INFLUENCE OF ENVIRONMENTAL WINDS ON PROPAGATION AND MOTION OF THUNDERSTORMS IN NORTHERN GREECE

The prediction of propagation and motion of thunderstorm activity, esp ecially when heavy rainfall accumulation amounts could occur, is a mat ter of great importance. The objective of this study is to examine the possibility of using the environmental wind data for the problem of a ssessing predictability of the propagation and movement of thunderstor ms in northern Greece, The propagation of radar thunderstorm echoes (r adar echo reflectivity maxima) and cell movement (fine radar echo patt erns) were determined for the spring and summer thunderstorms in 1992 and 1993 (April to September). Radar data were used by examining and a nalyzing digitally recorded plan position indicator for the entire tim e that the radar was operated, Multicell and single-cell thunderstorms were identified and thunderstorm propagation and cell motion were cal culated for each, Environmental winds at standard levels of 850, 700, and 500 hPa were considered and, additionally, a mean 0- to 6-km layer density-weighted wind was also examined, In northern Greece, winds an d radar thunderstorm echoes rarely moved from the clockwise sector 070 degrees to 220 degrees (ENE - SW). The movement of thunderstorm cells was not only steered by the 700-hPa level wind but was also well repr esented by the wind at the 500-hPa level and by the average wind in ti le layer 0-6 km, Using standard levels, cell motion can be determined by adding 5 degrees to 500-hPa wind direction and reducing the 500-hPa wind speed by 30%, With the use of the mean 0- to 6-km density-weight ed wind, cell motion can be represented by adding 5 degrees to the 0- to 6-km wind direction and increasing 35% the 0- to 6-km wind speed, T he propagation of multicell and single-cell thunderstorm echoes was ve ry similar, in spite of initial expectations, and was approximately eq ual to the cell motion, suggesting that the new cells grew on all side s of existing multicell thunderstorms, The majority of northern Greece thunderstorms do not propagate significantly and their motion is subs tantially translational and similar to cell motion. However, in cases of severe thunderstorms, propagation was indicated, Characteristic cas es of storm propagation are presented and vertical wind shear is inves tigated as a particularly important factor in influencing storm struct ure and evolution and the resulting storm propagation.