DIAGNOSIS OF THE ATMOSPHERIC CIRCULATION SYSTEMS ASSOCIATED WITH AN EPISODE OF EXTENSIVE FLOODS OVER GREECE

An attempt is made to study the basic characteristics of the atmospher ic circulation which caused the mixed-type (large-scale and convective -scale) elongated heavy rainfalls in the Greek area from 21 to 23 Octo ber 1994 leading to extensive floods in Attica, Karditsa-Trikala and K assos island territories and the loss of eleven human lives. The analy sis showed that during this time period a blocking-type anticyclone do minated the mid-tropospheric circulation of central and eastern Europe . Greece was situated at the south flank of the large surface anticycl one which was blocking the north-eastern progression of a synoptic-sca le frontal depression coming from western Mediterranean and rejuvenati ng over Sicily area by dynamical factors. Along with the blocking anti cyclone a quasi-stationary surface front crossing the northern coasts of Turkey and through Greek mainland and the eastern coast of the Adri atic Sea reached the Eastern Alps. This synoptic situation was associa ted 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, whereas in the lower part of the planetary boundar y layer cold and moist masses coming from north-east were passing over the Black and the Aegean Seas being warmed from below interleaving th e warm moist air over Greece which resulted in an unstable saturated m ixing layer. Strong low tropospheric convergence and positive advectio n of absolute vorticity at 500 hPa were bringing the saturated mixing layer up to the level of free convection releasing the real latent ins tability. So in the continuous upslide rain many thunderstorms occurre d. Their intensity depends partly on local factors. The combination of the elongated and of large-scale and convective-scale heavy precipita tion along with local factors resulted in the extensive floods over Gr eece. These rainfalls according to other Greek authors who studied the ir main parameters statistically can be classified in the category of 'natural disasters'.