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The Extratropical Transitions of Danielle and Earl (1998)

The Extratropical Transitions of Danielle and Earl (1998). Ron McTaggart-Cowan J.R. Gyakum, M.K. Yau. Outline. Motivation and ET background Case description Important upstream features Important downstream features PVmc introduction Importance of remnant vortex Discussion. ET Background.

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The Extratropical Transitions of Danielle and Earl (1998)

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  1. The Extratropical Transitions of Danielle and Earl (1998) Ron McTaggart-Cowan J.R. Gyakum, M.K. Yau

  2. Outline • Motivation and ET background • Case description • Important upstream features • Important downstream features • PVmc introduction • Importance of remnant vortex • Discussion

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Case Description • The MC2 model is used to sim-ulate the ETs of Danielle and Earl Valid 0000 UTC 5 Sept 00 h

  9. Case Description • The MC2 model is used to sim-ulate the ETs of Danielle and Earl Valid 1200 UTC 5 Sept 12 h

  10. Case Description • The MC2 model is used to sim-ulate the ETs of Danielle and Earl Valid 0000 UTC 6 Sept 24 h

  11. Case Description • The MC2 model is used to sim-ulate the ETs of Danielle and Earl Valid 1200 UTC 6 Sept 36 h

  12. Case Description • The MC2 model is used to sim-ulate the ETs of Danielle and Earl Valid 0000 UTC 7 Sept 48 h

  13. Upstream Features • PV features are identified and removed • Sensitivity of ET to each feature is determined

  14. Upstream Features • The upstream trough is the most important feature • The ex-hurricane plays a secondary role

  15. Downstream Features • 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

  16. Downstream Features • Simultaneous ET and intensifica-tion simulated with MC2 model Valid 0000 UTC 5 Sept 00 h

  17. Downstream Features • Simultaneous ET and intensifica-tion simulated with MC2 model Valid 1200 UTC 5 Sept 12 h

  18. Downstream Features • Simultaneous ET and intensifica-tion simulated with MC2 model Valid 0000 UTC 6 Sept 24 h

  19. Downstream Features • Simultaneous ET and intensifica-tion simulated with MC2 model Valid 1200 UTC 6 Sept 36 h

  20. Downstream Features • Simultaneous ET and intensifica-tion simulated with MC2 model Valid 0000 UTC 7 Sept 48 h

  21. Downstream Features • With a benign downstream state, the transition is tropical mode Valid 0000 UTC 7 Sept 48 h

  22. Downstream Features • With only a simplified NA jet/front, the transition is baroclinic mode Valid 0000 UTC 7 Sept 48 h

  23. 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

  24. PVmc Introduction 850 hPa PVmc and winds (kts) for control initial conditions • PVmc highlights regions of dynamic and thermodynamic importance 00 h

  25. 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

  26. Remnant Vortex Control 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.

  27. Remnant Vortex • Baroclinic mode (Earl) is insensitive to vortex removal • Tropical mode (Danielle) is influenced by the structure of the remnant TC

  28. 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

  29. Discussion • Advanced diagnostic tools are required for the investigation of ET dynamics and thermo-dynamics …

  30. Dynamic tropopause (2 PVU) and 850 hPa equivalent potential temperature

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