The Synoptic analysis of flood occurrence on November 2011 in Behbahan and Likak cities

The Recognition of behaviors of the atmospheric circulation patterns and effective climatic elements in occurrence of flood is very important، especially because abnormal climate changes such as global warming in recent years tend to change in atmospheric circulation patterns and outbreak climatic inelegances in many areas in the world. Intensive and torrential precipitations in anomalous time and space is very important for a country like Iran with specific climatic characteristics and is always among the factors which in different geographical regions has some irrecoverable effects on humans life and finance and has affected human beings and nature widely. The south-west region of Iran that has been located in the windward slops of Zagros mountains and in the path of westerlies and rainbearing air masses، is among the regions which due to different atmospheric circulation patterns and receives heavy precipitations and is exposed to destructive floods. That flood of 20 November 2011 which has occurred in Khuzestan and Kohkilou-e-va-buyer-Ahmad provinces is obvious among them. This flood imposed many death tolls and caused a big mass of financial damages to those regions. Coordination before occurring accidents and hazards is the key factor in crisis management. Therefore identification of flood causing patterns and their punctual prediction can facilitate better management of crises and hazards and minimize mortality and financial damages caused by these type of natural disasters. This flood in the study area created by an extreme storm in western and southwestern regions of Iran. In order to analyze the synoptic pattern of this flood، the data of sea level pressures، surface wind directions، surface temperatures، geopotential heights of 500 Hpa and specific humidity and wind directions of 850 and 700 hpa and data representing omega values from NCEP/NCAR website has been collected and then synoptic maps were drawn by using GrADS software which were related to 2 days before flood until a day after that. Then the causes of flood were revealed by synoptic analysis of the maps. Analysis of synoptic maps reveals role of some systems in creating the flood. By positioning the pressure center of Sudan low between European high pressure and Arabian subtropical high pressure has been resulted to form a convergence belt over the Sudan and the Red sea. European high pressure caused the advection of cold air from high latitudes and in other hand، Arabian subtropical high pressure caused to advection of warm and humid air from low latitudes into the Sudan low - Red sea’s convergence belt. Convergence of northern cold air and southern warm air in this belt، tend to intensify the belt. Gradually، with intensification of the system and its NE move، the convergence belt lied over SW regions of Iran. Pressure map of 500 hp shows a deep trough in North Africa. This trough intensifies temperature gradient in upper levels of the atmosphere and following of this event، the trough becomes deeper and its axis lies over the Red sea. Analysis of specific humidity and wind direction maps of 850 and 700 hpa indicated that the humidity sources for the rainfall systems were the Red sea، Arabian Sea and Persian Gulf in 850 hp level and Red sea and Persian Gulf in 700 hp level. Locating Sudan low pressure between Arabian subtropical high pressure and European high pressure tends to create an air convergence belt with SW-NE trend from Sudan to north of Arabian Peninsula. Cold Advection from European high pressure and warm advection from Arabian high pressure finally intensify temperature and pressure gradients in the convergence belt and the transformed the system into a dynamic system. In result of advection of cold air to back part of the system، its eastward movement became faster and pull the system over SW areas of Iran. Movement of cold air in back part of the system caused an extreme updraft of air and generated a severe rainfall. The Arabian subtropical high pressure provided required humidity for this rainfall from warm waters of Red sea and، Arabian Sea and Persian Gulf into the system and eventually resulted in a heavy rainfall that in a short time generated the disastrous flood with abundant damages in study area.