Thunderstorms: ‘ordinary’ or ‘single cell’ storms, multicell storms, supercell storms
Typical cumulonimbus – single cell thunderstorm – produces heavy shower, possibly with hail and lightning
Reading
• Ahrens, Chapter 14: Thunderstorms and Tornadoes
• This lecture + next (Lightning, tornadoes) will cover the topic.
What meteorological conditions precede a thunderstorm?
1. A conditionally unstable atmosphere2. Substantial boundary layer moisture3. A trigger to release the instability• On a skew T-log p plot:
CAPE:Convective Available Potential Energy= energy that can be releasedCIN:Convective INhibition:= energy barrier that has to be overcome
Real example tephigram – large amount of CAPE – thunderstorm v.likely
CA
PE
Higher dew-point T = more moisturePushes to higher SALR curve, i.e. higher CAPE
Td
CAPE is given by thearea between SALRand environmentallapse rate
An important forecaster tool for predicting thunderstorms: Maps of CAPE (contours) and vertical velocity (+)
Fri Nov 7 12Z2008
http://expert.woeurope.eu/cape_frame.htm
Sunday 1200 (8 Nov 2009)
Monday 31 Oct 2011 (03z)
‘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)
Stage 1: ‘Cumulus’
Cumulus Congestus(Cumulus with large vertical extent)
Cumulus stage (continued)• 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
Stage 2: ‘Mature’
Isolated cumulonimbus
Mature stage (continued)• 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
-40°C
0°C 5 km
10 km
Stage 3: ‘Dissipating’
Cumulonimbus dissipates, justleaving anvil – eventually leavingonly cirrus
Dissipating stage (continued)
• 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
Summary: ‘single cell’ storm
Cumulus Mature Dissipating
Vertical wind shear
• Why might this be important?
Approaching mature stageDissipating stage
Downdraught
Gust front
Multi-cell thunderstorms
• 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
Vertical Wind Shear
Shear ‘tilts’ the storm, helping it propagate, increases its lifetime and severityPromotes formation of new cells – i.e. a multicell storm
Shear and rotation
Since mass cannotaccumulate, there
must also be verticalmotion (red arrows)
Relative to flow at mid-levelFlow at mid-level
Shear isequivalent to
rotation
Updraught‘bends’ upwardsvorticity
Horizontal shear combined withan updraught can lead to a stormacquiring vorticity about a verticalaxis
Vorticityassociatedwithhorizontalshear
Generating a supercell storm
Supercell, Kansas, rotating updraught
Supercell thunderstorms
• 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…
