Numerical simulation of a tornado producing thunderstorm : A case study

Thunderstorms often cause serious damages due to the strong surface outflow or heavy precipitation. There are some weather patterns, which especially promote breaking out of severe thunderstorms. Radar and visual observations show that some of these thunderstorms can develop into supercell. One of these typical weather patterns is the prefrontal squall line moving from southwest direction (so called Slovenian Squall Lines). In this paper the results about the formation and development of a thunderstorm associated by this type of squall line is presented. Severe thunderstorms formed on May 18, 2005 over the eastern part of Hungary were investigated using radar observations and a mesoscale numerical model (MM5). The time and position of the most intensive thunderstorm coincide well with radar observations. The case study shows that there is a competition between thunderstorms for the wet and warm air. A thunderstorm which can collect the wet and warm air from larger area will have longer lifetime and more intensive updraft. The case study shows that in the case of absence of directional wind shear, the merging of the updraft cores can result in a supercell. Although the formation of the new cells frequently occurs by splitting, this process did not happen in this case. Analysis of the numerical simulation indicates the presence of three different types of downdraft regions in an intensive thunderstorm. The low level downdraft was generated by the precipitation loading. The intensity of this downdraft is also affected by melting and evaporation of the precipitation elements. The midlevel downdraft does not reach the surface, and it is driven by negative thermal buoyancy and set by an interaction of the updraft with the vertical wind shear. Downdraft cores at high level could be associated with the overshooting.