تحلیل همدید بارش های حدی و فراگیر در کرانه های شرقی خزر

در این پژوهش به‌منظور شناسایی و تحلیل بارش های حدی و فراگیر در کرانه های شرقی خزر، ابتدا پایگاه دادۀ بارش روزانۀ این پهنه، تشکیل و نقشه های هم بارش روزانه از تاریخ 1/1/1340 تا 10/11/1383 (15992 روز) بر روی یاخته هایی به ابعاد 14*14 کیلومتر به روش کریجینگ میانیابی و ترسیم شد. برای هر روز بارش، بیشینه و درصد مساحت زیر بارش محاسبه گردید. بر این اساس، حدی ترین و فراگیرترین بارش ها شناسایی و 101 روز از شدیدترین و فراگیرترین بارش های منطقه بر اساس شاخص پایۀ صدک 99 ام برای بررسی انتخاب شد. سپس، با بهره گیری از رویکرد محیطی به گردشی و تحلیل خوشه ای پایگانی انباشتی به روش ادغام وارد بر روی نقشه های فشار تراز دریا، 5 الگویی که در بارش های حدی و فراگیر کرانه های شرقی خزر موثر بودند؛ شناسایی گردید. به‌منظور تحلیل این بارش ها در هر الگو، یک روز به‌عنوان روز نماینده بر اساس ضریب همبستگی با آستانۀ 95 درصد تعیین و در این روزها نقشه های فشار تراز دریا، ضخامت جو در ترازهای 500-1000 هکتوپاسکال، الگوی گردشی تراز 500 هکتوپاسکال، تابع جبهه زایی برای ترازهای 1000 و 500 هکتوپاسکال و همگرایی شار رطوبت برای ترازهای 925 و 1000 هکتوپاسکال ترسیم و تحلیل گردید. نتایج این پژوهش نشان از نمود آشکار پرفشار بر روی دریای سیاه در اغلب الگوها دارد. بررسی الگوی ضخامت جو نیز نشانگر دو فرود عمیق بر روی شرق مدیترانه و شمال غربی دریای خزر است. تحلیل تابع همگرایی شار رطوبت نیز آشکار ساخت که رطوبت بارش های حدی و فراگیر منطقه عمدتاً توسط جریان های بادی که از روی دریای خزر می وزند تامین می شود و در درجه دوم دریاهای سیاه و مدیترانه در تغذیه رطوبت این بارش ها موثرند.

Synoptic Analysis of Extreme and Widespread Precipitations in Caspian Eastern Coasts

. IntroductionSince precipitation and its changes in each geographical area and also east coasts of the Caspian Sea make changes in human life and human activities directly or indirectly, this study wants to determine synoptic and dynamic patterns during precipitations and also appoint the resource of extreme and widespread precipitations in east coasts of Caspian. In this line, much research has been done in Iran and around the world. In the case of research done outside Iran, one can mention Lana &Borghino (2000), Huos et al (2008), Federico et al (2008), Keables (1989), and Lorenz et al (2008). In Iran, we can nameLashkary (1996), NajarSaligheh (2001), Khoshhal (1997), Mohammadi (2012), Bagheri (1993), Hosseini (2009), Azizi and Samadi (2007), Mofidiet, Zarrin and Janbaz Ghobadi (2008), and RezaiBanafshehet, Najafi, Naghizadeh and Abkharabat (2015).2. Material and MethodsIn this research, for synoptic analysis of extreme and widespread precipitations in Caspian eastern coasts, we interpolated and traced the daily Isohyet maps from 1340.1.1 to 1383.11.10 (15992 days) on the 14*14 km pixels by Kriging method and by use of daily precipitation data base of this region. These data provided the information of 108 studied local points for 15992 days to calculate maximum precipitation and area percentage under precipitation for every precipitation day. On this basis, we identified the most extreme and the most widespread of region precipitations on the basis of 99th centile base index for studying and analyzing. By applying environment – circulation approach and agglomerative hierarchical cluster analysis by "ward " method on the maps of sea level pressure, maps of the atmosphere thickness at levels of 500 and 1000 hPa and maps 500 hPa circulation patterns, circulation patterns which were effective and important for creating the extreme and widespread precipitations of Caspian eastern coasts. In continuation, for analyzing the precipitations in every pattern, the representative day defined on the basis of correlation coefficient with 95% threshold and in these days, sea level pressure maps, atmosphere thickness in 5001000 hPalevels, 500 hPa circulation patterns, functions of frontogenesis for 500 and 1000 hPa levels and moisture convergence flux for 925 and 1000 hPa levels were traced and analyzed.3. Results and DiscussionThe results of this research in the five patterns of sea level pressure indicate the presence of high pressure on the Black Sea that it’s tongue have been extended to Caspian coasts. Atmosphere thickness patterns indicate the deep trough on the Caspian Sea and Black Sea too. The analysis of frontogenesis function maps illustrate the existence of front on high latitude and Siberia that have had effect on studied area. The analysis of moisture convergence flux indicate that Caspian Sea is the most important source to providing humidity for precipitation in eastern coasts of this sea, and Aral Lake and Black Sea are in the next importance degree. Of course, mostly, the atmospheric flows importing the humidity of these two source in to the Caspian Sea that higher reinforcement results in occurring the extreme precipitation in eastern coasts of this.4. Conclusion The analysis of sea level pressure maps in extreme and widespread precipitations days, shows five patterns. In these patterns we can see depressure condition with high pressure dominance in the north of Iran and low pressure dominance in the south of Iran. Although frequency of the 5th pattern in more than others, the maximum of precipitation is seen in pattern number 2. The precipitations of the region begin from Ordibehesht and continue until Dey. The temporal peaks of precipitation are in Mehr, Aban, Shahryvar, and Azar. The patterns that are drawn, show high pressure systems on eastnorth of Iran and the Black Sea in all patterns, which enters cold air to the Caspian Sea from Siberia and Europe, and with the humidity of the sea, enters them into east coasts of the Caspian Sea. Atmosphere thickness patterns that are drawn for representative days, indicate the deep trough on the eastern Mediterranean and north west of Caspian Sea, which makes situation for falling cold air from Europe into the area. The analysis of moisture convergence flux shows the importance of the Caspian, Black and Mediterranean Seas as resource of humidity for extreme and widespread precipitations.