چکیده:
در این پژوهش، دو سامانه بارشی زمستانه که در روزهای نهم تا دوازدهم ماههای ژانویه و فوریه سال 2020، به ترتیب سبب سیلاب در استان سیستان و بلوچستان و بارش برف سنگین در استان گیلان شده است، از دیدگاه همدیدی بررسی شد. نتایج نشان داد هرچند این دو سامانه از نظر شکل ظاهری با هم متفاوتاند، ولی هر دو هسته سردی دارند و از نظر نحوه تشکیل و تقویت مشابه هستند. تقویت ناوه ارتفاع تراز میانی، ایجاد حرکت چرخندی و فرارفت تاوائی نسبی مثبت در شرق ناوه، وجود همگرایی در سطوح پایین، ایجاد یا تقویت کمفشار در سطح زمین در موقعیت شرق ناوه، در تشکیل و تقویت این دو سامانه بارشی نقش اصلی را برعهده دارند. این سازوکارها با نظریه تشکیل و توسعه سامانههای کمفشار عرضهای میانی هماهنگ است. یکی از تفاوتهای آشکار این دو سامانه، تداوم و افزایش سرعت باد شمالی در سامانه اولی است که در اثر تغییر نصفالنهاری جت قطبی ایجاد شده است. این شرایط سبب شده است تا انحناء چرخندی ایجاد شده در محور جت جنب حارهای، در سامانه اولی بیشتر شود و به سوی جنوب نیز امتداد یابد. این انحناء در سامانه اولی در شرق دریای سرخ و در سامانه دومی در شمال شرق دریای مدیترانه قرار گرفته است. همچنین در سامانه اول فرارفت هوای گرم و مرطوب از دریای عرب و دریای عمان و در سامانه دوم فرارفت هوای سرد از روی دریای خزر شرایط مناسبی را برای به ترتیب بارش همرفتی و بارش برف سنگین فراهم کرده است.
Introduction Analysis of a phenomenon occurrence frequency involves a set of methods that apply probability rules to estimate recurrence probability for the phenomenon in a time period. Torrential rains and heavy snowfalls are among the extreme weather events whose frequency of occurrence has increased in recent years, and therefore, have attracted the attention of meteorologists and climatologists. Some researchers have tried to investigate the effect of climate change on torrential rains and heavy snowfalls, while some others have simulated them using numerical weather prediction (NWP) models. Heavy rainfall events and heavy snowfalls are major meteorological disasters that occur around the globe. They pose a significant challenge for both scientific research and operational forecasts. From January 9 to January 12, 2020, an intense precipitation system passing through south of Iran, caused severe floods and inundation in different regions of Hormozgan, Kerman, and Sistan and Baluchestan provinces and resulted in substantial financial losses. From February 9 to February 12 of the same year, a cut-off low slowly crossed the Black sea and the northern Caspian Sea, which caused the temperatures to fall across the country and produced heavy snowfalls in the provinces around the Caspian Sea, especially Gilan. The purpose of this study is to compare the synoptic conditions that cause these two systems to strengthen. Materials and methods The 6-hour precipitation data from several synoptic stations in provinces affected by the selected systems were obtained from Iranian Meteorological Organization which measured in millimeters (mm). To analyze the amount of daily rainfall, the total recorded rainfall at 0600, 1200, 1800, and 0000 hours GMT (Greenwich Mean Time) were used. To investigate the synoptic of the two selected systems, the pressure field and mid-geopotential height, upward motion at mid-level, temperature advection and specific humidity in the lower level, and the wind field in the upper levels were analyzed during rainy days. The required data was extracted from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP/NCAR) archive on a regular network limited to zero to eighty degrees’ east longitude and ten to sixty degrees’ north latitude. This data sets were first interpolated to a common regular grid of 2.5o ×2.5o spatial resolution. With the help of computer software, relative vorticity, relative vorticity advection, and temperature advection were calculated based on theoretical foundations. Results and discussion Two precipitation systems with significant contrast in form were investigated from a synoptic perspective. The results from the synoptic investigation revealed the main difference between these two systems. It was found that the northern stream, generated as a result of meridional shift in the polar jet, was more persistent in the system that caused the floods in Sistan and Baluchestan province. Wind speeds are also higher in these streams. These conditions have caused the cyclonic curvature formed in the subtropical jet stream axis to increase and extend to the south. In this system, the advection of warm moist air from the south latitudes, the Arabian Sea and the Gulf of Oman has caused the thermal energy to increase and provided a suitable condition for convectional rainfalls. In the system responsible for the heavy snowfall in Gilan, the curvature of the subtropical jet stream is low, and the advection of cold weather in north latitudes has intensified the cut-off low and increased the cyclonic rotation. The transference of moisture from the warm waters of the Caspian Sea, the Black Sea and the Mediterranean Sea into the system has also provided the appropriate conditions for heavy snowfall. Conclusion In this study, a synoptic perspective was adopted to investigate two winter precipitation systems that caused a flood in Sistan and Baluchestan and a heavy snowfall in Gilan from January 9 to January 12 and February 9 to February 12, 2020, respectively. The results demonstrated that although different in shape, both systems were cold-core and similar in their formation and strengthening. Amplification of the mid-level trough, formation of cyclonic rotation, positive relative vorticity advection to the east of the trough, convergence in lower levels, and formation or strengthening of low-pressure at the Earth’s surface to the east of the trough, played a key role in development and strengthening of these two systems. These mechanisms are consistent with the theory of development and strengthening of mid-latitude low-pressure systems. A noticeable difference between these two systems is the continuity and increasing speed of the north wind in the first system, caused by the meridional shift in the polar jet stream. These conditions have caused the cyclonic curvature formed in the subtropical jet stream axis in the first system to increase and extend to the south. These curvatures are located to the east of the Red Sea in the first system and northeast of the Mediterranean Sea in the second system. Also, in the first system, the advection of warm moist air from the Arabian Sea and the Gulf of Oman, and in the second system, the advection of cold air from the Caspian Sea have provided suitable conditions for convectional rainfall and heavy snowfall, respectively.
خلاصه ماشینی:
عباس رنجبر سعادت آبادی- دانشیار گروه پیش آگاهی مخاطرات جوی پژوهشگاه هواشناسی، تهران ، ایران تاریخ دریافت : ١٣٩٩/٨/١ تاریخ بازنگری: ١٣٩٩/٩/٢٠ تاریخ تصویب : ١٣٩٩/٩/٢٦ چکیده در این پژوهش ، دو ســامانه بارشــی زمســتانه که در روزهای نهم تا دوازدهم ماه های ژانویه و فوریه ســال ٢٠٢٠، به ترتیب سـبب سـیلاب در اسـتان سیستان و بلوچستان و بارش برف سنگین در استان گیلان شده اسـت ، از دیدگاه همدیدی بررسـی شـد.
رنجبر و همکاران (١٣٩٧)، با اسـتفاده از مدل WRF و انتخاب سه دامنه با تفکیکی افقی متفاوت ١٨، ٦ و ٢ کیلومتر و بازه زمانی یک سـاعت ، سیلاب منطقه کن و سیچان مورخ بیست و هشتم تیرماه نود و چهار را شبیه سازی کردند و نشان دادند که نفوذ شمال سوی هوای گرم عرض های جنوبی و همراهی عبور ناوه سطوح میانی و بالایی وردسپهر و ریزش هوای سرد در این ترازها ناپایداری شدیدی را در منطقه ایجاد کرده است .
راسـتگو و رنجبر سـعادت آبادی (١٣٩٧)، با استفاده از داده های بارندگی ٨ ایسـتگاه هواشـناسی استان بوشهر در یک دوره آماری ١٤ ساله و به کارگیری شاخص پایه صدک، بیش ترین بارش روزانه حدی این اسـتان را ٨٤/٢ میلیمتر برآورد کردند و نشـان دادند که وقوع بارش های شدید و حدی در استان بوشهر، نتیجه استقرار یک سامانه بندالی از نوع رکس یا امگا در تراز میانی جو و سامانه کم فشار سودانی سطح زمین در جنوب غرب ایران است .