1. The Influence of the Downstream State on ET: Danielle and Earl (1998) Ron McTaggart-Cowan John Gyakum, Peter Yau

2. Outline Case description Modelling Earl and Danielle Transition modes Importance of the downstream jet Discussion

3. 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

4. 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

5. 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

6. Modelling Danielle/Earl MC2 is used for all simulations Initialization: 0000 UTC 5 Sept 48 h simulations at 35 km horizontal grid spacing Fixed SST SLP, 1000-500 hPa thickness, 250 winds valid 0000 UTC 5 Sept 00h

7. Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds 00 h Valid 0000 UTC 5 Sept Modelling Danielle/Earl

8. Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds 12 h Valid 1200 UTC 5 Sept Modelling Danielle/Earl

9. Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds 24 h Valid 0000 UTC 6 Sept Modelling Danielle/Earl

10. Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds 36 h Valid 1200 UTC 6 Sept Modelling Danielle/Earl

11. Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds 48 h Valid 0000 UTC 7 Sept Modelling Danielle/Earl

12. Dynamic tropopause (2 PVU) potential temperatures and winds Modelling Danielle/Earl 00 h Valid 0000 UTC 5 Sept

13. Two distinctly different structures evolve on the DT 48 h Valid 0000 UTC 7 Sept Modelling Danielle/Earl

14. Upstream modifications had little impact on ET structure Does a feature of the downstream state control the mode of transition? Sensitivity tests: –Remove all downstream features –Simple North Atlantic jet Transition Modes

15. IDEAL test – no downstream perturbations Dynamic tropopause (2 PVU) potential temperatures and winds Transition Modes 00 h Valid 0000 UTC 5 Sept

16. SIMJET test – no downstream perturbations Dynamic tropopause (2 PVU) potential temperatures and winds Transition Modes 00 h Valid 0000 UTC 5 Sept

17. Convective stability of the system is also modified by ET mode Transition Modes Baroclinic modeTropical mode PV and θ E-W sections at 36 h control IDEAL SIMJET

18. The secondary circulation around the jet asymmetrically enhances advection around the cyclone Transition Modes

19. The simultaneous ET and re- intensification events of Danielle and Earl display important structural differences The downstream jet plays the primary role in determining the mode of transition through enhancement of advection Discussion