تحليل سينوپتيكي بارش‎هاي شديد در استان اصفهان

Synoptic Analysis of Heavy Precipitation in the Isfahan Province

به‎منظور تعيين الگوي همديدي بارش هاي شديد در استان اصفهان، از آمار بارش 44 ايستگاه سينوپتيك، كليماتولوژي و باران‎سنجي استفاده شد. با استخراج داده هاي بارش در دوره‎ي آماري بيست‎ساله (2005-1986)، بارش هاي شديد در هر يك از ايستگاه‎ها دريافت شد. با توجّه به‎گستردگي و تفاوت موقعيّت بين شرق و غرب اين استان، ملاك بارش شديد، بيشترين بارشي بوده كه در طول اين دوره‎ي آماري در هر ايستگاه رخ داده است. براي تعيين الگوي همديدي اين بارش ها، داده هاي فشار، نم ويژه، مولّفه‎ي باد مداري (u)، مولّفه‎ي باد نصف‎النهاري(v) و سرعت قايم، در ترازهاي متفاوت به‎صورت شش ساعته و روزانه، از دو روز پيش از بارش از NCEP/NCAR به‎دست آمد. يافته ها نشان مي‌دهد، الگوي ادغامي كم فشار مديترانه و سوداني موجب بارش ‎هاي شديد در سطح استان اصفهان مي شود. اين الگو، همراه افت شديد فشار در مركز ايران و فرارفت تاوايي مثبت و حداكثر سرعت قايم منفي در نيمه‎ي غربي ايران است، بارش هاي شديد را در مركز و شرق استان، سامانه هاي سوداني ايجاد مي‎كنند كه به‎ترتيب از روي خوزستان و بوشهر وارد ايران شده اند. در اين الگوها، پُرفشار روي درياي مديترانه و زبانه‌هاي پُرفشار روي جنوب‎شرق ايران و شرق عربستان و همراهي آن با منطقه‎ي همگرايي تراز فوقاني جو و افزايش فشار و فرارفت تاوايي منفي در تراز دريا، نقش مهمّي در تقويّت و تعيين مسير اين الگوها دارند. الگوي ادغامي كم فشار سوداني و مديترانه اي روي شرق درياي مديترانه، موجب ايجاد بارش هاي شديد در غرب استان مي‎شود. يافته ها بيانگر آن است كه استقرار مركز فرارفت تاوايي مثبت در شمال‎شرق عربستان و روي خليج فارس در تراز سطح دريا و در تراز 500 هكتوپاسكال، از عوامل اصلي به‎وجود آوردنده بارش شديد در همه الگوها همديدي به‎شمار مي‎رود.

Introduction Iran is a dry and semidry country with precipitation less than one- fourth of the world because Iran is located in the vicinity of subtropical high pressure and is affected by such climate. Short time precipitation and falling as severe shower are climate patterns of this area. Because of topography condition and geography situation, the center of Iran has less precipitation than the other parts. Most of the systems that enter to Iran from south and south west lose more precipitation in the windward of Zagros. Sometimes these systems are invigorated after passing on mountain in some synoptic condition and cause heavy shower. Isfahan province with 10.6 million hectare area is located in the center of Iran between Zagros Mountain in the west and Central desert in the east. Climate condition and precipitation in Isfahan province is hardly affected by this situation. The mean annual precipitation about 120 mm is scattered over 85 percent of this area. Although the province vital artery that has named “Zayandeh Rood” is mainly dependent to water sources that placed in outside of this province, but supplying agricultural sector required water is dependent to precipitation and subsurface water (type of storage of precipitation). As well as Zayandeh Rood has have drought periodic. Therefore in this situation, precipitations have an important role and need management and schematization for water source to prevent wastage of water. Methodology For determination of synoptic pattern of heavy precipitations in Isfahan province, statistics relating to precipitation in 44 synoptic stations, climatology and rain gauge of meteorology organization as well as Ministry of Energy rain gauge station were used. Through extraction of precipitation data in stations for statistical period of 20 years (1986-2005), heavy precipitations were extracted in each one of the station. With regard to expansion of Isfahan province and position difference between east and west of this province, criterion for heavy precipitation was the most repetitive precipitation which had occurred in each station during this statistical period. Data were sorted in SPSS software from maximum to minimum and we used the first quarter of data as heavy precipitation. Through data extraction, the heaviest and the most pervasive precipitation were identified in the province. Considering this criterion, 37 patterns were extracted. Through this patterns 4 patterns were selected .In order to determine synoptic pattern of these precipitations, data of pressure, specific humidity, orbit wind component (u), meridian wind component (v), vertical speed (omega), were prepared in different levels for 6 hours and two days before beginning precipitation till precipitation climax from national center environmental precaution of America (NCEP/NCAR). Results and Discussion Considering meteorological maps show four patterns for heavy precipitation in the Isfahan province. Most heavy precipitation has caused by Sudan and Mediterranean low pressure merger pattern. Two low pressures have been merged due to eastern motion Mediterranean low pressure and northeast motion Sudan low pressure on Iraq. This system has accompanied with pressure reduction in Iran center and positive vorticity advection and maximum negative vertical speed on half of western Iran. Sudan systems have entered to Iran from two ways: Khuzestan (track A) and Boshehr (track B). In these patterns, high pressure situated on Mediterranean Sea and high pressure tongues have expanded on southeast of Iran and east of Saudi Arabia. These systems were accompanied with convergence zone of atmosphere upper level and pressure increase as well as negative vorticity advection in sea level. These ingredients play important role in nutrition and determination of track in these patterns. The fourth pattern is Sudan and Mediterranean low pressure that have merged on the east of Mediterranean Sea. In this pattern, high pressure tongues have strengthened on center and southern half of Iran and have caused Sudan low pressure moved toward north. They are merged with Mediterranean low pressure in the east of Mediterranean Sea. In spite of high pressure has weakened on southern half of Iran but has still located there. This system has moved to east and entered to Iran. Positive vorticity advection and humidity maximum in front of cyclone have caused heavy precipitations in west of the province. Conclusion Patterns that cause heavy precipitations on Isfahan province include: 1- Sudan and Mediterranean low pressure merger pattern on Iraq cause the heaviest and pervasive precipitations over Isfahan. 2- Sudan and Mediterranean low pressure merger pattern on the east of Mediterranean Sea cause heavy precipitations over the west of Isfahan. 3- Heavy precipitation in center and east of the province are caused by Sudan systems which have entered to Iran from Khuzestan (track A) and Boshehr (track B) respectively. Finding indicates that the positive cyclone advection center situated on northeast of Saudi Arabia and over Persian Gulf both in sea level and level of 500 Hectopascal are considered as the main reasons for generation of heavy precipitations in all synoptic patterns. In addition, two high pressures on Mediterranean Sea and south of Iran and west of Saudi Arabia in the sea level and two high height on Saudi Arabia and Libya have important role in the system arrangement.