Prezerakos, Nicholas G. (General Dept. of Mathematics, T.E.I. of Piraeus, Thivon)
Ralli, P. (Aigaleo 122 44, Athens)
In this presentation an attempt is made for a study of the atmospheric circulation basic characteristics which caused the mixed type (large scale and convective scale) elongated heavy rainfalls in the greek area from 21 to 23 October 1994 leading to extensive floods in Attica (the major region where Athens belong to), Karditsa-Trikala (in the central mainland) and Kasos island (close to Rhodes) territories and the loss of eleven human lives, nine of them in Athica.
The data used for this study include surface and upper-air observations from the GTS, vertical velocities (850, 700, 500 hPa), relative vorticity, absolute vorticity advection at 500 hPa, thermal advection at 850 and 700 hPa and divergence in 1000 and 850 hPa. Calculations are based on ECMWF initialised data, delivered by the Hellenic National Meteorological Service every 12 hours with a resolution of 1.5 (now 0.5). Cross sections of potential temperature, winds and PV as well, were provide by ECMWF. The 12 h precipitation heights, Tephigrams of Helliniko upper-air station and EUMETSAT imageries were kindly provided by HNMS.
The analysis showed that, during this time period, a blocking type anticyclone dominated the mid-tropospheric circulation of central and Esatern Europe. Greece was situated at the south flank of the surface large anticylone, which was blocking the northeastern progression of a synoptic scale frontal depression coming from western Mediterranean and rejuvenated in Sicily area by dynamical factors. Along with the blocking anticylone a stationary surface front crossing the northern coasts of Turkey and through greek mainland and eastern coast of Adriatic sea, reached eastern Alps. This synoptic situation was associated with the advection of warm and moist unstable air masses along the height of the whole troposphere towards Greece, coming from south over the Mediterranean, where as in the planetary boundary layer cold and moist masses coming from northeast passing over Black sea and Aegean sea being warmed from bellow, met the warm moist air over Greece resulting in an unstable saturated mixing layer.
Strong low tropospheric convergence and positive advection of absolute vorticity at 500 hPa were bringing the saturated mixing layer up to level of free convection releasing the real latent instability. So in the continous upslide rain many thunderstorms occurred the intensity of which depends partly on local factors. The combination of the elongated and of large scale and convective scale heavy rain precipitation along with local factors (e.g. orography) resulted in the extensive floods over Greece.
These rainfalls, according to other Greek authors papers which studied statistically their main parameters, can be classified in the category of "natural disasters".
The main depression which was created over Sicily remaining stationary initially moved slowly southeastwards.
At the framework of this depression a vortex was generated looking like a secondary polar low identified in the EUMETSAT imageries since 1830 UTC on 22 October 1994. This vortex being wholly over sea it was not possible to regist the weather associated with.