تحلیل همدید الگوهای جوّی توام با موج های سرمای زمستانه در ایران

در پژوهش حاضر تلاش شده با استفاده از رویکرد محیطی به گردشی، الگوهای همدید توام با موج های سرمای زمستانی در ایران شناسایی شود. براین اساس با استفاده از داده های میان یابی شدۀ دمای کمینه برای زمستان‌های 1339 تا 1388 ضمن شناسایی روزهای همراه با سرمای فراگیر، داده های جوی شامل فشار تراز دریا و ارتفاع ژئوپتانسیل تراز 500 هکتوپاسکال برای شناسایی الگوهای جوی همراه با موجهای سرمای فراگیر زمستانه در ایران بکار گرفته شد. با انجام تحلیل خوشه ای بر روی داده های فشار تراز دریا طی 487 روز همراه با سرمای فراگیر، درنهایت 5 الگوی همدید تشخیص داده شد. بررسی این الگوها نشان داد که تمامی موج های سرمای فراگیر در پهنۀ کشور با تکوین یک الگوی پرفشار گسترده بر روی ایران و مناطق همجوار آن توام بوده است. برهمکنش سامانه های پرفشار سیبری و پرفشارهای اروپایی با کم فشارهای جنب قطبی در انتقال هوای سرد از نواحی قطبی و شمال اسکاندیناوی به عرضهای جنوبی نقش مهمی داشته است. همچنین استقرار پشته ها و سامانه‌های بندالی تراز میانی جو در بخش های مرکزی و شرقی اروپا و استقرار ناوۀ شرقی آن‌ها بر روی ایران با ریزش هوای سرد به قسمت عقب این ناوه توام بوده و در ایجاد و تداوم موج های سرمایی در سطح کشور تاثیر زیادی داشته است.

Synoptic Analysis of Productive Patterns of Winter Cold Wave’s in Iran

1. IntroductionOne of the special temperature states in which extreme amounts of minimum temperature are observed, is the cold wave. The extreme cold waves due to the severity and sudden occurrence have an important impact on the ecosystems and human societies. There is a probability of occurrence of cold waves throughout the year and in each season, it creates its own special problems and damages. Because of the low angle of sunshine and the cold weather, these waves are stronger in winter and threaten the life of all living creatures. They also play a decisive role in economic, environmental and developmental issues such as road construction, damping and bridge construction, and can cause damage in various sectors. The intensity and weakness of the cold waves depend on the complex synoptic and dynamics factors, some of which are less wellknown and their analysis using a synoptic method can identify features such as the source, track, intensity and frequency of pressure systems and helps to recognize the fundamental factors and their characteristics and to increase our understanding of winter cold waves.2. Material and MethodsThe study area in this research includes the whole country of Iran. Iran is a rugged country with an elevation average of about 1250 meters. roughness plays an important role in Iran 's temperature; therefore, the temperature decreases wherever height increases. Iran’s average maximum temperature is 18 degrees Celsius and the average minimum temperature is 11 degrees Celsius.In the present study, the effective patterns in creating winter cold waves in Iran were identified using environmental to circulation approach. In order to achieve the specified purpose in this study, two environmental and atmospheric databases were used. Environmental data included the minimum temperature in winter between 1960 and 2011. In this research, it has been attempted to be defined the cold Somehow so that the principle of its relativity is considered in different regions; accordingly and according to Alijani and Hooshyar (2008) and Massoudian (2013) researches, Two conditions for the identification of cold waves were defined in the country. On the basis of the first condition, days were considered as a day with a cold wave that standard Z score for the 10th percentile of minimum temperature data is less than or equal to 1/2. By defining this condition, only very low temperatures were taken into consideration for each of the 7187 cells studied and the concept of relative coldness was considered for different regions of the country. The second condition is the continuity of the cold weather for at least 3 days; based on this condition, the systemic coldness from the local coldness (which is caused by environmental factors or overnight cooling) is separated. According to these conditions, cold days were identified on the ground and related synoptic systems were investigated in different layers of the atmosphere.3. Results and DiscussionIn the first pattern, about 45 percent of the country 's surface was affected by the cold wave, and the average minimum temperature was 4.4 Celsius degrees. Based on surface temperature maps in the northwestern part of the country, temperatures below 20 ° C was observed. The main factor behind the formation of this pattern is the Azores high pressure system, which has led to cold weather penetration from northern Europe to Iran.Due to the formation of the second pattern, about 21 percent of the Iran’s surface was affected by the cold wave. The average minimum temperature in the country is 2.9 Celsius degrees, which was the highest among other patterns. The lowest temperatures in this pattern were between 14 and 18 Celsius degrees that observed in the northwest, the central parts of the Alborz and northeast of the country.In the third pattern, the daily average of minimum temperature is 2.5 Celsius degrees, which is the lowest at the different patterns. In this pattern, 50 percent of the country 's surface was affected by the cold wave. In this pattern, cold weather penetrates the southern parts of the country and its impact was observed in many parts of the country. On the day the representative, the Siberian highpressure system is located at high latitudes and in the central parts of Russia. The weakening of the Iceland lowpressure and the displacement Azores highpressure towards the westerner parts, has strengthened the Siberian highpressure and increased its impact. The minimum temperature average in the fourth pattern was 3.5 Celsius degrees and the cold air coverage in the country was 30 percent. Lowest temperature values were observed in northern parts of the provinces of East Azerbaijan and Ardabil. This pattern is somewhat similar to the first pattern, but contrary to the first pattern, there is a negative pressure anomaly in the northern regions of Europe and throughout Russia. The Siberian high pressure has shifted to the east and the European high pressure is located throughout the region, while in the first pattern, this high pressure was located in Western Europe.The minimum temperature average in the Fifth pattern was 3.4 Celsius degrees and the cold weather coverage in the country was 36 percent. In this pattern, the core of the Siberian highpressure is located in west of Mongolia, and the Icelandic lowpressure was eastward movement and located in Scandinavia. This system has been located in large parts of northern Europe and central Russia as well as parts of the Atlantic in Western Europe. The expansion of this system and its displacement have caused more impact on Siberian highpressure.4. ConclusionPerforming synoptic analyzes on atmospheric patterns showed that all the pervasive cold waves in Iran were caused by the formation of massive highpressure patterns in the country, parts of the Middle East and Asia and SouthEast Europe. The position of the two Siberian and Azores highpressure systems has played a very important role in the transfer of cold air to the northern latitudes towards the country. The deployment of a polar lowpressure system in northern parts of Europe and Russia has led to the influx of cold air towards the southern regions. The formation of specific patterns of the intermediate layers of the atmosphere has had a significant effect on the creation and intensification of the winter cold waves. So that the extreme winter cold waves are formed when Blocking systems have been established in Eastern Europe and their eastern Trough has been located on Iran.