How do tornadoes form?

In Europe and Germany there is a rising interest in tornadoes, but how do the whirlwinds form?

Tornadoes generate the strongest near-ground winds on Earth, often causing catastrophic damage. Fortunately, they are very rare, and most of these whirlwinds are weak and short-lived. However, tornadoes do occur in Germany as well. According to the German Weather Service, there are approximately 25-70 confirmed cases each year (DWD, 2024), and even the strongest tornadoes are possible in this region. To enable more accurate forecasts and warnings in the future, the KIT Atmospheric Risks Research Group, as part of CEDIM, is studying tornado formation.

Tornadoes are whirlwinds beneath a cumulus cloud and in contact with the ground, typically with diameters of around 100 meters (Fig. 1). They should not be confused with the much larger and completely differently structured hurricanes. Tornadoes are classified into different types based on their formation: (1) along air mass boundaries (e.g., waterspouts), (2) in linear thunderstorm systems, or (3) in individual so-called supercells (Fig. 1). The latter produce almost all strong tornadoes and are the best studied in this context, as summarized earlier this year in collaboration with researchers from the USA in this scientific review article (Fischer et al., 2024; Supercell tornadogenesis: Recent progress in our state of understanding).

*Fig. 1: Evolution of tornadogenesis near Earth, Texas, May 2021 (© Photo courtesy Alex Schueth).*

Summary of the article: Supercell tornadoes form in four stages (see Fig. 2). First, a rotating updraft, known as a mesocyclone, is required. This can occur when wind changes significantly with height. The mesocyclone causes air at the ground level to be rapidly drawn upward into the thunderstorm. In the second stage, vortices form in the cold outflow of the storm, which, in the third stage, are stretched, intensified, and organized into a vortex by the vertical acceleration into the mesocyclone. In most cases, the intensification is insufficient, and the vortex dissipates before reaching tornado strength. However, if the organization and intensification are sufficient, a stable flow state can develop in which air spirals in near the ground and rapidly rises in the center as a vortex. A tornado has thus formed. Additionally, due to the rapid rotation, the air condenses in the vortex core, making the tornado visible as a "funnel" cloud (Fig. 1).

Fischer, J., Dahl, J. M. L., Coffer, B. E., Houser, J. L., Markowski, P. M., Parker, M. D., Weiss, C. C., Schueth, A. ( 2024): Supercell tornadogenesis: Recent progress in our state of understanding. Bull. Amer. Meteor. Soc., 105, E1084–E1097, doi:10.1175/BAMS-D-23-0031.1.

*Fig. 2: The four steps of tornadogenesis (Fischer et al., 2024).*

Associated institute at KIT: Institute for Meteorology und Climate Troposphere Research (IMKTRO)
Author: Jannick Fischer (Oct. 2024)