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, 1000-500 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 hValid 0000 UTC 5 Sept

Case Description• The MC2 model is used to sim-ulate

the ETs of Danielle and Earl

12 hValid 1200 UTC 5 Sept

Case Description• The MC2 model is used to sim-ulate

the ETs of Danielle and Earl

24 hValid 0000 UTC 6 Sept

Case Description• The MC2 model is used to sim-ulate

the ETs of Danielle and Earl

36 hValid 1200 UTC 6 Sept

Case Description• The MC2 model is used to sim-ulate

the ETs of Danielle and Earl

48 hValid 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

Downstream Features• Simultaneous ET and intensifica-tion

simulated with MC2 model

00 hValid 0000 UTC 5 Sept

Downstream Features• Simultaneous ET and intensifica-tion

simulated with MC2 model

12 hValid 1200 UTC 5 Sept

Downstream Features• Simultaneous ET and intensifica-tion

simulated with MC2 model

24 hValid 0000 UTC 6 Sept

Downstream Features• Simultaneous ET and intensifica-tion

simulated with MC2 model

36 hValid 1200 UTC 6 Sept

Downstream Features• Simultaneous ET and intensifica-tion

simulated with MC2 model

48 hValid 0000 UTC 7 Sept

Downstream Features• With a benign downstream state, the

transition is tropical mode

48 hValid 0000 UTC 7 Sept

Downstream Features• With only a simplified NA jet/front, the

transition is baroclinic mode

48 hValid 0000 UTC 7 Sept

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

PVmc Introduction

• PVmc highlights regions of dynamic and thermodynamic importance

850 hPa PVmc 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 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.

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