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 50% of Atlantic hurricanes ET The resulting storm is often more intense than the hurricane ETs are very hard to forecast in real time

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

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

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

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, hPa thickness, and SLP serve as an introduction to the case

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

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

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

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

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

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

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

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

Simultaneous ET and intensifica- tion simulated with MC2 model 00 h Valid 0000 UTC 5 Sept

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

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

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

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

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

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

PV mc 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

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

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

Remnant Vortex ControlDanielle 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.

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

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

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

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