Thunderstorms ordinary or single cell storms multicell storms

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. 2. 3. • A conditionally unstable atmosphere Substantial boundary layer moisture A trigger to release the instability On a skew T-log p plot: CAPE: Convective Available Potential Energy = energy that can be released CIN: Convective INhibition: = energy barrier that has to be overcome

Real example tephigram – large amount of CAPE – thunderstorm v. likely CAPE is given by the area between SALR and environmental lapse rate PE CA Td 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 12 Z 2008 http: //expert. woeurope. eu/cape_frame. htm

Sunday 1200 (8 Nov 2009)

Monday 31 Oct 2011 (03 z)

‘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) -40°C • Top of cloud extends to near tropopause levels (>10 km), well above 100% freezing level • Growth of drops & ice continues until updraught can 10 km no longer support them – start to fall • Entrainment of surrounding drier air tends to evaporate drops, cooling air 5 km • 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

Stage 3: ‘Dissipating’ Cumulonimbus dissipates, just leaving anvil – eventually leaving only 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 stage Dissipating 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 ‘tilts’ the storm, helping it propagate, increases its lifetime and severity Promotes formation of new cells – i. e. a multicell storm

Shear and rotation Flow at mid-level Relative to flow at mid-level Since mass cannot accumulate, there must also be vertical motion (red arrows) Shear is equivalent to rotation

Horizontal shear combined with an updraught can lead to a storm acquiring vorticity about a vertical axis Updraught ‘bends’ upwards vorticity Vorticity associated with horizontal shear

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…