Thunderstorms: ‘ordinary’ or ‘single cell’ storms, multicell storms, supercell storms
• Ahrens, Chapter 14: Thunderstorms and
Tornadoes
• This lecture + next (Lightning, tornadoes) will cover the topic.
Typical cumulonimbus – single cell thunderstorm – produces heavy shower, possibly with hail and lightning
What meteorological conditions precede a thunderstorm?
1. A conditionally unstable atmosphere
2. Substantial boundary layer moisture
3. A trigger to release the instability
• On a skew T-log p plot:
CAPE:
C onvective A vailable P otential E nergy
= energy that can be released
CIN :
C onvective IN hibition:
= energy barrier that has to be overcome
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Real example tephigram – large amount of CAPE – thunderstorm v.likely
CAPE is given by the area between SALR and environmental lapse rate
C
A
P
E
T d
Higher dew-point T = more moisture
Pushes to higher SALR curve, i.e. higher CAPE
An important forecaster tool for predicting thunderstorms:
Maps of CAPE (contours) and vertical velocity ( + )
Fri Nov 7 12Z
2008 http://expert.woeurope.eu/cape_frame.htm
Sunday 1200 (8 Nov 2009)
Monday 31 Oct 2011 (03z)
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‘Ordinary’ or ‘single cell’ thunderstorms
• Relatively small
• Isolated
• Typically just produce a single heavy shower, then dissipate.
• Very little vertical wind shear (come back to this later)
Cumulus Congestus
(Cumulus with large vertical extent)
• Buoyant updraught
• Vertical velocity increases with height, to ~10 ms-1 at top
• Surrounding air mixed in
(entrainment)
• Inside cloud, raindrops and supercooled drops grow, releasing latent heat
• At edges, drops evaporate into entrained air – moistens the surrounding air.
• As the environment moistens, successive updraughts sustain clouds to higher and higher levels
• No rainfall at this stage
Isolated cumulonimbus
-40
0
10 km
5 km
• Top of cloud extends to near tropopause levels (>10 km), well above 100% freezing level
• Growth of drops & ice continues until updraught can no longer support them – start to fall
• Entrainment of surrounding drier air tends to evaporate drops, cooling air
• Both these processes lead to development of a downdraught
• Updraught+downdraught=‘cell’
– ‘single cell’ thunderstorm
• Most intense stage – heavy rain, thunder, lightning
• Anvil starts to form at top
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Cumulonimbus dissipates, just leaving anvil – eventually leaving only cirrus
• Downdraught grows until it cuts off flow of air to the updraught – the storm has its ‘fuel supply’ stopped
• Rainfall declines and the lower part of the cloud evaporates
• Rainfall stops; all that is left is the anvil
• All 3 stages pass in typically about 1 hour - a rapid, heavy shower
• Why might this be important?
Cumulus Mature Dissipating
Approaching mature stage
Dissipating stage
Downdraught
Gust front
• This type of thunderstorm is where once one cell subsides, another grows in its place, adjacent to the last cell
• The downdraught causes a ‘gust front’ when it meets the surface. This may push up surrounding moist air and trigger a new cell to develop.
• The presence of vertical wind shear can help thunderstorm development and persistence by separating the updraught from the downdraught
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Shear ‘tilts’ the storm, helping it propagate, increases its lifetime and severity
Promotes formation of new cells – i.e. a multicell storm
Relative to flow at mid-level Flow at mid-level
Since mass cannot accumulate, there must also be vertical motion (red arrows)
Shear is equivalent to rotation
Updraught
‘bends’ upwards vorticity
Horizontal shear combined with an updraught can lead to a storm acquiring vorticity about a vertical axis
Vorticity associated with horizontal shear
Supercell, Kansas, rotating updraught
• Rotating updraught
– Rotation causes the storm to be more robust
– longer-lived, and therefore more dangerous
• Forms an area of low pressure at centre of rotation, called a mesolow
• Updraught centred on the low pressure
• Circulation around the low is in cyclostrophic balance…
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