اقلیم شناسی سیکلون های باران زای زمستانه ایران

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  spatial resolution  and temporal resolutions of 6-hourly for a 30 years period (1979-2009). The study domain includes Mediterranean and Middle East, from  E to  E and  N to  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  grid points. 2. For the intensity of the cyclone, the threshold of relative Vorticity, is considered equal or greater than . 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  can be written as , where  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.