2.3 Tornadoes
2.3.1. Supercell tornadoes

Tornadoes that occur with supercell storms are to be considered the most dangerous. They occur under the mesocyclone of the storm that may be detected with Doppler-radar. If the storm is close to the radar, a smaller, more intense circulation may be observed at the location of the tornado, the tornado cyclone or tornado vortex.

Several environmental characteristics have been associated with the occurrence or of tornadoes. In some cases, answers are missing or incomplete as to why these characteristics are so important. Most important is, naturally, the presence of a supercell storm. Remember that those storms become more likely with increasing wind shear in the 0-6 km layer.

Secondly, strong tornadoes seem to be favoured by strong low-level wind shear. When the vertical wind shear in the lowest kilometre of the troposphere is around 10 m/s of shear or more, tornadoes are more likely with supercell storms (Brooks and Craven, 2002).

Thirdly, storm-relative helicity in the lowest kilometre appears to have predictive skill as well (Rasmussen, 2003). Many strong tornadoes in Europe occur in situations of strongly veering winds in the lowest kilometre, indicative of high storm-relative helicity.

Additionally, the amount of CAPE that is released in proximity to the earth's surface - say, below 3 km above the surface - is found to have a relation with the occurrence of tornadoes. When much CAPE is converted into upward motion nearby the surface, strong accelerations result that intensify any vorticity present, by stretching the air column. It turns out that many tornadoes in the U.S. occur when around or above 100 J/kg of CAPE is released below 3 km (Davies, 2004). Another parameter that represents the same effect is the altitude of the level of free convection (LFC). If this level, where a parcel becomes warmer than its environment is low, strong upward accelerations and amplification of vertical vorticity can be expected. Statistal relation has been found as well between vey low values of convective inhibition CIN and the occurrence of significant tornadoes.

Finally, it has been shown that the lifted condensation level (LCL), which is a proxy for the cloud base is important to consider. If the LCL is high, say above 1500 m, the chance of tornadoes drops quite rapidly. The reason behind this is not yet well-understood, but has probably a lot to do with strong evaporative cooling in the storm's downdrafts, that prevent the cold air to flow up into the storm again - something that usually happens in and near the tornado.