Tropical Depression One formed on the 31st July, 2004 in the western Atlantic at 30.6N 78.6W, approximately 500 KM east of the Georgian coast and with a central pressure of 1010 hPa.
Tropical cyclones that develop in the North Atlantic Ocean are steered by the dominant flow circulating the sub-tropical Azores high pressure belt.
Moving slowly in a northwesterly direction and deepening, it became a Tropical Storm on the 1 st August. Turning to the northeast and intensifying further, Alex achieved hurricane status on the 3 rd, slightly earlier than average, brushing the north-eastern United States at Cape Hatteras . It reached a maximum depth of 957 hPa on the 5th which it maintained as far north as 42 N. Maximum sustained winds reached 105 KT.
Although tropical cyclones generally weaken as they move pole ward due to increased wind shear, lower sea surface temperatures and lower tropospheric humidities / landfall, of the average 40% of tropical cyclones in the Atlantic Basin that undergo ET about half develop explosively into extra-tropical depressions. Low latitude tropical cyclones are more likely to re-intensify after transition than those that develop in the sub-tropics. Due to the higher sea temperatures and their longer track in the deep tropics they have more time to develop and are charcterised by lower surface pressures. For this reason they can also spend longer in the transitional phase than less intense storms and as a result are more likely to enter an area of favourable development. Hart and Evans (2001).
The time of year is important because of the related factors of sea surface temperatures, which are highest in September, and the strength of the mid-latitude“westerlies”, which generally increase through the season. The time of highest sea temperatures frequently coincides with the peak in tropical cyclone activity, when re-intensification after transition is most likely to occur at higher latitudes. The latitude of transition increases from 30-35 degs N early and late season to 40-50 N mid season, in response to the rise and fall of sea surface temperatures / jet stream movement and intensity.
By the 5 th August, Alex was beginning to accelerate away to the northeast.
Phase 2 - Transformation - 6 th August, 2004
Characteristics
The time of transformation can be difficult to forecast accurately several days in advance, principally due to the uncertainty of the tropical cyclone track over the same timescale.
It can also be difficult to model because of :-
a) the structural unknowns, the cyclone having originated and tracked over oceanic/data sparse areas.
b) the rapid changes in this structure that occur during the transformation phase.
c) the contrast between a relatively compact, self-contained cyclone and an expanding, interacting mid-latitude depression.
d) Some of the physical processes of the ET process are as yet still uncertain.
During transformation the cyclone is accelerating under the increasing influence of the mid-latitude “westerlies”. These begin to strengthen from the end of July/early August, in response to high latitude radiative cooling and the resultant increase in thermal contrast. They then continue to strengthen through the duration of the hurricane season, approaching peak values at the end of the season as winter approaches. If this jet is too strong the organised convective cell structure / PV tower will rapidly break down due to vertical wind shear before the storm can interact with potential sources of development, such as an upper trough.
If the jet is too weak the hurricane will move only slowly over colder ocean temperatures. The tropical cyclone’s energy source is cut off and any remaining structure then dissipates before it reaches a development area. At the same time, the advection of the warm core ( low level +PV / high level –PV) over progressively lower sea surface temperatures moves the storm into, and further enhances, a region of thermal contrast / baroclinicity with a resultant increase in tropospheric gradient and jet stream strength.
Study the area of transition on appropriate satellite imagery.
Phase 3 – Re-intensification - 7 th August, 2004
If the developmental criteria have been met, Phase 3 of the process is when Re-intensification occurs. Many of the characteristics of cyclogenesis are then a familiar feature of mid-latitude development, although remnants of the tropical cyclone can still add some degree of uncertainty, as can the interaction between a transitioning cyclone and existing frontal features.
Characteristics
If the PV anomaly associated with the storm core is weaker, this would have the effect of decreasing the downstream divergence and upstream convergence associated with the upper trough. The weaker trough would then produce a more zonal flow, thereby delaying storm / trough interaction and with resultant cyclogenesis timing, tracking and detail differences.
If the steering flow is more zonal, perhaps also in association with a high North Atlantic Ascillation (NAO) index, the resultant extra-tropical cyclone will head eastwards towards Europe . If the flow is meridional, as with a low NAO index, it is more likely to be steered in a more northerly direction parallel to western Europe .
Recent studies have suggested that the most important factors for rapid cyclogenesis of a transitioning cyclone are the 4–dimensional structure of :-
Harr and Elsbury (2000), McTaggart – Cowan et al (2001), A Agusti-Panareda, CD Thorncroft, G C Crag, S L Gray (2002-2003)
Alex transited in a zonal westerly flow before re-curving northwards during cyclogenesis to the west of Ireland .
Heavy and prolonged rain on slow moving occlusion linked to tropical origin of air. A prolonged spell of unsettled weather and associated flooding, Wittering for example recording 90.8 mm in the 24 hours up to 0900 hrs on the 10 th, Pembrey Sands had 44.2 mm in 3 hours on the 12 th.
Main Characteristics of Transition:-
Oceanographical Aspects of ET
The occurrence of heavy seas and large swell through the autumn, winter and early spring seasons is a regular feature of mid-latitude weather, well documented and readily appreciated.
What can make the transition of a tropical cyclone important is that energy levels associated with these “winter” conditions can occasionally, and suddenly, occur in the late summer. They typically occur during a spell of relatively quiet weather and often after a lengthy period of low or even flat sea states. The holiday season is in full swing, coastal populations both on land and at sea are high, beaches are crowded but hazard awareness is often low.
The acceleration of a transitioning tropical cyclone can allow the accompanying wind field to move and increase at a speed in phase with the growth of the ocean waves it is generating, variously called dynamic, trapped or phase linked fetch.
Swell generally travels along the shortest route between 2 points on the Earth’s surface; great circles. The track of transitioning Atlantic tropical cyclones can occasionally follow a great circle for some distance. These are often the same great circles that radiate out from the coast of western Europe into the transitioning zone, which is one of the main reasons dynamic fetch waves affect this coastline.
The "Pink Lady" whilst crossing the Atlantic was hit by 9 m waves and was badly damaged and the crew had to be rescued.
Lundy Wave Buoy Data Wave Height Wave Period
The diagrams above graphically indicate how quickly the swell front from Alex arrived, in this case recorded in deep water at the Lundy Wave buoy off the west coast of Devon. The actual wave heights seen on Fistral Beach, Newquay increased from approx. 0.5 m to 1.5 m between 1600 and 1800 Z. Whilst the measured deep water wave heights from the Lundy Buoy indicate only a slight increase in size at this time, the period increased from a modest 9 seconds to a far more energetic 15 seconds, with the result that wave heights on the beach increased by a factor of 3 over the same time scale.
This sort of increase can rapidly generate rip-currents, cause difficulties of exit from enclosed beaches, especially if it coincides with an incoming tide, and can pose a threat to pleasure craft or divers as the swell begins to break over formally "hidden" reefs.
More dramatic instances than that of Alex have certainly occurred. The swell from ex-hurricane Irene in 1998 increased from 1m to 4m within an hour, resulting in an air sea rescue operation in Cornwall.
Fatalities around the UK coast could be linked to the arrival of swell fronts.