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Location: Buckingham, Bucks | I, like many get excited when we hear that a plume could come our way. I know the spanish plume usually leads to some decent convective activity but why?
Is it something to do with unstable air converging with a hot airmass, or something similar? I am very keen to learn but as you all probably know my knowledge is VERY limited so I would be greatful if any explainations could be simplified somewhat.
Many thanks in advance  |
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Location: North Herefordshire. | Er... the warm moist air gets lifted as it moves northwards, then becomes unstable as an upper trough and/or dry tongue over runs it from the west. We often get higher based elevated storms from this set up I think. That's as far as my knowledge goes. And often lots of AcCas. June 28th last year was an example I think... edit: current models show storms breaking out Central Wales and W Midlands next Sat evening from this.
Edited by Howard Kirby 3/6/2006 15:05
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Location: Buckingham, Bucks | So is it simply a case of warm moist air colliding with dry air and shooting upwards? Does there have to be a large temperature difference between the moist and dry air? You will have to excuse my lack of knowledge.
Thanks for the reply by the way. |
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Location: North Herefordshire. | I think the dry air has to come above the warm moist air as it has a steeper lapse rate for the moist air to rise up into. Only then does it become unstable. If you imagine a surface based thunderstorm only the warm air has been imported and its all happening higher up and not due to whats happening down at our level, if you know what I mean. Its OK I'm no expert, its only what I've learnt from here and by looking at models. Someone else can probably explain this better than me....
Edited by Howard Kirby 3/6/2006 15:12
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Location: North Herefordshire. | Just found this... Spanish Plume: A catchy name applied to what is in reality quite a complex process producing the conditions necessary for severe local storms over maritime N.W. Europe. Strictly, the 'spanish plume' is the warm/dry ex-Saharan air, that has passed over the Iberian peninsula; been lifted by forced ascent (due to near-jet level forcing), cooling and moistening and producing outbreaks of thundery rain from medium level cloud. Initially providing a 'lid' (see 'loaded gun') which inhibits deep/vigorous convection, its breakdown allows the sudden release of potential instability, with the fuel for the subsequent severe storms being provided by air of a high theta-W value often running in from the SSE. Thunderstorms, often severe, are most likely within the tongue of highest theta-W air (> 18 degC or so), and where there are low-level forcing agents: e.g. isobaric troughing, sea breezes, coastal convergence etc. from here |
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Location: New Milton, Hampshire (55m) | Howard, that link to the glossary is great. Do you know how to get the rest up (A-R)? |
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Location: Buckingham, Bucks | Indeed a superb link. I can sort of understand how it works now. The moist air (if conditions are right) eventually 'bursts' it's way up through the dry air, decent convection starts at a rapid rate. I never thought dry air above warm moist air could act as a barrier as it were. I think I have grasped the concept.
Many thanks by the way. Truely educating |
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Location: Plymouth | Lightning Hunter - 3/6/2006 20:31
Howard, that link to the glossary is great. Do you know how to get the rest up (A-R)?
Tracking back through the URL, I have found the index page...
http://homepage.ntlworld.com/booty.weather/FAQ/Glostop.htm
Will
Plymouth |
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Location: North Herefordshire. | Paul Radon - 3/6/2006 20:42 Indeed a superb link. I can sort of understand how it works now. The moist air (if conditions are right) eventually 'bursts' it's way up through the dry air, decent convection starts at a rapid rate. I never thought dry air above warm moist air could act as a barrier as it were. I think I have grasped the concept. Many thanks by the way. Truely educating :) Its OK. I'll have to read it again myself I think!  P.S or here.... http://homepage.ntlworld.com/booty.weather/FAQ/subindex_FAQ.htm
Edited by Howard Kirby 3/6/2006 16:19
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Location: Middle Assendon, S. Oxfordshire, UK. 65m AMSL | Paul Radon - 2/6/2006 19:40I, like many get excited when we hear that a plume could come our way. I know the Spanish plume usually leads to some decent convective activity but why?Is it something to do with unstable air converging with a hot airmass, or something similar? I am very keen to learn but as you all probably know my knowledge is VERY limited so I would be greatful if any explainations could be simplified somewhat.Many thanks in advance :)
The "Spanish Plume" (i dont like the term- because the plume is not always or whole Spanish! among other reasons) is a warm season, warm sector phenomenon in which subtropical continental air is advected northwards by the warm conveyor of a mid latitude cyclone over the north Atlantic, from as far south as NW Africa across NW Europe (eventually into central Europe). Advection occurs along the (western flank) of an upper ridge extending northwards from NW Africa/the foreward (eastern flank) of an upper trough extendng south across the North Atlantic. The stronger the geopotential gradient along the forward flank of the upper trough, the stronger the warm air advection.
The significance of subtropical contiental airmasses for convective storm forecasting, is that the air being advected north wards is invariably warm and for the most part dry or at leaset initially dry within its lower levels. This is especially true if the air has originated over the Sahara. Importantly, the dryness fo the airmass allows it to remain umodified during its northward journey, by synotptic and orograpghic lifting.
At low levels the subtropical contiental airmass may contain what is know n as an elevated mixed layer. This is dry air that was formerly the convectively mixed but very dry boundary layer over a dry region, such as the Sahara or the Iberian Plateua. During ithe advection process, this dry former boundary layer air is synoptically lifted by and by orographic ascent over the mountains of North Africa, or the those of southern and northern Spain or all, from the surface (the point wehere it is lifted from the surface is the "dryline" BTW). Once lifted this dry boundary layer air becomes an elevated mixed layer.
The elevated mixed layer because it is warm and dry will act to inhibit deep moist convection from the boundary layer below, preventing deep mixing of the atmosphere. As a result, moisture evaporated from land or seas, or transpired from plants, cannot mix with the atmopshere above the EML. Asa result the boundary layer below the EML becomes increasing moist and because the skies are usually cloudless, very warm - creating a potentially conditionally unstable atmopshere in which the boundary layer parcels have high CAPE by virtue of the presence of commonly steep lapse rates within and commonly above the EML. This moist boundary layer invariably flows northwards below the capping inversion of the EML.
This CAPE will be released from the boundary layer, where and when the boundary layer becomes warm and moist enough that boundary layer parcels have enough positive bouyancy that they can rise through it. This may occur as a result of insolation heating the BL so that thes temperature of parcels therein become sufficiently high that they can rise through the EML. If these parcels are also very moist they will contientues to ascend as a result of the relese of latent thermal energy, creatign deep mosit convection and perhaps a convective storm. Alternatively, the same effect may be triggered by the influx of sufficiently warmer air from elsewhere (thermal advection). Alternatively, as the EML trvels north from itis origin it will gradually cool, where and when it has cooled sufficiently, boundary layer parecels may ascend through it. Alternatively, the EML may be cooled by the deep lifting of the atmosphere by synoptic processes, or by it s ascent over high ground. Dry air that is lifted will cool much more for the same amount of lift than moist air. Thus if the EML is lifted sufficiently this will eliminate the capping inversion at its base, releasing instability from the boundary layer. Alternatively, CAPE may be released from the boundary layer as a result of it becoming sufficiently moist, either gradually or by the influx of moister air from elsewhere (moisture advection). More often than not, CAPE is released from the BL by a combination of the above processes.
Often during the advection of subtropical contiental airmasses, the above scenario is complicated by the fact that not only is the dry boundary layer lifted as it is advected northwards, but also, the warm moist boundary layer that develops below it. When this happens the EML caps an elevated warm moist layer. CAPE can be released from the elevated warm moist layer just as it can from a capped warm moist boundary layer at the surface. This will result in eleavted deep moist convection (Ac castellanus) and convective storms (ACbs) that are elevated (ie their inflow is not from the boundary layer). Release of CAPE from an elevated warm moist layer can occour for exactly the same reasons as its from a boundary layer, ecept it is not normally triggered by insolation close to the site of convective activity (but it can be by insolation upstream over the source region).
However, Convective storms that develop from condtionally unstable subtropicalcontiental airmasses do =not= invariably produce severe weather. Ideally, for this to be likely requires three things. Firstly, a capping inversion at the base of the EML that is sufficiently strong that the requirements for the release of CAPE and initiation of deep moist convection will likely only be met in a few locations. Secondly, that lapse rates above the EML have be steep so that warm moist parcels that can ascend through it, acquire high positive bouyancy, above. For this to occur often requires the atmosphere above the EML to be dry and for it to then be subjected to strong deep lifting (remember dry air cools much faster than moist for the same amount of lift). Thirdly, that the release of siginificant CAPE occurs in a highly kinematic environment, exhibiting at least moderate (40kts) deep layer speed shear, and also preferably high speed shear in the lowest 1Km and high positive storm relative helicity there too. I would point out that all these requirements are rarely met over the UK, occuring on perhaps only a few days per decade. Instead more often than not, perhaps only one or two are met.
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Location: North Herefordshire. | That's a marvellous explanation Nick. Many thanks. I will print it out for my own education and reference assuming you don't mind. |
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Location: Buckingham, Bucks | Yes, a truely interesting read Nick
Thanks to yourself and Howard for the replies
Edited by Paul Radon 4/6/2006 10:18
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Location: Worcester | Nice explaintion . Looks intereseting next weekend  |
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Location: Reading | I agree - a good description. I also agree with the comments about calling plumes, "Spanish Plumes"...just because a plume of high theta-e air has moved from the south doesn't make it a "classic" SP. Any "plume" of warm, moist air can be destabilised (by a number of processes) to create showers or thunderstorms. |
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Location: Verdun Montreal Quebec Canada H4G 2A8 | Maybe Spanish plumes should be renamed Saharan plumes, after all doesn't sand or dust sometimes come down with the rain when the resulting storms are over NW Europe?
I was wondering, given the prevailing west to east circulation, whether areas like Greece and the Balkans and maybe Ukraine might be even more exposed to incursions of hot dry subtropical air from the Sahara than NW Europe is. Perhaps areas around the Black Sea could be particularly exposed to storms from plume events. |
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Spanish Plumes and thunderstorms
