Thunderstorms are local-scale convective storms invariably produced by a cumulonimbus cloud and always accompanied by lightning and thunder (AMS, 2015). These weather systems usually produce strong wind gusts, heavy rain, and sometimes hail. They form in an environment with a strong vertical decrease of temperature, which lead to ascend of moist and warm air masses up to the top of the tropopause (approx. 10 – 12 km in mid-latitudes). This process is referred to as deep moist convection. The spatial extent of thunderstorm systems can reach from a few kilometers to up to more than 100 km. Worldwide there are about 40.000 – 50.000 thunderstorms daily (NOAA, 2010). In Germany, severe thunderstorms predominately occur during the summer months from April to September. Besides winter storms and flooding, they make up for the most significant and most damage-relevant natural hazards in Germany. In Southern Germany, most of the insured building damage by natural hazards (so called elementary insurance) is caused by severe convective storms, mainly by hail.

For the formation of deep moist convection, the following three requirements need to be fulfilled (Doswell, 1987):

(a)    High moisture content in the lower troposphere allows for substantial latent heat release during condensation, which represents the convective energy of the thunderstorm.  .
(b)    An unstably stratified troposphere leads to free ascending of a lifted air parcel above the level of free convection (LFC).
(c)    A lifting mechanism (trigger mechanism) to lift the air parcels to the LFC; this mechanism can be of either of dynamic (front, mountain range) or thermal nature (heating up of near-surface air).

Furthermore, the vertical change of the horizontal wind (wind shear) is decisive for the kind of thunderstorm system that may develop and its life time.

Different forms of  thunderstorm cells

Convective systems may form either as single, isolated convective storms, or as cellular and larger complexes. The figure below shows the influence of the vertical wind shear on the development of different thunderstorm systems. Each system has characteristic features and features a different hazard potential. Depending on their flow- and precipitation dynamics, thunderstorms are often categorized into the three basic forms of single-, multi- and supercells (Houze, 1993). Single-cell thunderstorms are most common but not a major hazard. Multicell thunderstorms are clusters of single cells at different stages, repeatedly spawning new cells. Mesoscale convective systems (MCS) are ensembles of thunderstorms with contiguous precipitation of 100 km in diameter in at least one direction. Special types of an MCS are Squall Lines, which are narrow bands of convective cells extended in one direction. Rotating supercell thunderstorms are the most dangerous convective storms, capable of producing the largest hailstones and the most violent tornadoes (EF3 and higher).

Especially for rotating supercells and for Squall Lines mostly linked to fronts, wind shear plays an important role.

Literature:

AMS, 2015: Glossary of Meteorology. American Meteorological Society, glossary.ametsoc.org

Doswell III, C. A., 1987: The distinction between large-scale and mesoscale contribution to severe convection: A case study example. Wea. Forecasting, 2, 3 – 16.

Houze, R., 1993: Cloud dynamics. Academic Press, San Diego.

Markowski, P. und Y. Richardson, 2010: Mesoscale meteorology in midlatitudes, Vol. 3. John Wiley & Sons, Chichester, Großbritannien.

NOAA, 2010: National Weather Service Weather Forecast Office, Thunderstorms – Introduction. http://www.srh.noaa.gov/jetstream/tstorms/tstorms_intro.html.

Text: Dr. Susanna Mohr & Dr. Michael Kunz, Karlsruhe Institute of Technology