SUMMER 2006 - part 2 (a): Destabilisation Alley!


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Into early July the heat built and built until BANG! the heat and humidity gave way to some torrential thunderstorms - Mid-Wales getting its fair share over four days that were difficult to forecast due to the complicated set-up.

Very hot weather in the UK is not just down to days of sunshine but to the flow north or north-eastwards of very hot air from the Continent. Under certain circumstances, this air may have originated as far to the south as the Sahara Desert.

The air's origin, and the weather in its area of origin, are both highly relevant factors with regard to how it behaves once it's arrived here, so it's worth looking at the mechanisms in a bit more detail, prior to the photos! Firstly we'll have a quick look at what aids/hinders the development of low-based thunderstorms. These are storms rooted in the Planetary Boundary Layer, that bit of the atmosphere no more than a kilometre or two thick at the most, where the air is modified by its interactions with the planet's surface:

In the above diagram, hot, dry, well-mixed air originating from the Sahara is lifted by mountains in North Africa and Spain, to form a layer aloft - a few thousand feet up - the Elevated Mixed Layer or EML. Because it's a little warmer, at first, than the Boundary Layer air below, this layer acts as a block - or cap - to any air trying to convectively rise. This serves to effectively isolate the Planetary Boundary Layer below from the rest of the atmosphere for a period of time. Within the Boundary Layer, heat builds up, and interaction with the surface adds moisture to that air - water evaporates, plants transpire, we all sweat and get irritable and so on. Moisture is of course essential to storm formation!

Southwards, a sharp humidity divide is found, to the north of which the air is increasingly humid and to the south of which the warm dry air reaches right down to the surface. This is called the dryline, and is more important in the USA E of the Rockies than it is here in the UK.

As the heat and humidity build within the Boundary Layer, air temperatures near to surface may exceed those of the EML. Eventually, and sometimes locally, air is warmed enough to rise through the EML and explosive convection then soars upwards. This is known as the cap "breaking". An analogy can be drawn with having a lid on a saucepan of boiling water, then moving it very slightly to one side. Steam billows up through the gap thus made for it. Other mechanisms, like convergence of low-level air or forcing upwards over mountains, may do for the cap in a similar way.

On other days, the cap may hold, despite the forecast for widespread storms. Strict adherents to charts showing CAPE (Convective Available Potential Energy) and Lifted Index values (chart below, for example) will therefore be disappointed. Such charts in fact show what CAPE can be released if the cap does break. If it doesn't - then no release of CAPE = no thunderstorms!

Often the cap fails to break because the amount of expected heating may not occur. A very common summertime mechanism (or "spanner in the works") can be due to elevated thunderstorms drifting into the UK overnight from France. Sometimes these are bunched together, forming an extensive area of heavy, thundery rain - a Mesoscale Convective System or MCS. Either way, what you get is a lot of moist, rain-cooled air - storm outflow as it's known. Dull, damp conditions under stratiform, cloudy skies result, and a lot of solar energy just gets used up "burning-off" the cloud, instead of heating the near-surface air. Such days can only be forecast on an hourly basis, looking for clear areas on satellite images - where storms are most likely to form.

On days where the cap is too strong, towering cumulus may be seen rising upwards and looking quite mean, but they "top out" against some invisible barrier. The hard-looking cauliflower-like tops flatten and become fluffy and the whole process fizzles. Yet on other days, rock-hard towers shoot up, form extensive clumps, grow in size sideways and upwards until the familiar dark, rumbling cumulonimbus storm-cloud has formed and the scene is set for spectacular - and sometimes severe - weather conditions. The cap's broken!

One scenario for severe weather development involves very high CAPE present at low levels but a strong cap above, that eventually breaks in just one or two places in the late afternoon when peak heating has been attained. Such storms have an awful lot of fuel available from the surrounding low-level air, and will, with decent wind-shear available aloft, organise into multicells capable of producing flash-flooding, large hail and severe wind-gusts. In exceptional shear conditions, supercells - organised very severe storms with persistent rotating updraughts or mesocyclones - can occur, although they are rare in the UK.

Now let's have a look at elevated thunderstorms - a very common member of the thunderstorm family seen every summer in the UK:

If rather moist air - originating, for example, from the seas west of Spain and/or North Africa, is lifted over the European mountains, what we end up with is an Elevated Moist Layer (diagram above). This time, because the warmth, moisture and thus potential instability is present high up, if a storm forms its cloudbase will appear to be much higher up - 10,000 feet or more. These storms are referred to as high-based and are often preceeded by high-up fields of small towers of cumulus cloud - Altocumulus Castellanus, known to most as "ACCAS". High-based storms take none of their fuel from the Boundary Layer, although because they dump lots of rainwater into it they can add to its moisture content and at the same time lower its temperature. On other occasions, rainfall may be intense under the storm-bases but light or non-existant at ground-level. This is because it evaporates as it falls through warm, dry air below.

Instability in these higher-based situations may be released by heating, by interaction with a cooler airmass causing lift or by the over-running of the area by a low-pressure trough, among other things. It's important to remember that those CAPE/Lifted Index charts need to be interpreted with caution with respect to elevated storms, as they depict values closer to the surface - and can thus miss significant CAPE/instability aloft!

On many occasions both elevated and low-based storms are potentially possible. Forecasters trying to untangle such headaches typically examine weather-balloon radiosonde ascents (soundings) taken upstream - i.e. in a SSW airflow, a sounding taken at 0600 at Camborne in Cornwall is sampling an air profile which is likely, given a certain forward speed, to have arrived over, say, Wales by midday. Obviously various processes may modify the profile during its northward journey but it does give a framework from which elements of a convective forecast may be based. It is usually possible to work out how hot and humid it needs to get for Boundary Layer rooted storms to fire up and how severe they are likely to be.

The period July 2-6 was one such episode, with spectacular storms across Mid-Wales on some days and nothing on others. The following images on this and the next page follow the fates and fortunes of this period - a forecaster's nightmare, but fascinating all the same!

July 2 began with rather unexpected overnight elevated thunderstorms drifting up across the SW. Outflow from these resulted in a lot of murky cloud which only started to move off N by lunchtime. Temperatures shot up in the warm sunshine - would they be enough to bust through the cap?

Early afternoon - July 2nd. Up on top of the mountain road - will it fire? Remains of outflow to L, clear skies to R....

Not looking very likely despite the sea-breeze!

Maybe... mid afternoon overlooking Llyn Clywedog.

This lot all fizzled out again unfortunately!

Finally it fired off at around 2030! This is looking N from Melin Byrhedyn. Chase on! Best bet seemed to be head back up to the top of the Machynlleth-Llanidloes mountain road....

...where the setting sun was lighting up the tops of the developing towers...

Nice! Worth the evening out, this lot!

The Cb to the R gave several good long rumbles of thunder......


...whilst to the north a strange lenticular-like cloud sat squarely over the top of a collapsing updraught tower....


This one looked as though it had a halo!


The setting sun brilliantly lit the thunderstorm cumulonimbus cloud, even after the sun's illumination had left the lower slopes....



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