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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
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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)
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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
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2315 UTC 09 Oct 2001
2315 UTC 10 Oct 2001
2315 UTC 11 Oct 2001
2315 UTC 12 Oct 2001
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SLP, 900 hPa winds
200 hPa height, 330-340 K PV
Global (1ox1o) data from FNL
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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
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WRF HOURL
Y PRECIP
.
9 km grid
IR SAT.
1800 UTC 11 October 1500 UTC 12 October
Model Evaluation
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HourlyRain Rate
(mm h-
1)
Precipitation
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10 m winds
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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
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F15
AdvectionTendency (10-4 s-1 h-
1)
F19
StretchingTendency (10-4 s-1 h-
1) 900 hPa
p v v p
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Frontogenesis
F12 F15
F18 F21
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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
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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
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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)
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F18
F24
Advective
Tendency
(PVU h-
1)
Diabatic Tendenc
y (PVU h-
1)
335 K surface
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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