Abstract
Ghare Sou basin is one of the sub-basins of Karkeh River located in Kermanshah, West of Iran. In spite of not being a flood-prone area in terms of physiography, severe floods have occurred in this region. The main aim of the present study is to present an overview of thermodynamicand synopticconditions affecting precipitations causing extreme flows. Accordingly, two databases were made use of: a) data including discharge and daily precipitation measured at the basin area covering the period of 1389-1351 and b) data including air temperature, air pressure, geopotential height, zonal and meridional wind speed, specific and relative humidity related to the levels of 1000, 850, 700, 500, 600, 400, 300, 250, 150, and 100 hPa provided on the NCEP / NCAR for each six-hour monitoring. Then, with application of the probability distribution method, the ultimate limit of type I with a ten thousand-year return period of 02/247 cubic meters per second per day discharge threshold was specified as diagnostic criteria for exceptional flows. Finally, five floods were extracted for explanation, among which one sample was analyzed. The obtained results revealed that during rainy days, blocking systems such as Rex, Omega, cut-off low, and high-pressure stack systems in the middle levels of the atmosphere were dominant over Mediterranean and Middle East region; the mentioned patterns revealed disruption of normal atmosphere. Moreover, analyses of the indexes and thermodynamic charts were indicative of non-modular arithmetic of maximum indexes and maximum conditional instability with maximum precipitation. Hence, acquaintance with synoptic conditions of systems leading to floods can be of great significance in presenting flood warnings before occurrence of any crises.