cyclone is a likely area of low slantwise stability and region of efficient vertical PV redistribution

• Upshear convection – generates intense western-end vorticity max

that is eventually organized into smaller vortex– cycles cooler air into the BL which forms a cold

tongue that encompasses the warm anomaly– redistributes PV to the surface and introduces

a circulation that removes PV gradients