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:
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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:
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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?
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Early
afternoon - July 2nd. Up on top of the mountain
road - will it fire? Remains of outflow
to L, clear skies to R....
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Not looking
very likely despite the sea-breeze!
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Maybe... mid afternoon overlooking Llyn Clywedog.
This lot all fizzled out again unfortunately!
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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....
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...where the setting sun was lighting up the tops
of the developing towers...
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Nice!
Worth the evening out, this lot!
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The Cb to the R gave several good long rumbles of
thunder......
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...whilst to the north a strange lenticular-like
cloud sat squarely over the top of a collapsing
updraught tower....
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This one looked as though it had a halo!
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The setting sun brilliantly lit the thunderstorm
cumulonimbus cloud, even after the sun's
illumination had left the lower slopes....
PART
2 |
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