EUMeTrain: Case Study - Extra Tropical Transition

Authors

Met Office College - UK
Jason Bradley , Ian Mills , Nigel Emery

Introduction

Hurricane Alex makes the transition from a Category 3 Hurricane to an Extra-Tropical Cyclone

This is a case study on Extra Tropical Transition (ETT): the process by which a hurricane can change from a tropical cyclone to a mid-latitude depression. During this process the storm changes from warm to cold core.

We will investigate the case of hurricane Alex as it progressed though transition and crossed the Atlantic as an extra tropical cyclone. Between 31 July and 6 August 2004 Hurricane "Alex brought category 1 hurricane conditions to North Carolina Outer Banks as its center passed just offshore, and later strengthened to a Category 3 hurricane while near 38°N latitude. Only one other hurricane (Ellen of 1973) reached major hurricane status farther north than Alex". (Franklin, 2004)

"By late on 5 August Alex had moved north of the Gulf Stream over sub-20°C waters and was weakening rapidly. Moving at 40-45 KT, Alex weakened to a tropical storm after 0600 UTC 6 August and became extratropical a few hours later about 830 NM east of Cape Race Newfoundland. The circulation of Alex was absorbed into a larger extratropical low by 0000 UTC 7 August". (Franklin, 2004)

GOES-12, 4 KM infrared imagery on 3 August 2004 at 07:08. Courtesy NOAA Satellite and Information Service

This case study deals with the meteorology of extratropical transition of Hurricane Alex.

The aim of this case study is to:

• Define ET
• Investigate the available conceptual models that may help explain the process of ET.
• Explore any similarities to and differences from conventional baroclinic cyclogenesis.
• Recognise and quantify the indicators within model fields that signal the likelihood of ET.
• Diagnose developments using real time satellite imagery, as well as other available data.

Pre-requisite knowledge:

Some basic understanding of the following would be of benefit, though reference materials will be available.

• Satellite imagery interpretation. “Manual of Synoptic Satellite Meteorology”.
• Hurricane formation, structure and climatology."NOAA"
• NWP analysis and forecast products.
• Plotted synoptic charts.

We will investigate 6 main threats of Extra Tropical Transition which are:

1. The system is accelerating and the track of extra tropical transitioning cyclones is inherently unpredictable
2. In Europe extra tropical transition cyclones bring weather more akin to winter but can actually occur in the summer months
3. Storm surges
4. High waves
5. Strong winds
6. Heavy rainfall

The process of extra tropical transition is not yet fully understood. This case study will explore the current level of understanding and theories.