واكاوي همديد رخداد سيلاب آبان‌ماه 1390 در شهرستان‌هاي بهبهان و ليكك

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

در پژوهش حاضر به منظور واكاوي همديد سيلاب 29 آبان 1390 (20 نوامبر 2011) كه در شرق خوزستان و غرب كهگيلويه و بويراحمد رخ داد، تغييرات الگوهاي همديد در نقشه‌هاي سطح زمين و تراز 500 هكتوپاسكال از دو روز پيش از رخداد سيلاب تا يك روز پس از آن واكاوي شد. بررسي‌هاي انجام شده نشان داد كه تشكيل و گسترش مراكز كم‌فشار مديترانه- سوداني به‌دليل قرارگيري بين مراكز پرفشار اروپا و پرفشار جنب حاره‌اي جنوب‌شرق عربستان، سبب تشكيل كمربند همگرايي روي سودان و درياي سرخ مي‌شود. وجود دو مركز پرفشار يادشده به‌ترتيب سبب فرارفت هواي سرد و گرم به درون كمربند همگرايي درياي سرخ و سامانه سوداني مي‌شود و موجبات تقويت آن را فراهم مي‌كند. همچنين، وجود ناوه عميق شمال آفريقا كه محور آن از شرق مديترانه تا شمال سودان امتداد دارد، سبب تشديد تضاد دمايي در ترازهاي بالاي جوّ و پويايي بيشتر اين سامانه شد. قرارگيري منطقه پژوهش در خروجي جت‌باد جنب‌حاره‌اي نقش مهمي در تقويت و عميق شدن كم‌فشار مديترانه- سوداني و حركت شرق‌سوي آن داشته است. در اين شرايط آرايش الگوهاي همديد به نحوي بود كه منجر به ورود رطوبت از منابع رطوبتي درياي عرب، درياي سرخ و خليج فارس به منطقه پژوهش شد و سبب شد تا در روز بيستم نوامبر در منطقه پژوهش بارش شديد و در نهايت سيلاب رخ دهد.

Introduction 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 rain bearing 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 death tolls and financial damages caused by these natural disasters. Materials and methods This flood 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 pressure, surface wind direction, surface temperature, geopotential height of 500 hPa and data of zonal wind and meridional wind of 200 hPa for drawing jet stream map, specific humidity and wind direction of 850 and 700 hPa and data representing omega values has been collected from NCEP/NCAR website and then synoptic maps were drawn by using GrADS software which were related to 2 days before flood-maker rainfall until a day after its occuarrence. Then the causes of flood were revealed by synoptic analysis of the maps. Result and discussion Analysis of synoptic maps reveals role of some systems in creating the flood. Lying low pressure center of Sudan 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 of higher latitude and in other hand, Arabian Subtropical high pressure caused to advection of warm and humid air of 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 movement, the convergence belt lied over SW regions of Iran. Pressure map of 500 hPa 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. Locating the study area in exit part of Subtropical Jet Stream (STJ) causing to intensify Sudan Low and transferred it to east. 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 hPa level and Red sea and Persian Gulf in 700 hPa level. Conclusion Locating Sudan Low between Arabian subtropical high and European high 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 finally intensify temperature and pressure gradients in the convergence belt. Furthermore, locating study area in northeast of exit part of subtropical jet stream (STJ) causing to intensify of Sudan Low and transformed it into a dynamic system. In result of advection of cold air to back part of the system, its easward movement became faster and pulled the system over SW area of Iran. Movement of cold air in back part of the system caused an extreme updraft of humid air and generated a severe rainfall. The Arabian Subtropical High provided required humidity for this rainfall from warm waters of Red sea, Arabian Sea and Persian Gulf into the system and eventually resulted in a heavy rainfall in a short time that generated the disastrous flood with abundant damages in study area.