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Nick F

Guide To ..... Uk Thunderstorms Setups

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There are three basic types of thunderstorms that the UK may encounter at anytime of the year:

Single Cell Storms

A single-cell thunderstorm forms when there is little wind shear (neither in direction nor strength) with height. The storm will be poorly organized, and without the slant of the downdraft from wind shear away from the updraft, the downdraft of cool air and precipitation, cuts off the updraft and terminates the storm growth cycle. A single cell storm will typically last around 30 to 50 minutes. If there is windshear with altitude, the storm usually becomes multicellular, having several growth cycles and lasting longer.

Multicell Storms

Multicell thunderstorms are likely to form when wind shear is stronger, they usually consist of multiple convective updrafts and downdrafts. Multicell storms form when new convective cells develop along the boundary of a cold pool from the downdraft on the downshear side of an old cell. In severe multicell storms, there are usually several cells active at any one time, and a storm complex may develop called a Mesoscale Convective System or MCS which is an area of deep convective cloud that has an anvil which can be several hundred kms across in diameter. These 'storm complexes' can sometimes affect the UK in summer during Spanish Plume set-ups, usually drifting up from France. A MCS can also, under the right conditions, develop supercell characteristics aswell.

Supercell Storms

These tend to be a rare visitor to the UK, but are not unkown to occur most years under the right conditions. The supercell is the most severe of all thunderstorms. It can form out of a generic multicell storm, if there is enough directional windshear in the atmosphere to slant the updraft and separate the cool downdraft from the updraft. The main updraft can strengthen and start rotating due to the air vorticity and mesoscale inflow/outflow pattern (mesocyclone); rotation in a storm is an indicator for supercellular behaviour. Supercells create their own small low-pressure systems with fronts (usually in the form of a flanking line and gustfront). They can last several hours and spawn tornadoes, wall clouds and drop very large hail (in rare cases up to 10 cm or 4"). Supercells in the UK normally form in summer when warm moist air at low-levels is over run by colder drier air aloft, usually in the vicinity of a cold front moving in from the West, and also with a strong jet aloft creating strong vertical wind shear .

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THUNDERSTORM SET-UPS IN THE UK …

In a 'Spanish Plume'…

During the summer, when low pressure stalls in the Atlantic to the West or NW of the UK and high pressure resides to the East, warm and moist air of high Theta-W (Wet-bulb potential temperature) of up to 20C can be advected N or NE at low-levels and mid-levels across the UK from France either from the Mediterranean or from the Atlantic Bay of Biscay area (see below). This warm moist air is often forced to ascend if it arrives from the South by the mountains of Spain and France and also by hot dry low-level air heated by hot sunshine being fed from the SE from central Europe. This often leads to mid-level instability, especially when a mid-level trough passes through. Altocumulus castellanus are often a sign of this mid-level instability and these can grow into Cumulonimbus clouds to great heights given enough heat and forcing. The rain from these high-based mid-level storms sometimes doesn't reach the ground because it evaporates in the warm dry air beneath.

Sometimes, when warm moist air has been feeding in across the UK for a few days from the Atlantic Bay of Biscay or further SW from the azores - warm moist air of high Theta-W values begins to stagnate at low-levels. Intense insolation over the Spanish Plateau causes a thermal low to develop over the Iberian Peninsula - this advects elevated warm dry off the plateau which then drifts North towards the UK above the warm and moist air at low-levels. Initially this warm dry air above the high Theta-W air nearer the surface forms a stable layer 'lid' causing convective inhibition (CIN). However, large amounts of Convective Available Potential Energy (CAPE) builds up as a result, to form a 'loaded-gun' situation or conditional instability. Then the final stage for this convective energy to be released into Thunderstoms, is some strong ground heating from the sun and some sort of suitable triggering, this normally happens as a result of a forcing mechanism, either from convergence of low level flows; forcing from short wave troughs; or forcing from cold fronts - these will be explained later on.

Spanish plume thunderstorms often form over France in the late afternoon, during peak temperatures of the day. These storms then push N or NE and cross the channel after dark, and Nocturnal cooling of the cloud tops/storm anvil further intensifies the storms that arrive overnight by steepening mid-level lapse rates and destabilising warm moist air present. Continuous advection of warm moist air in the mid-levels from the South during a Spanish Plume keep these thunderstorms 'fuelled' throughout the night over the UK as it destabilises - this process is more technically known as Differential Thermal Advection

The Dynamics of a Spanish Plume:

Mid-level instability

Midlevelinst.gif

Loaded 'gun' conditional instability

SpanishPL.gif

An example of a chart showing a plume of high Theta-W/E values(black contours):

WBPT2.jpg

Thunderstorms often form in the areas of high Theta-w values if there's a sufficient trigger such as a trough or cold front.

In a Tropical Maritime airmass …

The Spanish plume is not always needed to trigger severe thunderstorms over the UK. If there is a good supply of warm and moist air feeding off the Atlantic from the SW, and a trigger from a passing upper short-wave trough, cold front or from orographic forcing (uplift over mountains) then deep convection can occur, especially in spring and summer with the help of strong sunshine. A good example of this was on 17th July 2004. That day saw a SW/W'erly airstream across the South brought by a shallow secondary Atlantic low crossing the UK. The cold front passed through in the afternoon as the low moved into the N Sea and triggered Thunderstorms across London and East Anglia, one storm dropped golf ball size hail over SE Essex before it moved into the North Sea, where it went on to develop supercell characteristics as it hit Holland.

Synoptic situation 00z 18th July 2004:

18072004.gif

In a Polar airmass …

These can form at any time of year on the poleward side of the polar front in either a Polar Maritime air (pM), Polar Arctic air (pA), Winter only Continental Polar air (cP) or Returning Polar Maritime air (rpM). The redeeming feature of these airmasses is cold air aloft, and if warm moist air is available at the surface either from surface heating of moist air in spring/summer or moisture and warmth picked up over the sea during the winter, then convection is likely due to parcels of relatively warm air from the surface rising upwards in the cold air aloft.

Late March to early October

During this period, the sea temperatures around the UK are likely to be similar in temperature or cooler than cold polar crossing it, so convection over maritime areas tends to be less likely. However during April, the sun becomes noticeably stronger and therefore tends to heat the ground more readily -and with cold polar aloft, steep lapse rates (drop of temperature with height) are likely which allows parcels of warm moist air from the surface to rise upwards over inland areas and form cumulus and cumulonimbus clouds. Polar airstreams are fairly common in spring though become less common in summer, though not unkown. Thunderstorms that form in polar airmasses tend to be non-severe and short in duration generally - but with cold dry air aloft a common feature, hail often accompanies storms.

Late October to Early March

This 'winter' period is when polar airmasses are likely to be colder than sea areas they cross, therefore the sea surface 'warms' the surface air layer causing parcels of air to rise up through the colder polar air above it and form cumulus and cumulonimbus clouds. Coastal areas of the UK during the winter see a higher frequency of thunderstorms than further inland, as the weaker sun during the period has little effect at warming the ground. NW, N, NE, E winds , and even W and SW winds via returning polar maritime air circulating around lows to the West commonly bring heavy showers of hail, sleet and snow accompanied by thunder to coastal areas of the UK in winter if cloud tops reach high enough. Sometimes, if the temperature gradient along a cold front is steep enough along cold fronts during winter thunder may accompany rain or even snow over inland areas aswell.

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TRIGGER MECHANISMS FOR UK THUNDERSTORMS

Convergence -either from a sea breeze front or thermal wind or moisture confluence.

An example of sea-breeze type convergence type thunderstorms happened over Southern England on the afternoon and evening of May 7th 2000. The afternoon of the 7th saw a slack surface NE'ly airstream across much of SE England. However, sea breezes developed from the South coast and Thames Estuary, and as a result, a sea breeze front developed along the South Coast, and with temps reaching 22-24C to the North of the front, thunderstorms began to break out over Hampshire around 1400z along this convergence line of sea breezes. The outflow of these storms along the South Coast combined with the movement of the sea breeze front Northwards triggered thunderstoms further North causing an intense storm over Bracknell. 65mm fell in an hour at Beaufort Park, Bracknell at the height of the storm.

This is the radar image for that day:

http://www.metoffice.com/climate/uk/intere...ar7may2000.html

Synoptic analysis shows a mesoscale trough and sea breeze front across Southern England causing convergence and uplift:

SeaBFront.gif

An account of the Bracknell storm can be seen here:

http://www.metoffice.com/climate/uk/intere...torm.html#synop

Short-wave troughs

Positive Vorticity Advection PVA ahead of a trough forces warm moist air upwards. These troughs normally are the result of upper atmospheric disturbances downstream of cold fronts/longwave troughs and are often found in association with jet streaks and/or cold air advection above the 500mb level.

An example of thunderstorms caused by a short wave trough were the severe storms which broke out over Southern and Central England on August 3rd 2004. With low pressure to the NW of the UK and high pressure over Scandinavia, a warm and moist air flow with high theta-W values was being advected North from France. Severe thunderstorms had already been rumbling away over the Midlands at breakfast time, these slowly died away leaving mid-level cloud across the North. Further South, the skies cleared, leading to some very warm sunshine in a light Southerly breeze. Temperatures rose to over 27C by the early afternoon, and this was likely to break the break the 'cap' of warm moist air around 800 mb, suggested by morning radiosonde ascents over the South, and with a suitable trigger cause intense convection. The 12z Herstmonceux (E Sussex) sounding (below) suggested that cloud tops could reach 12,000m (38,000ft) or 200mb with cloud tops over -50C. The trigger or forcing for the convection that afternoon came from an upper short-wave trough that moved NNE from Northern France, PVA was created ahead of it leading to a rapid release of CAPE and an explosion of intense thunderstorms to break out Northwards from the M4 corridor between Bristol and West London. The 12z soundings that day also revealed dry mid-level air (see Herstmonceux sounding below) - which is well known to assist hail formation, and indeed there were reports of large hail and frequent lightning. High Wycombe in Buckinghamshire recorded 42.4mm in just over 40 minutes during the storms. The storms merged to form an MCS as they moved North into the Midlands.

Synoptic situation at 00z showing a trough over France which would then later move North over S England and intiate PVA to cause intense convection:

03082004.gif

12z Herstmonceux Sounding showing the 'cap' erosion would lead to cloud tops reaching 200 mb with >600 j/kg CAPE provided there is a forcing mechanism - this arrived along a short-wave trough that afternoon:

Herst03080412zb.gif

Cold Front/Long wave-troughs - The arrival of a cold front from the Atlantic intensifies the thermal gradient strengthening the wind and wind shear, intensifies the warm air advection (WAA) and ascent of warm moist air ahead of the front. Over-running of cold air at upper levels further intensifies the instability. Sudden changes in wind direction (directional shear) at low-levels along a cold front causes much helicity and combined with strong deep layer shear can result in tornadoes.

Example 1. The 22nd July 2004 saw a slack Southerly flow across England and Wales bringing warm and moist air from the continent, while a cold front to the West was moving East into Wales and the West Country during the afternoon, as shown on the synoptic chart at 1800 hrs (below):

22072004.gif

Thunderstorms broke out in the afternoon ahead of the cold front initially from a single cell over Bristol which developed further cells and formed a MCS which tracked NE towards the SW Midlands. It produced localised flooding in it's path, with 28mm falling at Lyneham up to 1800hrs, near Stow-in-the-Wold. The system reached its peak near Daventry, Northants - where marble sized hail was reported and a tornado observed around 1600hrs in a nearby village. The storm exited over the North Sea by 1900hrs.

Example 2. The 28th January 2004 saw a classic example of 'thundersnow' over much of the UK. This event was brought about by sharp temperature contrasts either side of a cold front which was moving South from Scotland during the day. Ahead of or to the South of the cold front was a broad moist Westerly flow which brought temperatures as high as 6C in the afternoon in the South with cloud and rain. However, to the North of the cold front was much colder and drier air below 0C, and its rapid movement South forced the relatively mild and moist air upwards into towering cumulonimbus clouds. As a consequence, a band of intense heavy and squally snowfall was accompanied by thunder and lightning over many areas as the cold front moved South. The temperature behind the cold front dropped to -1C widely in the early evening, causing rush-hour chaos as up to 10-15cm of snow fell in places in the space of an hour.

Synoptic chart for 28th Jan 2004:

20040128.gif

Sferics (lightning) plot for 28th Jan 2004:

Rsfloc20040128B.gif

Warm air advection/Warm front

Not all summer thunderstorms form in association with a cold front or a trough, and the next example is of more unusual 'warm front' thunderstorm, where thunderstorms are triggered by warm air pushing up from the continent colliding with colder air from the North or North East and being forced to rise.

The afternoon and evening of May 9th 2001 was overcast, grey and cool in SE England with stratus lapping off the North Sea in a cool ENE airstream. However, to the South over France a warm and moist plume was steadily being advected Northwards. As the front of the warm air collided along a convergence line with the cool air that had been feeding in from the NE at the surface over the Dover Straits and SW North Sea, the warm moist air was forced to rise and rapidly built into towering cumulonimbus cells around Calais in the late evening. Further cells then rapidly multiplied NW across SE England around midnight with warm air continuing to be forced aloft by cooler air flowing in from the N Sea at the surface. These cells produce spectacular lightning displays and merged to form an MCS which practically engulfed the SE corner. As the storms cleared NW, instead of the air becoming fresher, the air turned more humid, and the following day saw a sticky 25C reached compared to the cool day previous. The funny thing was, the Met Office forecasters were rather caught out and left with red faces as they had predicted a small chance of some thundery showers for the South that evening. They didn't forsee a rapid explosion of severe thunderstorms and blamed it on cool air from the North Sea colliding with warm air off the continent.

Synoptic situation at 00z 10th May 2001:

10052001.gif

Sferics (lightning) for 9th May 2001 show intense lightning activity across SE:

Rsfloc20010509b.gif

Orographic forcing

Convection over mountainous areas of the UK in Wales Northern England and Scotland is quite a common feature, particularly in spring and summer. Basically warm moist air from the Atlantic is forced to rise up over mountains, and as it cools when rising it condenses and forms cumulus and cumulonimbus clouds. Such convection is further enhanced by strong solar heating in sheltered valleys which concentrates rising updrafts without mixing from general surface winds. Orographic forcing has been shown to occur over more modest hilly areas even in the South above 100m. In still conditions in summer with little wind, subtle rises in the land can help warm air to rise and continuely feed warm moist air upwards into Storm clouds which may be in situ above.

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Absolutely Fantastic read Nick. Thank you for all the time you put into that and Boy was that a Storm on Saturday 17th July 2004!! My best in over 5 years here in South East Essex.

Many Thanks

Paul Sherman

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Well done Nick ,clearly explained and covers just about everything.

We just need to get a glossary of thunderstorm terms like RFD,wall cloud,Debris Cloud,gustanado,inflowtails etc together although the following link does a pretty good job.

Photographic Glossary

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