Met Office College - Course Notes
Cumulonimbus clouds and severe
storms
Contents
1.
Introduction
2.
The parcel theory
3.
Single-cell clouds
3.1
3.2
4.
Conditions of zero wind shear.
Conditions of uni-directional shear
Multicell storms
4.1
4.2
Gust fronts
Daughter cells
5.
Squall lines
6.
Supercells
7.
Large hail and tornadoes
7.1
7.2
Large hail
Tornadoes
8. Further reading
Appendix A Severe convection over the USA and UK
Appendix B Lightning
Further reading
 Crown Copyright. Permission to quote from this document must be obtained from The
Principal, Met Office College
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1. Introduction
When an upward impetus is given to a parcel of air in an unstable
environment, it will continue to rise for as long as it is warmer than its
environment. The initial upward motion is commonly provided by
buoyancy resulting from heating through contact with a warm surface,
but may also arise from convergence on a range of scales, mass ascent or
orographic forcing.
The upwardly mobile parcel will cool at the dry adiabatic lapse rate, the
work done in expanding resulting in a reduction in temperature. As the
parcel cools it loses its capacity to contain water vapour, and once below
its dewpoint condensation will start to occur onto suitable atmospheric
aerosol. This slows the rate of cooling by the addition of latent heat,
giving an extra boost to the buoyancy force.
The cloud is composed principally of water, but some droplets may
freeze, depending on the temperature and the number of suitable ice
nuclei present. In general there will be a small minority of ice particles
once the temperature falls below about -100C. If the parcel continues to
rise and cool, more ice nuclei will become active and a larger proportion
of cloud particles will be composed of ice. The proportion of ice particles
in the cloud will also tend to increase through the growth of the particles
by deposition (Bergeron-Findeisen), some growing large enough to
develop appreciable fall speeds and continue their growth by colliding
and merging with other cloud particles.
At some temperature a significant proportion of the cloud particles in
our parcel of air will be composed of ice crystals. This may occur at a
temperature of around -200C depending on conditions, and marks the
transition of the cloud from a large cumulus to a cumulonimbus. At
around -400C virtually all the cloud particles will be composed of ice.
Because ice particles evaporate more slowly than water droplets, they
survive longer as they mix into the dry ambient air that surrounds the
cloud. This results in the diffuse look to the cloud boundary that is
characteristic of cumulonimbus cloud.
Because such clouds are frequently associated with locally heavy
precipitation, strong up and downdraughts and lightning, they are
important phenomena to be able to understand and forecast. In their
extreme forms they may also give rise to damaging hailstones, strong
winds and tornadoes.
2. The parcel theory
To attempt to quantify the thermodynamics of cumulonimbus clouds
and at the same time present a simple model of the processes which
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occur, we may use a thermodynamic diagram to plot the progress of a
parcel of air rising through its environment, as shown in Figure 1. All
the time the parcel curve is to the right of the environment curve,
buoyancy forces are acting to increase its vertical velocity. The energy
gained by the parcel, represented by the shaded area below A, is
converted into kinetic energy. As it passes A, a negative buoyancy force
comes into play, decelerating the parcel until it comes to rest at B and
sinks
ELR
=constant
Figure 1 Parcel theory.
back down. The two shaded regions in this idealised model have the
same areas, the lower area being referred to as Convectively Available
Potential Energy, or CAPE for short.
This simplified model assumes frictionless and adiabatic flow. In reality
the parcel of air is not completely insulated from the environment and
entrainment reduces its buoyancy by diluting it, reducing its moisture
and buoyancy. At the same time the momentum of the updraught is
reduced by small-scale turbulent motions occurring around the edges of
the rising parcel. Point A is rarely passed (except in severe organised
convection), and the parcel generally loses its buoyancy at some lower
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level. The larger the cloud the more realistic the adiabatic assumption is
larger clouds will tend to rise higher since they contain parcels of air
near their centres which are protected against entrainment of dry air by
those nearer the cloud edges.
-
Another important factor is the interaction of the cloud microphysical
processes. As cloud particles grow into precipitation particles, they fall.
As they fall they exert a downward drag on the rising air within the
cloud. Where melting or evaporation of the precipitate occurs there is
cooling of the air by latent heat exchange. Both these factors lead to a
precipitation-induced downdraught that acts against the updraught.
Sophisticated numerical models have been formulated to study the
structure and life cycle of convective clouds. They are non-hydrostatic,
because vertical acceleration is something that cannot be ignored, and
they parameterise the microphysical processes that govern the latent
heat exchanges so important in driving much of the motion.
Conceptual models have been constructed using results from such
models together with observations taken in the real atmosphere, and
these will be described below.
3. Single-cell clouds
CAPE indicates how much energy there is waiting to be released by
convective activity, and generally the larger the CAPE the bigger the
storm. However, the energy may be released in many small convective
clouds or a few large ones. It is the interaction between the updraught
and the downdraught within the clouds that determines their longevity,
and in turn their longevity that determines their size and severity. To
decide how the updraught and downdraught will interact we must
consider the vertical wind structure through the storm environment.
3.1 Conditions of zero wind shear.
In conditions of zero vertical wind shear, i.e. no change of wind with
height, air parcels will rise vertically. Precipitation will form in the cloud
and start to fall when its fall speed exceeds the updraught speed. As it
falls and grows by accretion it will exert a drag on the updraught
feeding the cloud, weakening it. Further reduction in updraught
strength will occur as precipitation melts or, below the cloud base,
evaporates, as these processes will extract heat from the air, cooling it.
At some stage the developing downdraught will swamp the updraught.
Because there is no vertical shear in the cloud the downdraught occurs
over the core of the updraught and acts directly against it. Once the
downdraught reaches the surface, generally around the commencement
of precipitation, the inflow of warm air to the cloud is cut off and the
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cloud rapidly decays. The process is self-destructive and self-limiting,
taking about 30 minutes to grow and 30 minutes to dissipate.
3.2 Conditions of uni-directional shear
Convective clouds tend to move with the environmental wind at some
mid level. This steering level as a general rule occurs roughly a third of
the way up the cloud, and may be calculated as:
Steering level = Height of base + Height of top – Height of base
3
When considering wind flow in the cloud environment it is useful to
take wind velocities relative to the moving system rather than to some
fixed point on the earth’s surface. Figure 2 shows a cloud that has
developed in an environment of uni-directional shear. In this
configuration the updraught slopes along the shear, and the
precipitation-induced downdraught is removed from a position directly
above the updraught.
Figure 2 Cumulonimbus cloud in uni-directional shear
Winds relative to
earth’s surface
Winds relative to
cloud motion
Inflow
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Thus the downdraught does not initially interfere with the
downdraught. However, on reaching the surface the down-draught will
spread out and cut off the supply of warm moist surface air that is
feeding the updraught in this case the system is longer-lived but still
self limiting. With clouds in such an environment the active growing
stage may last as much as an hour before the dissipation stage sets in.
However, if the shear is strong, the cloud organisation may be destroyed
and convection thus limited.
-
4. Multicell storms
If winds change direction with height a different configuration of up
and downdraughts is possible. With directional shear the downdraught
falls to one side of the inflow feeding the updraught. If in Figure 2, for
instance, there were an increasing component of wind with height out of
the diagram added to that depicted in the plane of the diagram,
the downdraught would not fall directly into the inflow, but would hit
the ground first on the right-hand side of the direction of travel, before
spreading out laterally and eventually cutting off the inflow (Figure. 3).
This increases the life span of the cloud further still, allowing more
precipitation to form in the updraught and thus a stronger
downdraught to develop.
Figure 3 Cumulonimbus cloud in directional shear
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4.1 Gust fronts
When air from such a well-developed downdraught spreads out it
travels through a warm environment as a cold density current, rather
like a mini cold front. At the leading edge of the front there is
convergence and uplift of the warm moist boundary layer air, as shown
in Figure 4. At the surface the passage of a gust front is marked by a
sudden increase in wind and very often a change in direction. Wind
vectors across a gust front commonly change by 10 ms-1 over a distance
of a few hundred metres and may reach 30 ms-1 or so.
Figure 4 Leading edge of a gust front
Temperature commonly falls by several degrees C and pressure may
rise by up to 4 hPa. On average the gust front extends about 5 km ahead
of the precipitation area, so very often it is a precursor to rain, and can
still be recognisable after it has travelled a distance of 30 km or more.
Downdraughts hitting the ground are referred to in aviation circles as
microbursts and are hazardous to aircraft. A microburst occurring
directly above an aircraft will act to accelerate it towards the ground.
Also, an aircraft flying through a region of sudden changes in horizontal
wind associated with the spreading out of the downdraught will
experience sudden changes in airspeed which may induce stalling or a
reduction in lift. These effects have led to aircraft crashes, particularly
close to take-off or landing.
The convergence at the leading edge of a gust front may provide the trigger
action to initiate a new convective cell, particularly where it is enhanced by
environmental flow in the opposite direction or where two such gust fronts
from different clouds meet.
4.2 Daughter cells
Considering the case shown in Figure 3, it can be seen that the
maximum convergence around the gust front will occur on the northeast
side of the cloud. Here a new convective cell is likely to develop and
take over from the old one, as the source of air supplying its updraught
is cut off by its spreading downdraught. This new cell is termed the
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daughter cell, as it has been spawned from the original, or parent, cell.
This daughter cell is likely in turn to follow a similar life cycle giving
(b)
(a)
(c)
Figure 5 (a) Multi-cell storm travelling to right of individual cells.(b) Wind
environment for multicell storm represented on a hodograph. (c) Configuration
of up and downdraughts from consideration of system-relative winds
rise to its own daughter cell on its northeast side before dissipating.
Thus the storm, i.e. the area of maximum convective activity as tracked
for instance by radar, moves to the right of the individual cell motion,
typically by about 20 or 30 degrees (Figure 5(a)).
In Figure 5(b) a hodograph shows the environmental winds at low,
medium and high levels corresponding to Figure 3, showing typically
large changes of wind direction with height. Taking the mid-level flow
as the steering level, system-relative winds may be derived by
considering wind plots relative to this. This gives a low-level inflow
from the northeast and a high level outflow from the west-southwest, as
shown in Figure 5(c). The downdraught then falls to the east-northeast
of the cloud, and it is here that the daughter cell forms, where the gust
front converges with the low-level flow.
If winds veer with height, the storm will propagate to the right, and this
is the commonest case. However, if winds back with height, the storm
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will propagate to the left. Notice that with these multi-cell storms no
external trigger action is required after the initial convective
development, and storms that are initiated by daytime heating, for
instance, may continue into the night.
Very large point rainfall totals can occur if the lifetime of each cell is
short and regeneration occurs over the same area. This occurred during
the so-called Hampstead Storm in 1975, when cells formed to the south
of London and moved northwards as they developed, maturing over
north London before dissipating and generating a daughter cell to the
south. This happened 5 or 6 times, giving over 150 mm of rain within a
few hours in Hampstead.
5. Squall lines
In the multi-cell conceptual model considered so far, the cumulonimbus
cells are characterised by having updraughts that slope down-shear with
height, that is in the direction of the wind shear. However, other
configurations are possible where the updraught slopes up-shear, i.e.
against the direction of the wind shear, and these give rise to a more
organised internal circulation, shown two-dimensionally in Figure 6.
Lines of cumulonimbus clouds sometimes form with such a flow
pattern, and these are known as squall lines. They are typically about 30
km wide and 200 km long. Often there is some capping inversion at low
levels in the pre-storm environment that prevents widespread
development of convection and spreading up of high w surface air.
As these systems advance, the gust front forces this low-level air up
above the inversion to the level of free convection where it joins the
updraught. These lines in effect suck up warm surface air as they
overtake it, transporting it to the high troposphere whilst at the same
time bringing down cooler air from aloft behind them. Thus the centre of
gravity of a column of air is lowered by the passage of such a system
and the environment is left considerably more stable.
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Cloud
Figure 6 High w surface air entering from the left rises up the sloping
updraught. The cloud formed drops its precipitation into low w air entering at a
higher level from the right. Evaporation cools the low w air, and this, together
with the drag imposed by the precipitation, forces it to descend. At lower levels its
horizontal direction is partly reversed to the direction of the low-level flow, but a
component spreads out as a gust front to undercut the low level warm air,
providing uplift to feed the updraught. Note that streamlines shown are relative
to the system movement, which is generally from right to left in the diagram.
Since such systems generally move in the direction of the shear (i.e. right
to left in Figure 6), their arrival is preceded by their extensive anvils
spreading out before them in the upper flow. Sometimes this spreading
upper and middle level cloud extends over tens of thousands of square
kilometres forming a giant cloud canopy, with precipitation falling from
much of its area. These are then classed as a type of mesoscale convective
complex or MCC. They appear on satellite imagery as vast, long-lived,
areas of thick cloud, hiding the elongated shape of the squall line
producing them, and show as large areas of precipitation on a radar.
Figure 7 indicates a wind environment that would favour line
squall/MCC development. Rather than large changes of direction of
wind with height through the troposphere, as is the case with an
environment favouring multi-cell development, there is large directional
shear up to, say, 3 km with large uni-directional shear above.
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Figure 7 Hodograph of wind environment favouring line squall development
This type of squall line organisation occurs in mid-latitudes, particularly
in the USA, and a similar phenomenon occurs in some tropical areas of
Africa, southeast Asia and central America.
On occasions splitting storms have been observed in an environment of
large, mainly unidirectional shear. Two daughter cells form on either
side of a downdraught and move away to the right and left of the mean
wind direction, the right moving one being longer-lived and more
severe.
6. Supercells
The term ‘supercell’ was coined by Browning (1962) when describing an
intense travelling storm that affected southeast England in 1959, known as
the Wokingham Storm. This storm, which consisted of a single cell which
propagated continuously rather than in discrete jumps, appeared to have a
highly organised internal circulation with the downdraught co-existing
with the updraught. It was far larger and more persistent than a normal
mature cell and gave more severe weather.
Since then many others have been identified and described, particularly
in the USA. They are long-lived (a life span of 12 hours having been
recorded), and are much larger than normal, often around 50 km in
diameter. Where they occur they are often associated with tornadoes
and large hail stones, leaving long swathes of hail damage in their
wakes. It should be stressed that even in areas where they are relatively
common, such as the mid-west of the USA, they constitute only a very
small proportion of local storms.
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This type of storm is most likely when a capping stable layer delays
release of energy until late in the day, then breaks down allowing the
development of one or a few isolated storms. The very efficient
updraught can exceed 50 ms-1 (100 kn) and creates a continuous
overshooting dome that pokes up above the spreading anvil into the
stratosphere, to a level of perhaps 15 km (50000 ft). A cyclonic
circulation is present within the storm, first developing at mid-levels
near the updraught. The formation of this mesocyclone may be due to
rotary motion about a horizontal axis caused by the strong shear
between the up- and downdraughts. It is thought that this spin about a
horizontal axis is tilted to some degree, producing rotation about a
vertical axis. The convergence created by air rushing in towards the
cloud to feed the updraught enhances this rotary motion.
Figure 8 Simplified cross-section of a supercell. The shaded areas represent
precipitation, darker shades indicating greater rates. The rear flank
downdraught has not yet reached the ground. Inset shows cut away of
precipitation area across line AB and updraught/cloud outline from above.
Plane of main diagram cross section is marked CD.
Two downdraughts exist on either side of the updraught, the forward and
rear flank down-draughts. The updraught enters beneath and to the right of
the rear flank downdraught, which initially does not reach the ground. It
ascends so rapidly
into the cloud that precipitation particles do not have time to grow and so its
radar profile shows a characteristic vault’ or weak echo region. The rear flank
downdraught is colder and drier than the forward one, and originates from
between 4 and 10 km where the mid to upper level flow is deflected
downwards on encountering the precipitation. Evaporating rain from the
section of the anvil spreading out to the rear of the storm aids this rear
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downdraught to work its way down to the ground. Figure 10 shows a 3dimensional representation of the major flow regions in a supercell.
The boundaries between the high w updraught air being drawn from the
surface and the low w down-draught air originating from aloft form a mini
frontal system with the mesocyclone at its apex, the relationship being
similar to that which occurs in a warm sector depression. The outflow from
the rear flank downdraught may catch up with the boundary between the
forward flank downdraught air and the warm updraught air, occluding
this warm air off the surface and forcing the updraught to rebuild slightly
forward of its old position (Figure. 9).
Figure 9 Supercell seen from the top. The rear flank downdraught has descended to the ground and
the dividing line between low w downdraught air and high w updraught air resembles mini frontal
system. (a) shows forward flank downdraught (FFD), updraught (UD) and rear flank downdraught
(RFD). Arrows show system-relative surface flow. Hook shape to precipitation area in (b) is a
characteristic radar signature of severe convection, indicating risk of tornado genesis. Convection
along trailing cold front gives rise to a line of flanking cumulus clouds.
A feature that distinguishes supercell convection from all other types is the
fact that the cell movement is in a direction different from the winds at any
level through its depth, normally to the right. The reason for this is not fully
understood, but it may be due to the fact that the storm is rotating,
inducing motion due to the Magnus effect, just as topspin imparted to a
tennis ball will cause it to dip. This cell motion from outside the ‘envelope’
of winds allows the strong convergence zone created by the downdraught
to be collocated with the existing updraught the main downdraught
provides uplift for the updraught, which in turn feeds the downdraughts;
their relationship is symbiotic and long-lasting (Figure. 11)
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UD
9 km flow
RFD
FFD
4 km flow
surface flow
Figure 10 Three-dimensional representation of a supercell with fully-formed
mesocyclone, the centre of which is shown by the thin vertical cylinder between
the updraught and the rear flank downdraught. If a tornado were present it
would be at the base of the cylinder. Flow lines are system-relative.





Supercell type storms occur typically in the following synoptic-scale type
environment:
Strong instability with parcel theory indicating more than 4 0C excess
buoyancy at 500 hPa.
Strong mean sub-cloud winds (of the order of 10 ms-1)
Strong environmental shear through the cloud layer of 2.5 m s-1 to
4.5 m s-1 per kilometre, and therefore strong upper winds.
Strong veer of wind with height.
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Figure 11 ; (a) Wind environment for supercell storm represented on a
hodograph. (b) Configuration of up-and downdraughts from consideration of
system-relative winds.
7. Large hail and tornadoes
Large hail and tornadoes cause considerable damage to personal
property and animal life as well as significant economic damage to crops
in parts of the world. In the USA on average 2% of the nation’s
agricultural crop is destroyed by hail annually (quoted at 680 million
dollars in 1983), and ‘in some areas within the USA the average figure is
20%. Only one death through hail has been recorded in the USA, but in
India and China there have been occasions when 200 or more deaths
have been attributed to individual hail-storms. This high figure is
presumably due to the fact that in these countries larger numbers of
people are likely to be out in the open away from any shelter.
Although economic damage due to tornadoes is less than that caused by
hail, the number of fatalities is greater. In the period 1916—53 the
average annual number of lives lost was 230 in the USA, which
compares to 75 in hurricanes, though both figures have since decreased
due to improved warning services.
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Figure 12 Hatched regions show the areas affected by severe local storms
producing large hail and damaging winds. The arrows represent the mean
positions of the strongest mid-tropospheric winds in the storm season, with
which they are strongly correlated in the northern hemisphere. In the southern
hemisphere the storms occur in the southeastern parts of continents during
northward excursions of the strong winds from their mean positions associated
with penetration by cold fronts into their latitudes. Stippled areas in the tropics
are affected by line squall type storms in which large hail is extremely rare but
strong winds due to outspreading downdraughts occur.
7.1 Large hail
The way in which large hailstones are formed is still the subject of some
controversy. One theory is that they grow to a large size by repeated recirculation within the cloud, and where updraughts are strong enough
to support them. Reference to Figure 8 on page 10 shows that
precipitation occurring in the rear flank downdraught can fall back into
the sloping updraught to be carried aloft again. The radar echo
associated with this rear flank downdraught is sometimes referred to as
the embryo curtain as it contains small precipitation particles, some of
which eventually grow into hailstones. The echo-free vault around the
updraught is a region of high liquid water content contained in cloud
droplets, and here the developing hailstone can grow rapidly by
accretion.
Where accretion is rapid, water droplets spread out in a film over the
surface of the hailstone, because even where temperatures are below
freezing point, release of latent heat on freezing warms its surface to 0
0C. As the hailstone is carried aloft and away from the updraught core,
accretion becomes less rapid. Latent heat release is reduced and the film
of water freezes into a translucent layer.
In the cold regions of the cloud where water content is lower,
supercooled water droplets will freeze onto the hailstone surface
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without melting. Thus a layer is formed where the air is trapped in the
ice between individual frozen droplets, and this appears opaque. A
hailstone cut in half therefore shows alternating layers of opaque and
translucent ice.
Eventually the hailstone’s trajectory will be such that it fails to fall back
into the strong updraught core, or it will grow to such a size that its fall
speed exceeds the updraught speed. It then falls to the ground around
the edges of the updraught area as shown in Figure 9b.
In areas where hailstorms are more frequent, such as inland Europe, the
USA and northern India, the normal upper limit to hailstone size is a
diameter of 10 cm and a weight of 0.5 kg. In the UK such sizes have not
been reliably recorded, though hail-stones of around 8 cm diameter have
been recorded several times this century.
7.2 Tornadoes
The mesocyclone previously mentioned is the precursor to damaging
tornadoes. Forming aloft in the cloud, the circulation works its way
down to lower levels. Its visible manifestation is an area of persistent
rotating low cloud base known as the wall cloud (shown in Figure 9
where the updraught enters the cloud). The exact mechanism whereby a
tornado forms is not fully understood. It is thought that strong rotation
about a horizontal axis between the forward flank downdraught and the
updraught at the surface gust front may be tilted into the vertical as the
rotating cylinder of air is pulled into the updraught (represented by the
tilting term in the vorticity equation). Local enhanced convergence due
to air rushing in to feed the updraught increases spin-up (like a spinning
ice skater drawing in their arms).
Although there has been at least one documented case of anticyclonic
rotation, the vast majority of tornadoes rotate cyclonically. This, together
with the fact that they are very rare or do not occur within 20 degrees of
the equator, suggests that the earth's spin plays a part in their formation
as it does in larger low-pressure systems.
Most tornadoes have diameters of 100 - 500 m and estimated wind
speeds in the range 40 - 80 ms-1 but as much as 140 m s-1 has been
estimated. The funnel is made visible by condensation brought about by
adiabatic reduction of pressure as air enters its circulation. Estimates of
the central pressure of tornadoes vary a lot, some sources quoting 600
hPa, others which are probably more realistic indicating a local
reduction of pressure of some 100 hPa.
A typical tornado path is about 10 km in length and 150 m wide and
travels at about 15 m s-1. The paths are usually fairly straight, but
sometimes show a looping pattern, and occasionally there is evidence
for multiple suction vortices rotating about the main centre of rotation.
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Damage is caused not only by the high wind speeds, but also by suction
created by the low pressure centre and twisting effects due to large
horizontal wind shears.
In the USA the annual average of reported tornadoes is 700, whilst in the
UK it is around 50. They occur in Great Britain more frequently than was
previously thought, most commonly from the north Midlands southwards.
They are most likely in the winter half of the year in association with cold
air mass convection in the circulation of intense depressions and in active
cold fronts, where they may pass unnoticed in generally windy conditions.
In these circumstances it is the well-marked trail of destruction that
provides evidence of their existence after-the-fact. Tornadoes associated
with active cold fronts or cold air convection are not as powerful as those
found accompanying outbreaks of severe warm-air convection, and
sometimes only a funnel cloud appears that does not actually reach the
ground.
Water spouts are a mild form of tornado occurring over the sea or large lakes,
where funnels of water are lifted high into the air in the suction vortex. Like
tornadoes they axe associated with convective clouds, but not necessarily
with large cumulonimbus.
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Appendix A Severe convection over the USA and UK
The topography of the USA is unique in the way it favours the
formation of severe storms. With a southerly surface flow air of high w
feeds up from the Caribbean (Figure. 13). The winds generally veer with
height, so that at medium levels there is northeastward advection of air,
which has originated over the Rocky Mountains or their southern
extension into the Mexican Plateau. This air is comparatively hot but,
being dry, has a low w,. So a situation of potential instability arises
where air at low levels has high w and that at higher levels has a low
w; convection is prevented from breaking out because the upper level
air is warm and a stable layer separates the two air streams.
Figure 13 Trajectories favouring development of severe convection over the
midwest of the USA. Hatched area is high ground.
However, as the upper level stream flows northeastwards it rises in the
large-scale flow and cools. When it has cooled sufficiently the stable
layer is removed and the potential instability can be realised. Storms
commonly break out close to what is known as the dry line, that is a line
separating the moist south-southeasterly surface flow of tropical
maritime air from a dry south-southwesterly flow of tropical continental
air. This is similar in some respects to a front with no clouds or weather,
but a well-marked dewpoint change of 10 degrees or so across a narrow
zone. Strong shear to organise the convection is often present due to the
proximity of trailing cold fronts from depressions crossing America to
the north.
Over western -Europe, and the UK in particular, severe convection is not
as common as in the USA. Long-lived severe travelling hailstorms occur
on average once or twice a year somewhere in central and southeast
England, being about five times as common over much of southern
France and Austria and also probably northern France and Germany.
Severe travelling storms are likely to occur somewhere in western
Europe on between 10 and 20 days per summer.
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When they do occur over Britain they are often associated with a cold
front over or close to northwestern parts of the UK with an attendant
upper trough to the west (Figure. 14). The baroclinicity due to the
proximity of the front ensures a fairly strong flow at upper levels from
the southwest. At medium levels air which has originated over the
Sahara has been drawn into the flow ahead of the trough so that it
passes over the Spanish plateau. This air is similar to that coming from
the southern Rockies, being hot and dry through heating over the Sahara
and the high ground of Spain.
Figure 14 Trajectories associated with outbreaks of severe convection over
southern UK. Hatched area is the Spanish plateau
The warm dry air aloft limits the depth of convection over southwest
France, where the subsidence to the lee of the Pyrenees also acts to
confine surface daytime heating to the lowest kilometre or so of the
atmosphere, allowing the attainment of w values of as much as 24 oC
here. These high surface temperatures lead to the formation of a shallow
heat low over France, familiar to forecasters as a 'thundery low'.
Overnight the low-level high w air is advected northwards around the
low, approaching southern England from the south or southeast the next
day.
In the meantime, the warm dry air of lower w at higher levels coming
from Spain, sometimes referred to as the ‘Spanish plume’ has undergone
mass ascent with northward progression. It cools and moistens,
sometimes producing bands of altocumulus castellanus and thundery
outbreaks as it approaches the UK across the Bay of Biscay and northern
France. By the time it reaches the UK the ‘lid’ produced by warming
over the high ground of Spain has been removed and the potential
instability can be realised.
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Cumulonimbus Clouds And Severe Storms
Sometimes thunderstorms break out over northern France and advect
northwards across the English Channel, becoming more organised and
severe upon encountering the stronger upper-level winds.
Thunderstorms are most likely to break out within the ‘tongue’ of
highest w air and where there is low-level convergence due to isobaric
troughing or mesoscale effects such as sea-breezes.
Appendix B Lightning
Lightning occurs in vigorous convective cloud where sufficient electrical
charges are separated to give rise to thunderstorms. As a phenomenon it
does not affect the dynamics of cumulonimbus clouds, though there is
evidence to suggest that the presence of a strong electric field enhances
the coalescence efficiency of cloud droplets.
Although there have been observations of lightning emanating from allwater clouds, the vast majority of thunderstorms extend above the
freezing level and contain ice particles. The more popular theories on
how charge separation comes about assume that ice particles play a key
role, initially by the thermoelectric effect of ice.
Some water molecules in ice are dissociated into positive (H) and
negative (OH) ions, the number decreasing with decreasing
temperatures. If an ice particle is warmer at one end than at the other,
the warmer end will contain more ions. Since ions tend to migrate from
regions of high concentration to regions of low concentration, there will
be some movement of ions from the warmer to the colder end of the ice.
However, the mobility of the negative (OH) ions in ice is essentially
zero, so that it is only positive ions that migrate, creating a positive
charge at the cold end of the ice and a negative charge at the warm end.
A hailstone falling through a cloud of mixed super-cooled water and ice
crystals will have a surface warmer than the surroundings due to latent
heat release. On colliding with a small ice particle, for an instant the two
may be considered as one piece of ice; the hailstone, being the warmer
end, will receive a negative charge and the small ice particle a positive
charge due to the thermoelectric effect (Figure. 15a). On rebounding, the
negatively charged hailstone will continue falling towards the base of
the cloud whilst the positively charged ice particle is swept up in the
rising currents towards the top of the cloud.
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Figure 15; (a) Small ice particle in contact with a falling hailstone receiving a
positive charge through the thermoelectric effect before being carried upwards.
(b) Charges induced on cloud particles are such that their lower surfaces are
positively charged. Small particles colliding and rebounding from larger ones
therefore receive a positive charge before being carried upwards
As a charge separation increases, negative below and positive aloft, the
cloud droplets and precipitation particles within the cloud will have
opposite charges induced on them, positive in their lower parts and
negative in their upper parts. While larger particles briefly collide with
smaller particles, they will instantaneously become one body with the
smaller, lower particle receiving a net positive charge and the upper larger
particle receiving a net negative charge (Figure. 15b). If they bounce away
from each other, the small positively charged particle will tend to be swept
towards the top of the cloud whilst the larger negatively charged particle
will tend to fall to lower regions. This induction stage of charge separation
will increase in effectiveness with time, as the greater the charge separation
in the cloud, the greater the charge separation induced on precipitation
particles.In some thunderstorms there is a small pocket of positive charge
in the lower part of the cloud just below the 0 0C level. This is because
melting particles receive positive charges due to the bursting of air bubbles.
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Cumulonimbus Clouds And Severe Storms
Figure 16 Lightning stroke from negatively charged lower portion of cloud to
surface which has positive charge induced on it.
The strong negative charge in the base of the cloud induces a positive
charge at the earth’s surface. At some stage the potential gradient
between the cloud and the ground, or between various regions of the
cloud, exceeds that which the air can sustain and a lightning stroke
results, either to the ground (Figure. 16) or to another part of the cloud.
The lightning discharge consists of several strokes back and forth, the
first coming from the cloud base, but the highly luminous discharge
being a return stroke back to the cloud base. This raises the air in the
lightning channel to above 30000 0K in such a short time that it has no
time to expand. The pressure therefore increases to 10 or perhaps 100
atmospheres. This high-pressure channel then expands rapidly, setting
up a powerful shock wave that travels faster than the speed of sound
and creates a sound wave, which is heard as thunder.
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8. Further reading
Ludlam, FH
1980
Clouds and Storms The behaviour and
Effect of Water in the Atmosphere
Browning, KA and
Foote, GB
1975
Airflow and hail growth in supercell
storms and some implications for hail
suppression
Scorer, RS and Verkaik,
A
1989
Spacious Skies
Atkinson, BW
1981
Mesoscale Atmospheric Circulations
Wallace, TM and
Hobbs, PV
1977
Atmospheric Science an Introductory
Survey
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