اقليم شناسي سيكلون هاي باران زاي زمستانه ايران

Climatology of Mediterranean winter season rain producing cyclones in Iran

در اين مطالعه با استفاده از داده هاي چرخندگي نسبي مركز اروپايي پيش بيني ميان مدت جوي(ECMWF)، سيكلون هاي باران زاي فصل زمستان (دسامبر، ژانويه و فوريه) ايران مورد مطالعه قرار گرفت. بدين منظور، داده هاي چرخندگي نسبي سطح 700 هكتوپاسكال در بازه زماني 30 ساله (2009-1979) با تفكيك مكاني 1/5 درجه جغرافيايي و زماني 6 ساعته، جهت شناسايي، مسيريابي و تحليل سيكلون ها بكار گرفته شد. با تعريف آستانه مشخص چرخندگي نسبي، مراكز سيكلوني بالقوه شناسايي شد و با روش رهگيري در 8 همسايه مجاور و با داشتن طول عمر حداقل 8 گام زماني 6 ساعته، سيكلون ها تعريف و مسيريابي گرديد. نتايج مطالعه نشان داد كه بخش شرقي درياي مديترانه بخصوص جزيره قبرس، درياي اژه، درياي آدرياتيك، درياي سرخ و منطقه سودان و همچنين كشور عراق از مهمترين كانون هاي سيكلون زايي زمستانه ايران مي باشند. مسيرهاي اصلي ورودي سيكلون ها شامل مسير شمال غرب، غرب مياني و نيز جنوب غرب مي باشد. عمر متوسط سيكلون ها چهار و نيم روز، ميانگين سرعت جابجايي آنها در حدود 20 كيلومتر بر ساعت و نيز متوسط جابجايي سيكلون ها در حدود 1700 كيلومتر است. از نتايج قابل توجه ديگر كاهش تعداد سيكلون هاي باران زا در طول سه دهه اخير مي باشد.

Cyclones as the most important factor in the turbulence of mid-latitude regions are low pressure centers that have one or more closed Isobaric curves. Mid-latitude cyclones are responsible for transfer of the heat and moisture between the tropics and the polar regions. Mediterranean cyclones have the most important role in Iran's rainfalls and get from different tracks to the country and affect Northwest، West and South West of Iran in the cold period of the year. Cyclones are the prevailing weather phenomenon in Mid-latitude region and studying the characteristics and effects of these phenomena has always been of interest to climatologists and meteorologists. Iran which is located in mid-latitude region، actually is located in the path of one of the world's largest cyclone formation and the Mediterranean Sea has a main impact on the rainfall occurrence in Iran. Regarding the importance of cyclones in rainfall occurrence in Iran، especially in cold season، studying this atmospheric event seems very necessary. The aim of this study، is to identify، tracking and analysis of the winter season (December، January and February or DJF) rain producing cyclones using special database and algorithm during three recently decades (1979-2009). In this study، in order to specify، tracking and statistical analysis of the DJF rain producing cyclones، relative Vorticity of 700 hPa were received from ECMWF data center with AWT IMAGE spatial resolution and temporal resolutions of 6-hourly for a 30 years period (1979-2009). The study domain includes Mediterranean and Middle East، from AWT IMAGE E to AWT IMAGE E and AWT IMAGE N to AWT IMAGE N. Cyclone identification will follow the approach presented in Blender et al. (1997) and Flaounas et al. (2014)، including two important points and conditions: 1- A cyclone is considered to be a relative Vorticity extrema، thus the aim will be to look for the maximum value of relative Vorticity at the 700 hPa level in a box of AWT IMAGE grid points. 2. For the intensity of the cyclone، the threshold of relative Vorticity، is considered equal or greater than AWT IMAGE. Then by 6-hour tracking the maximum amount points of Vorticity and connecting them to each other، identification and routing of cyclone is implemented. The track AWT IMAGE can be written as AWT IMAGE، where AWT IMAGE describes the cyclone position in sequential time steps t = 0، … ، T، which are given at 00، 06، 12 and 18 GMT. The initial position is considered the first detected maximum of relative Vorticity. During the cyclone detection and tracking method، different threshold values are commonly applied to eliminate، for instance، the weakest centers and short living systems. For this reason، the threshold of equal or greater than 8 time steps (3 days and more) is considered based on Alijani (1366) to eliminate short living cyclones. In this study، climatology of cyclones such as genesis and lysis centers، tracks and frequencies in the Mediterranean Basin and Middle East were analyzed. The results showed that، center and eastern part of the Mediterranean region include Aegean Sea، Cyprus، and northern borders of Mediterranean Sea، Adriatic Sea and also Red Sea، Sudan and Iraq، are most important cyclogenesis centers in winter season. These cyclones which form in these cyclogenesis centers، move to the east with two eastward and northeastward distinguished directions. In northwest of Iran، the cyclones by passing over West and East Azerbaijan and Ardebil provinces in eastward and northeastward directions، move into western and southern shores of Caspian Sea. In southwest of the country، the cyclones are entered into Iran over Khozestan، Chaharmahal and Kohkilouyeh provinces. Part the cyclones are moved into center and northeast of the country with northeastward direction and other part by crossing over southern shores of the country move to the southeast of the country. In mid-west of the country، the cyclones are arrived into Iran in three different tracks. The most important path is Kordestan's path which the cyclones in the origin of Mediterranean are entered eastward into Iran by crossing over Zanjan and Hamadan provinces، under the Alborz ranges are moved into east and northeast of the country. Two other paths are observed in Ilam and Kermanshah Provinces which in these tracks also the cyclones are approximately moved eastward into center and northeast of the country. The results also showed that eastern part of the Mediterranean region specially Cyprus، Aegean sea، Adriatic sea، Red sea and Sudan and also Iraq are most important wintertime cyclogenesis centers of Iran. The main entrance paths of the cyclones includes northwest، mid- west and southwest. The averages cyclones lifetime are 4.5 days، movement speed is about 20 km/h and also travelling length is about 1700 km. Of the other notable results we can refer to decreasing of the rain producing cyclones during 3 last decades.