روابط بندال‌هاي كم‌فشار بريده و دوام يخبندان‌هاي بهاره ايران

The relationships between cut-off low blockings and durability of spring frosts across Iran

بادهاي غربي ضعيف در عرض هاي جغرافيايي بالا و مياني سبب بندال هاي كم فشار بريده مي شوند و اين بندال ها پديده هاي جوي بادوام مانند موج هاي يخبندان را پديد مي آورند. ايران در جنوب عرض هاي مياني هر ساله درگير موج هاي يخبندان بهاره است. هدف ما در اين تحقيق بررسي اثرات بندال هاي بريده بر دوام يخبندان در ايران بود. براي رسيدن به اين هدف داده هاي دماي كمينه روزانه 69 ايستگاه هواشناسي طي 20 مارس 1968 تا 21 ژوئن 2014 (برابر با فصل بهار در تقويم ايران) از سازمان هواشناسي تهيه شد. بر پايه جداول پراكنش دماهاي گزارش شده و نمودارهاي تغييرات فراواني آنها، تعداد 114 موج با دوام 3 تا 25 روز شناسايي شد. بررسي نقشه هاي هواي تراز 500 هكتوپاسكال طي روزهاي اوج بادوام ترين يخبندان ها نشان داد كه سامانه غالب در 18 موج يخبندان بندال ها از نوع كم فشار بريده بوده اند. اين سامانه ها در پيرامون درياي خزر با مركزيت شرق اين دريا نمايان و براي روزهاي متمادي ماندگار شدند و بادهاي غربي را به دو شاخه اصلي در شمال و فرعي در جنوب تقسيم كردند. نتايج نشان داد چهار عامل شامل «جابجايي مورب كم فشارهاي بريده از جنوب اسكانديناوي به سوي فلات پامير» و «حركت شرق سوي ناوه ها»، هر دو در تراز 500 هكتوپاسكال و «موقعيت رشته كوه زاگرس» و «مسيرهاي ورود زبانه هاي هواي سرد پرفشار سيبري»، هر دو در تراز درياي آزاد؛ زمينه ساز نفوذ متقارن امواج يخبندان در غرب و شرق ايران بوده اند.

IntroductionWeak westerlies over high and mid latitudes cause cut-off low blockings (CB) and CBs create durable phenomenon such as frost waves. Iran, in the south of mid latitudes is engaged with annual spring frost waves. So far, most of the researches in Iran have been done with statistical and synoptic methods, but most of these studies, especially the synoptic ones, are case study samples. The apparent role of large-scale atmospheric systems such as cut-off low blockings in the emergence and durability of spring frost-waves, requires great research to undertaken throughout Iran and in a long period. So our goal was the study of effects of cut-off lows on frost durability over Iran. Data and methods For this purpose, the data of minimum daily temperature of 69 weather stations from March 20th 1968 to June 21st 2014 was collected from the meteorological organization of Iran. It was determined that 114 spring frosts with a duration between 3 to 25 days exist and in the next step 47 enduring ones based on tables of daily temperatures and fluctuation of their frequencies were determined. All of these 114 waves were determined based on the frost duration criteria which lasted at least two consecutive days and their spatial criteria occurrence in more than two stations. This allowed the determination of the most durable frost index wave of each year during spring. Analyzing the peak-days of all index waves, distinguished the waves affected by the cut- off low blocking system. The final synoptic patterns were designed for analysis by examining the 500 hPa maps of the distinct days. Results and discussionThe height contours on 500 hPa air-charts from ENCEP/ENCAR showed cut-off low blockings were dominant patterns during the peak days of the most enduring frosts (18 ones). The first results indicate that the appearance of this blocking which directs the cold air flows to Iran from high latitudes has minimized over the last three decades. Designed synoptic patterns show that during the peak days of spring, frosts affected by cut-off low blockings, appeared around Caspian Sea, especially its eastern basin and stayed for many days and then divided into two branches including the main one in northern latitudes and minor in southern ones. Although the southern branch of these winds appeared relatively weaker than the main and northern one, but its effects were cold and sub-zero airflow from the Scandinavian Basin, Eastern Europe, Caucasus Basin and the Ural Mountains towards northwest, north, and sometimes northeast of Iran, and penetrated till Central Iran. Lack of high summits and mountains, lower inclination of sunlight and proximity to the southern warm waters are factors that have moderated the violence of these flows in the southern coastal provinces and there have been no reports of temperatures of zero and below zero degrees Celsius. The results showed four factors including “the diagonal move of cut-off lows from Southern Scandinavia to Pamir Plateau” and “eastward movement of troughs from Mediterranean Basin” both at 500 hPa level and “the position of Zagros mountains” and “the entrance of cold high pressure tongues of Siberia” at the surface, caused the symmetric influence of durable frosts over west and east of Iran.ConclusionBased on this research, the decrease in the occurrence of cut-off low blockings during the last three decades in Iran can be justified by the tangible and searchable expansion of warm air territory from low latitudes to the high latitudes in Southwestern Asia.