Lithuanian Hydrometeorological Service - Lithuania
According to the data of Lithuanian Hydrometeorological Service, there were 23 tornadoes recorded in Lithuania since 1950 to 2001.
It is known that tornadoes occur mostly in the warm period of the year, from April to September inclusive. Most of them occurred in August (6). In the course of the day, the most favorable hours for them to occur are in the afternoon, when the most-heated soil surface combines with the best-developed atmospheric convection, i.e. between 13 h and 16 h UTC (12 cases), and between 16 h and 19 h UTC (7 cases).
It is determined that atmospheric convection is the most active when the warm upper ridge is replaced by a cold trough, most often moving from the west and pushing the heath eastward. Under the conditions of extremely unstable atmospheric stratification, the release of accumulated energy occurs on a mesoscale (over limited areas) where near-surface atmospheric streams come together. A tiny detention layer - the inversion - restricts vertical dissipation of energy. Then, the energy reaches values necessary to 'break' through the inversion layer. Before the tornado occurs, the well-developed powerful Cb almost reaching the tropopause, begin to disintegrate. On their tops, the air becomes colder than its surroundings and clouds begin to disintegrate, unable to bear their vertical shape. This 'fall" of cloud-tops initiates fast downward movement. Together with alongside upward movement of the heated air, it gives a momentum for development of the vertical vortex.
There exists a distinction between two basic synoptical situations leading to the development of tornados in Lithuania: the areas of convergence besides cold-front wave tops (61 percent of all cases) and the lines, or zones, of instability (13 percent of cases).
Besides this, it has been determined that tornadoes develop in the warm and humid air with dewpoint temperature >10 - 12 °C and the wet bulb temperature >12 - 15 °C, while in 2/3 of all cases the latter being at least 18 °C; potential temperature of lower troposphere (in a layer not less than 400 hPa) falling with height more than by 5 °C; the vertical wind turn in the 850 - 500 hPa layer exceeding 10 m/s, the upper troposphere wind speed being >20 - 25 m/s with air streams exceeding 35 - 40 m/s, and the near-surface layer often under inversion condition.
The poster presentation will give the detailed case study of the very strongest tornado (Fujita 2 class) recorded in Eastern Lithuania on 29 May 1981.