Abstract:
بارشهای سنگین بهعنوان عامل مخاطرهآمیز در بروز سیل مطرح بوده و به طور مداوم خسارتهای جبرانناپذیری در بیشتر مناطق، از جمله استان لرستان وارد میکند. در این پژوهش به بررسی نقش تعامل الگوهای توام بریدهی کمفشار و رودباد جبههی قطبی در رخداد بارش سنگین فروردین ماه 1398 در استان لرستان پرداخته شد. برای نیل به این هدف، از چهار پایگاه داده شامل: فراسنجهای پایش جوی روزانهی (نقشههای فشار، دما، باد، تاوایی، رطوبت نسبی و...) پایگاه NCEP/NCAR با قدرت تفکیک مکانی 5/2 درجه جهت شناسایی الگوهای همدیدی موثر بر وضعیت هواشناختی منطقهی مورد مطالعه، نقشههای همپوشانی مدل عددی جهانیGFS با قدرت تفکیک مکانی 13 کیلومتر برای بررسی تطبیقی جریان باد با فراسنجهای مورد مطالعه، دادههای بارش ماهوارهTRMM با قدرت تفکیک مکانی 25/0 درجه، جهت پایش توزیع فضایی بارش و دادههای مشاهداتی ایستگاه هواشناسی خرمآباد بر اساس کدهای هواشناسی همدیدی (ww)، در راستای تحلیل وضعیت هوا در هنگام رخداد بارش سنگین، استفاده شدهاست. واکاوی همدیدیـ دینامیکی سامانهی بارشی مورد نظر نشان داد؛ ناپایداری و صعود هوای مرطوب توسط دو واداشت جوی بریدهی کمفشار و رودباد جبههیقطبی و همگرا شدن جریانات مرطوب از سه کانون رطوبتی شمال اقیانوس هند، دریای سرخ و شرق مدیترانه شکل گرفت. این ویژگیها، شرایط همدیدیـ دینامیکی لازم را برای وقوع بارش سنگین مهیا نمود. البته عوامل محیطی متعدد دیگری، از قبیل اشباع بودن لایه سطحی خاک به دلیل میزان بالای بارش در روزهای قبل از وقوع سیل و... در تشدید سیل مذکور نقش داشتهاند.
IntroductionOne of the most dangerous atmospheric hazards that can cause so much damage is the flood phenomenon. Heavy rainfall has been a risk factor for flooding. Floods are one of the most severe and destructive atmospheric hazards that have financial and physical harmful effects and affect humans and nature on a large scale. Heavy rainfall and thunderstorms are among the climatic features of the arid and semi-arid regions causing floods. Iran is also affected by the flood phenomenon because of this feature. Synoptic patterns, distances and proximity to sea, altitude, slope and direction of roughness are the most important backgrounds for the formation of precipitation events. Interaction of these factors can exacerbate this phenomenon and increase heavy rainfall. Studying the climatology of precipitation, while recognizing the characteristics and behavior of precipitation, it is possible to plan based on it. Therefore, the mechanism of precipitation, especially in critical conditions (such as drought and flood), has been of interest to researchers in Iran and worldwide. Many studies have been conducted in this regard with different attitudes and goals regarding the dynamic, thermodynamic and synoptic analysis of heavy rainfall. Therefore, despite the fact that floods are recognized as one of the natural hazards and their occurrence every year in Iran, not only the damage caused by this phenomenon has not diminished but it is also increasing. Therefore, planning and trying to reduce the damages caused by it is essential. Lorestan province in southwestern Iran, due to its geographical location and geological and geomorphologic conditions, is affected by devastating floods every year due to heavy rains and floods, which often cause a lot of casualties and financial losses. Therefore, understanding the regime and pattern of these types of precipitation and the synoptical-dynamical mechanisms that lead to their occurrence will help to identify and predict these types of precipitation.The purpose of this study was to investigate the synoptical - dynamical analysis of one of the heaviest rainfall in Lorestan province (April 2019), which was unprecedented in the last two hundred years, based on meteorological statistics.Materials and methodsBelieving that heavy rainfall in each region is associated with high-altitude convective synoptic patterns and meteorological conditions in the region, assuming occasional heavy rainfall in the western half of Iran is affected by the Cut Off Low and Polar Front Jet Stream, Selected as the case study. For this purpose, four databases for a wide area focusing on Lorestan province include: daily atmospheric monitoring parameters such as maps of pressure, temperature, wind, precipitation, relative humidity, etc. at different levels from the National Center for Atmospheric Research and The National Center for Environmental Forecasting (NCEP / NCAR), GFS global numerical model overlay maps, TRMM 7 Level 3 satellite precipitation data, and weather data at the time of precipitation were used based on synoptic signs and meteorological codes (ww). In the final step, data are used with time steps (hourly and daily) and different locations (from 0.25 to 2.5 °) for different atmospheric levels (sea level, 850, 700 and 500 hPa) for a range of 0 to 70 East longitude and 10 to 60 degrees north latitude were extracted and processed using various statistical and graphical software.Results and discussionLorestan is a mountainous land that, with the exception of a number of alluvial valleys and a few small plains, has no smooth area, and almost all of it encompasses the central Zagros Mountains, which regularly extend northwest to southeast. These topographies associated with deep converging valleys are often compact, tall, and uneven, which increase the flood potential due to the high slope and the low permeability. The amount of rainfall that resulted from the floods of April 2019 was much higher than other flooding events in Lorestan province, according to unprecedented meteorological statistics.The results show the instability and ascension of humid air by two Cut Off Low (COL) atmospheric forcing and the Polar Fron Jet Stream (PFJ) and the convergence of moist currents from the three moisture centers of the northern Indian Ocean (Arabian Sea), the Red Sea and the eastern Mediterranean. These features provided the dynamical-synoptical conditions necessary for heavy rainfall occurrence in April 2019 in Lorestan province. However, other environmental factors such as saturation of the soil surface layer due to the high amount of precipitation in the days before the floods, etc. have also contributed to the intensification of the flood.ConclusionThe results of the synoptic-dynamic analysis of the flood system (30 March to 1 April 2019) in Lorestan province showed that the precipitation was in the synoptic patterns middle level of the atmosphere and the occurrence of convective upward movements depend on the lower levels. In this rainfall, Cut Off Low formation in the vicinity of polar front Jet Stream provided the dynamic conditions necessary for the formation of significant Heavy rainfall in Lorestan province.In such conditions, the fusion of the unstable region due to the jet stream with the unstable region of the eastern part of the Cut Off Low has a great potential for destabilizing and uplifting the humid air below which the convergence of moisture flux originates from the foci of the northern Indian Ocean (Arabian Sea). Red and Mediterranean have increased its humidity to saturation. In other words, the aforementioned patterns provide the potential for sufficient and dynamical- synoptical potential for heavy rainfall events in the study area.On the other hand, Lorestan province is on the located of entry of numerous western rainy and continuous rainfall caused by these masses during most of the cold season (region saturation), mountainous region and topography with deep and converging Compact valleys, very low permeability of the region Due to its special lithology, reduction of surface runoff time due to deterioration of vegetation in the area and some other human factors has doubled the flood potential of Lorestan province.
Machine summary:
بررسي نقش تعامل الگوهاي توأم بريدة کم فشار و رودباد جبهۀ قطبي در رخداد بارش سنگين فروردين ماه ١٣٩٨ در استان لرستان کوهزاد رئيس پور١ - استاديار اقليم شناسي ، گروه جغرافيا، دانشگاه زنجان ، زنجان ، ايران .
عليجاني و همکاران (١٣٨٩) با بررسي شرايط همديدي بارش سنگين ٦ ژانويه جنوب شرق ايران ، نشان دادند که رخداد بارش سنگين موردمطالعه ، با شرايط مطلوب همديدي نظير وجود رطوبت زياد در تراز پايين جو، به ويژه تخليه رطوبتي شــديد از خليج فارس ، بي هنجاري هاي منفي شـديد در ترازهاي مياني جو و اسـتقرار رودباد جنب حاره اي در غرب منطقه مرتبط بوده است .
از ديگر مطالعات انجام شـده در اين خصـوص که نشـان از تلاش محققان براي شناسايي عوامل مؤثر بر رخداد بارش هاي سنگين در گسترٔە ايران دارد ميتوان : به بررسي اثر ناؤە عميق شرق مديترانه بر بارش هاي سنگين ايران (مفيدي و زرين ، ١٣٨٤)، اثر فرودهاي عميق مديترانه اي بر بارش هاي سـيل زاي ايران (اميدوار، ١٣٨٦)، اثر ناؤە شمال آفريقا بر بارش هاي سنگين جنوب و جنوبغرب ايران ، تعيين درياهاي عمان و عرب به عنوان مهم ترين کانون هاي رطوبتي مؤثر بر برخي از بارش هاي زمســتانۀ ايران (فرج زاده و همکاران ، ١٣٨٨)، بررسـي شـار رطوبتي سـنگين ترين بارندگي زمستاني در ايران مرکزي (براتي و همکاران ، ١٣٩٣) و مطالعات متعدد ديگري توســط محمدي، ١٣٨٨؛ عزيزي و همکاران ، ١٣٨٨؛ کرمپور و همکاران ، ١٣٩٢؛ مظفري و همکاران ، ١٣٩٦؛ مفاخري و همکاران ، ١٣٩٦؛ مصـطفيزاده و همکاران ، ١٣٩٧؛ شـادماني و همکاران ، ١٣٩٧ اشاره کرد که نتايج حاصــله به خوبي بيان گر اين اســت که الگوهاي همديدي متفاوتي در عرض هاي جغرافيايي مختلف ســبب وقوع بارش هاي سـنگين ميشـوند.