The Influence of the Downstream State on ET: Danielle and Earl (1998)

Ron McTaggart-CowanJohn Gyakum, Peter Yau

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

• 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

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

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

Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds

00 hValid 0000 UTC 5 Sept

Modelling Danielle/Earl

Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds

12 hValid 1200 UTC 5 Sept

Modelling Danielle/Earl

Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds

24 hValid 0000 UTC 6 Sept

Modelling Danielle/Earl

Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds

36 hValid 1200 UTC 6 Sept

Modelling Danielle/Earl

Sea Level Pressure, 1000-500 hPa thickness and 250 hPa winds

48 hValid 0000 UTC 7 Sept

Modelling Danielle/Earl

Dynamic tropopause (2 PVU) potential temperatures and winds

Modelling Danielle/Earl

00 hValid 0000 UTC 5 Sept

• Two distinctly different structures evolve on the DT

48 hValid 0000 UTC 7 Sept

Modelling Danielle/Earl

• 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

IDEAL test – no downstream perturbations

Dynamic tropopause (2 PVU) potential temperatures and winds

Transition Modes

00 hValid 0000 UTC 5 Sept

SIMJET test – no downstream perturbations

Dynamic tropopause (2 PVU) potential temperatures and winds

Transition Modes

00 hValid 0000 UTC 5 Sept

• Convective stability of the system is also modified by ET mode

Transition Modes

Baroclinic mode Tropical mode

PV and θ E-W sections at 36 h

control control

IDEALSIMJET

• The secondary circulation around the jet asymmetrically enhances advection around the cyclone

Transition Modes

• 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