بررسي اقليمي سامانه‌هاي بندالي نيمكرۀ شمالي و ايران

Climatology Analysis of Blocking System in Northern Hemisphere on Iran

سامانههاي بندالي پديدههاي مقياس همديد منطقة برونحارهاند كه موجب انقطاع و تضعيف بادهاي غربي ميشوند و در رخداد فرينهاي آبوهوايي نقش مهمي دارند. براي بررسي فراواني، توزيع زماني و مكاني سامانهها از دادههاي ارتفاع ژئوپتانسيل تراز 500 هكتوپاسكال با تفكيك مكاني˚5/2 × ˚5/2 و تفكيك زماني ششساعته براي دورة 1951 تا 2012 در نيمكرة شمالي و نماية فيلترشده (TM) استفاده شد. يافتهها نشان داد كه رخدادها در اطلس دو برابر آرام و سه برابر امريكا و غرب اروپا است و كمترين رخدادها در شرق آسيا و شرق اروپا است. همچنين، از نظر زماني، در اقيانوس اطلس سال 1954 بيشترين و 1968 كمترين و در اقيانوس آرام سال 2011 بيشترين و 1957 كمترين فراواني رخداد را دارند. در ايران يازده سال بدون رخداد و سال 1975 بيشترين درصد فراواني مشاهده شد. توزيع فصلي نشان داد بيشترين فراواني رخداد بهترتيب در فصل تابستان، پاييز، زمستان، و بهار است. رخدادهاي بندالي در فصل زمستان بيشتر روي مناطق اقيانوسي و سواحل و در فصل گرم روي مناطق قارهاي نيز مشاهده شد. همچنين، مشخص شد كه رخداد سامانههاي بندالي در 90 موقعيت از 144 طول جغرافيايي با كاهش بارش سالانه در ايران ارتباط معنادار دارد.

Introduction Blocking is one of the most protruding flow patterns and has received attention during the last decades due to its effect on seasonal weather characteristics. The first qualitative conditions for the blocked atmospheric flow was introduced by Rex in 1950 telling the atmospheric jet becomes divided into two separated parts with a considerable connected mass transport, the flow to be meridional at the splitting point and downstream, and the state to continue for at least ten days with a zonal width of at least 45 degrees. The new ideas formulated by Rex became the basis for several blocking indices. Most of the indices detect typical atmospheric patterns. For shorter time series, particular criteria have been used, although, the majority of the analyses are in the 500 hPa geopotential height. In the face of the differences between the index classifications, they draw similar conclusions regarding the patterns of the Northern Hemisphere blocking climatology. However, the blocking frequency is the percentage of time steps an assured longitude. It can be as low as 5% or higher than 20%. The main aims for this wide range of frequencies in automatic recognition are modifications of the Rex criteria and the use of different parameters. Due to its simplicity, the index of Tibaldi and Molteni (TM-Index) has become a standard in automated blocking detection analyses. An important problem of the TM-Index is that it cannot effectively distinguish between blocking and cut-off low patterns, since both fulfill the TM-Index criteria. The adapted TM-Index MTM has a higher rejection rate for cut-off lows. Limits in the Rex conditions can be measured as implementations of independent filters which show synergetic effects if they are used in run. Materials and methods We have used the 500 hPa geopotential height of the NCEP/NCAR reanalysis dataset (1951–2012) with a spectral truncation of (2.5*2.5on a Gaussian grid). We have defined blocking in north hemisphere by Tibaldi and Molteni index. The blocking can be defined at a longitude as GHGS > 0, GHGN < −10 m lat, with the geopotential height gradients in a northern and southern region.