Shear and Thunderstorms - The Basics

[Nick L]

This is just a basic guide as to why wind shear is so important when it comes to thunderstorm development and lifespans.

 

So, why is shear important when it comes to thunderstorms? It's first useful to discuss the structure of a normal thunderstorm in a non-sheared environment. In the early stages, the updraft (which "feeds" the storm) is dominant, intensifying the thunderstorm. Once the thunderstorm reaches its mature stage, rain begins to fall and enhances a downdraft. In a non-sheared environment, the downdraft works against the updraft, thus cutting off the fuel source for the thunderstorm, and dying away fairly quickly. This can be shown in the next figure:

 

 

 

Now, if we add a bit of wind shear, we can get much more prolonged storms. This is because the change in wind speed/direction with height causes the storm to tilt, and this tilting separates the updraft and downdraft, meaning the fuel source is sustained for longer. The image attached shows it quite nicely. The rain shaft is normally associated with the downdraft, primarily through drag causing descending air.

 

 

Typical wind shear values for single cell storms are below 15m/s, multicell storms 15-25m/s and supercell storms >25m/s. HOWEVER, shear is not the only ingredient that determines the nature of a thunderstorm. The Convective Available Potential Energy (CAPE) is also a key factor. This is essentially a measure of the positive buoyancy or instability of the atmosphere. If you've got plenty of shear but no CAPE then you're not going to get much!

 

This is all fairly basic and not at all exhaustive. There are other and more complicated factors regarding the cycle of supercells, but this is just a broad overview of typical UK thunderstorms. If anyone wants to add/correct anything I've said then feel free! 

 

(Figures found in various lecture notes...)

Edited April 30, 2014 by Nick L

[Vorticity0123]

A great summary Nick! I think this will be of benefit to many  .

 

One addition: another important type of thunderstorm, which is also very frequent in the UK is a squall line. This type of storm develops when wind shear is at least 10 m/s between the surface and 2 kilometres height, or more than 20 m/s between the surface and 6 kilometres height^1. Usually, these kind of storms are triggered due to a trough of low pressure or a frontal zone.

 

The most recent occurence (in the UK) I can remember is 25 January 2014 (which was a very unusual occurence based on the fact that the severe weather event occured in winter). Below is a link to the radar image of the event:

 

http://europa.buienradar.nl/image/archive?timestamp=201401251600

 

The image contains a radar image of Europe, as of 25 January 2014. 17 UTC. The squall line is located over central-eastern England.

 

Furthermore, an excellent guide about thunderstorms and their specifications can be found in the following link (from ESTOFEX):

 

 http://www.estofex.org/guide/1_4_3.html

 

Sources:

¹: http://www.estofex.org/guide/1_4_3.html 

http://europa.buienradar.nl/image/archive?timestamp=201401251600

http://www.knmi.nl/cms/content/117884/extreem_weer_tussen_vorst_en_dooi (in Dutch)

Edited April 30, 2014 by Vorticity0123

[Chris K]

Thanks again! Very helpful for people learning, such as myself.

[Nick L]

A great summary Nick! I think this will be of benefit to many  .

 

One addition...

 

Yes add to this by all means, it's certainly not exhaustive. Hopefully I'll add some stuff to this myself soon

[Cakie]

Really useful and informative

Please feel free to add any extra info anyone

[East_England_Stormchaser91]

Brilliant post Nick. I was always wondering what the actual principles were behind wind shear and exactly what happens. Think June 28th 2012 storms were a key example for the UK.

[Nick L]

Thanks for the good feedback I'll hopefully add some more stuff in time, it's an excellent means of procrastination