The Extratropical Transitions of Danielle and Earl (1998)
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Transcript of The Extratropical Transitions of Danielle and Earl (1998)
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The Extratropical Transitions of
Danielle and Earl (1998)
Ron McTaggart-Cowan
J.R. Gyakum, M.K. Yau
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Outline• Motivation and ET background• Case description• Important upstream features• Important downstream features• PVmc introduction• Importance of remnant vortex• Discussion
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ET Background•50% of Atlantic hurricanes ET
•The resulting storm is often more intense than the hurricane
•ETs are very hard to forecast in real time
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Case Description• Danielle reaches five
intensity maxima – 55 ms-1
• Earl develops in the Gulf of Mexico and only reaches Category 2 before landfall
Danielle
Earl
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Case Description• Both storms transition and
reintensify rapidly between 0000 UTC 5 September and 0000 UTC 7 September
Earl: water vapour image from 1200
UTC 6 September
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Case Description• Both storms transition and
reintensify rapidly between 0000 UTC 5 September and 0000 UTC 7 September
Danielle: infrared image from 1200
UTC 6 September
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl• Traditional synoptic diagnostics
provide insight into the transition process
• 250 hPa winds, 1000-500 hPa thickness, and SLP serve as an introduction to the case
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl
00 hValid 0000 UTC 5 Sept
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl
12 hValid 1200 UTC 5 Sept
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl
24 hValid 0000 UTC 6 Sept
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl
36 hValid 1200 UTC 6 Sept
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Case Description• The MC2 model is used to sim-ulate
the ETs of Danielle and Earl
48 hValid 0000 UTC 7 Sept
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Upstream Features
• PV features are identified and removed
• Sensitivity of ET to each feature is determined
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Upstream Features• The upstream trough is the most
important feature• The ex-hurricane plays a secondary
role
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• Simultaneous ET and intensifica-tion simulated with MC2 model
• The dynamic tropopause (DT, 2PVU) potential temperature describes the upper-level dynamics
• Near-surface potential (850 hPa) temperatures act as surrogate PV at lower levels
Downstream Features
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Downstream Features• Simultaneous ET and intensifica-tion
simulated with MC2 model
00 hValid 0000 UTC 5 Sept
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Downstream Features• Simultaneous ET and intensifica-tion
simulated with MC2 model
12 hValid 1200 UTC 5 Sept
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Downstream Features• Simultaneous ET and intensifica-tion
simulated with MC2 model
24 hValid 0000 UTC 6 Sept
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Downstream Features• Simultaneous ET and intensifica-tion
simulated with MC2 model
36 hValid 1200 UTC 6 Sept
![Page 20: The Extratropical Transitions of Danielle and Earl (1998)](https://reader036.fdocuments.in/reader036/viewer/2022062723/56813f6a550346895daa3c6e/html5/thumbnails/20.jpg)
Downstream Features• Simultaneous ET and intensifica-tion
simulated with MC2 model
48 hValid 0000 UTC 7 Sept
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Downstream Features• With a benign downstream state, the
transition is tropical mode
48 hValid 0000 UTC 7 Sept
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Downstream Features• With only a simplified NA jet/front, the
transition is baroclinic mode
48 hValid 0000 UTC 7 Sept
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PVmc Introduction
• Our upstream study found small sensitivity to the remnant TC
• Removal of both dry dynamics and moisture is required
• Moist Component PV links dynamics and moisture in a balanced way
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PVmc Introduction
• PVmc highlights regions of dynamic and thermodynamic importance
850 hPa PVmc and winds (kts) for control initial conditions
00 h
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Remnant Vortex• Combining PV and PVmc
modifications allows for the complete removal of the TC remnants
• The tropical mode ET (Danielle) should be more sensitive to the moisture modifications
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Remnant VortexControl Danielle Removed
850 hPa geopotential heights (3 dam) and specific humidity (g/kg) in the initial conditions before (left) and after (right) the complete removal of ex-hurricane Danielle.
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Remnant Vortex
• Baroclinic mode (Earl) is insensitive to vortex removal
• Tropical mode (Danielle) is influenced by the structure of the remnant TC
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Discussion• Two transition modes identified:
tropical and baroclinic• Upstream trough is necessary and
sufficent for baroclinic mode ET• Tropical mode ET also requires
remnant TC vortex• Downstream jet modulates the
transition mode
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Discussion• Advanced diagnostic tools are
required for the investigation of ET dynamics and thermo-dynamics …
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Dynamic tropopause (2 PVU) and 850 hPa equivalent potential temperature