Reading. What meteorological conditions precede a thunderstorm? Thunderstorms: ordinary or single cell storms, multicell storms, supercell storms

Transcription

1 Thunderstorms: ordinary or single cell storms, multicell storms, supercell storms Reading 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: Convective Available Potential Energy = energy that can be released CIN: Convective INhibition: = energy barrier that has to be overcome 1

2 Real example tephigram large amount of CAPE thunderstorm v.likely CAPE is given by the area between SALR and environmental lapse rate An important forecaster tool for predicting thunderstorms: Maps of CAPE (contours) and vertical velocity (+) Fri Nov 7 12Z 2008 CAPE T d Higher dew-point T = more moisture Pushes to higher SALR curve, i.e. higher CAPE Sunday 1200 (8 Nov 2009) Monday 31 Oct 2011 (03z) 2

3 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 -40 Mature stage (continued) 10 km 0 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 3

4 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 Vertical wind shear Why might this be important? Cumulus Mature Dissipating Approaching mature stage Gust front Dissipating stage Downdraught 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 4

5 Vertical Wind Shear Shear and rotation Flow at mid-level Relative to flow at mid-level Shear tilts the storm, helping it propagate, increases its lifetime and severity Promotes formation of new cells i.e. a multicell storm 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 Generating a supercell storm Updraught bends upwards vorticity Vorticity associated with horizontal shear 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 5