واكاوي توزيع مكاني كانون‌هاي تمركز ارتفاع برف اَبَر سنگين در جلگۀ گيلان با استفاده از مدل WRF (سامانه‌هاي برف سنگين سال‌هاي 1383، 1386 و 1392)

Analyzing the Spatial Distribution of Heavy Snow Fall Depth in Gilan Plain (February 2005, January 2008, and February 2014) Using WRF Model

جلگۀ مركزي گيلان طي دهه اخير تحت تأثير سه سامانه بارشي با بارش فوق سنگين برف قرار گرفته است. شناسايي مناطق تحت مخاطره و تعيين عوامل آب‌و‌هوايي مقياس منطقه‌اي تاثيرگذار روي شكل‌گيري الگوي مكاني بيشينه عمق برف، نقش مهمي در مقوله مديريت بحران و تسريع خدمات‌رساني به جوامع آسيب‌پذير ايفا مي‌كند. بدين منظور، سازوكار اين سامانه‌ها با استفاده از اجراي مدل عددي WRF با تفكيك افقي 7 و 21 كيلومتر شبيه‌سازي و بررسي شد. خروجي مدل دقت قابل قبولي در شبيه‌سازي مقادير بارش و آشكارسازي دو هسته بيشينه ارتفاع برف يكي در جلگه مركزي گيلان و ديگري حوالي تالاب انزلي دارد. منشأ اين سه سامانه، توده هواي سرد و پرفشار قطبي از سمت شمال كشور روسيه و يا زبانه پرفشار نيمه‌دائمي سيبري و همراهي آن با ناوه‌هاي عميق سطوح مياني جو است. گردش واچرخندي با هسته قوي روي شمال و شمال‌شرقي درياي كاسپين موجب فرارفت هواي سرد در لايه‌هاي زيرين وردسپهر مي‌شود‌‌. واداشت سرمايشي ناشي از گسترش زبانه توده هواي سرد روي رشته‌كوه‌هاي قفقاز و شارش جريان‌هاي سرد كوه به دشت، موجب شكل‌گيري واچرخند ثانويه در مقياس محلي روي جلگه كورا در غرب كاسپين مي‌شود. تباين دمايي بين جلگه كورا و پهنه آبي كاسپين و همچنين شيو فشاري بين جلگه كورا و سواحل جنوب‌غربي كاسپين با سوي شرقي ميدان باد از جانب پرفشار ثانويه كورا همراه است كه در برخورد با جريان‌هاي غرب‌سو ناشي از گردش ساعتگرد واچرخندسرد روي كاسپين، موجب همگرايي جهت باد سطحي به صورت باند همگرايي در امتداد ساحل غربي پهنه كاسپين مي‌شود. جريان‌هاي همگرا شده، حامل شارهاي رطوبت بوده و به پهنه كوچكي در جنوب‌غربي سواحل كاسپين وارد مي‌شود كه دقيقاً منطبق بر كانون بيشينۀ عمق برف در جلگۀ مركزي گيلان است. در سامانه برف 1392 در ناحيه همگرايي جريان باد در تصوير سنجنده موديس ماهواره ترا و همچنين تصوير شدت بارشِ خروجي رادار گيلان، باند ابري مشاهده مي‌شود كه نتايج حاصل از شبيه‌سازي مدل عددي و تحليل‌هاي همديد را تأييد مي‌كند.

Introduction Without any doubt, the Gilan plain in the northern regions of Iran plateau has the special conditions faced with heavy snowfall hazards in terms of damage amount. In the recent decade, the increase in frequency of heavy snowfall events in contrast with earlier decades, has caused a huge attention to this atmospheric hazard in Gilan province and also Iran. In most studies that were done on heavy snowfall events, the target was analyzing the synoptic-dynamic structure of case study and long term events. Overall, the main question about heavy snowfall in Gilan plain was the cause of this event in the central plain and another was the movement of the maximum snow depth area in this region from east to center and west. So, what is the structure for super heavy snowfalls in Gilan central plain in recent decade? What were the causes for the formation of two different spatial patterns of these three snowfalls? The snow depth of the west, central and east regions of Gilanplain in the super heavy snowfall events (Gilan Meteorological Office). Total snow fall (cm) Western plain Central plain Eastern plain Station Astara Talesh Bandar Anzali Agricultural Lahijan Roodsar 7th -12th Feb 2005 10 No data 3 170 120 No data 7th -14th Feb 2008 36 59 204 136 91 66 30th Jan-4th Feb 2014 34 3 95 40 45 75 2. Material and Method 1- GFS-FNL data with horizontal resolution of 0.5 degrees as an input to the WRF 3.5.1 version. 2- Hourly data of meteorological parameters in the 12 synoptic stations of Gilan province for statistical analysis of used atmosphere characteristics 3- Visible band images and 1-2-7 MODIS sensor for Terra and Aqua satellites at the time of the snow falling. 4- Kiashahr (Gilan) radar outputs at the time of the snowfall 3. Results and Discussion In this investigation, the role of effective regional factors and geographical components on formation of the maximum snow depth patterns in three systems with super heavy snowfall in Gilan province of Iran is investigated using numerical model of WRF with horizontal resolution of 7 Km. The overall results are as follows: In large and mesoscales, the source of these atmospheric systems are the cold anticyclones of North Europe and the semi-permanent anticyclone of Siberia that extend to lower latitudes with strong depth and its cold flow affects the northern part of Iran and also the south coast of the Caspian Sea. The precipitation output of WRF numerical model for 7 Km horizontal resolution, perfectly enhances the spatial pattern of snow depth in these three recent systems that includes Gilan central plain and near the south of Anzali wetland. The important point in 10-meter wind output in the region is that in all three systems with the southerly movement of cold high pressure center to lower latitudes, cooling over the Caucasus and the high mountains of this area is intensified. These situations are seen appropriately in the 2-meter temperature pattern. Formation of cold flow from mountain to plain f caused by the existence of cold cores over the big and small Caucasus Mountains, led to form secondary high pressure in local scale over the Kura plain. Settlement of secondary high pressure over western coast of the Caspian Sea (Kura plain) and the weakening of pressure counter over the southern coast of the Caspian and also the temperature difference between water surface of the Caspian Sea and the land surface temperature in the west of the coasts (Kuraplain) causes the formation of easterly flows from the Kura to the Southern Caspian water surface. These easterly flows acts as a forcing in local scale in contrast with the westerly wind that is from the high pressure mass in Northeast of the Caspian Sea and causes the convergence of wind flow in the conjunction part of them and towards the lower latitudes. The convergence of 10-meter wind flow in the form of convergence band along the western coast of Southern Caspian enters to the small area of Southwest of the Caspian in the Gilan central plain or near the Anzali wetland and causes the intensification of instability in the lower layers of Troposphere. The maximum snow concentration area dependson the convergence part of the flows in Gilan central plain. The images of MODIS Terra and Aqua and also the Kiashahr (Gilan) radar output in the 2014 snowfallconfirm the formation of cloud band coincident to the wind convergence band over the western part of the Caspian coasts that enters to the southern parts. 4.Conclusion In recent decade, Gilan central plain has been affected by three main precipitation systems with super heavy snowfall. In order to determine the effective weather factors in regional scale in organizing the spatial pattern of the maximum snow depth event, the structure of these systems is investigated using the WRF numerical model with horizontal resolution of 21 and 7 Km. The output of the model has an appropriate accuracy in simulating the amount of precipitation and enhancing the two cores of maximum height of snow depth. (One in the central plain of Gilan and another near the Anzali wetland). The source of these three systems is the cold and high pressure polar air mass coming from the northern part of Russia and the semi-permanent high pressure of Siberia and accompanying with the deep troughs of atmosphere medium levels. The anticyclonic circulation with strong center over the North and Northeast of the Caspian Sea causes advection of cold air in lower layers of Troposphere. The cooling forcing, caused by the development of cold air mass over Caucasus Mountains and spread of mountain toplain cold flows, caused governing of secondary anticyclone in local scales over Kura plain in the West of the Caspian Sea. The temperature difference between Kura plain and the Caspian water surface and also thepressure gradient between Kura plain and the Caspian Southwest coasts, is accompanied with the eastern flow of the wind field from Kura secondary high pressure. That in conjunction with westerly flows caused by clock wise circulation of anticyclone over the Caspian Sea, causes convergence of surface wind direction to convergence band along with Western coast of Caspian water surface. The Converging flows are bearers of humidity flux and enteredto the small part in Southwest of the Caspian coasts that exactly matched with the highest snow depth in Gilan central plain. In the convergence part of the cold flow in MODIS sensor images of Terra satellite and also the image of precipitation intensity of Gilan radar output, thecloudy band is seen that confirms the results from numerical simulation and synoptic analysis