The Climatology Analysis of Blocking System in Northern hemisphere and Iran

The Climatology Analysis of Blocking System in Northern hemisphere and Iran 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 in 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 study 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, which is the percentage of time steps an assured longitude, is blocked, 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 (Schalge and et al,2011, Elliott, R.D., T.B., Smith, 1949, Knox, J.L, J.E., Hay, 1984, Pelly, J.L, 2001, Antokhina and et al,2016, Rex, D.F, 1950).
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 (Tibaldi, S., F., Molteni, 1990, Schalge and et al,2011, Barriopedro and et al,2006). The data used is the 500 hPa geopotential height of the NCEP/NCAR reanalysis dataset (1951–2012) with
a spectral truncation of (2.5*2.5 on a Gaussian grid) (Tibaldi, S., F., Molteni, 1990, Schalge and et al,2011).
We used to define blocking in north hemisphere Tibaldi and Molteni index. For blocking define at a longitude are GHGS > 0, GHGN < −10 m lat, with the geopotential height gradients in a northern and southern region (1)   In the present study, the TM-Index is modified by introducing different ranges for the gradients (this will be denoted as the basic setup in the following):
And for Iran:
Which:Z is the geopotential height and φ indicates latitudes.
So, the 500 hPa geopotential height field is analyses for a pattern with a positive geopotential height gradient in the southern region and a strongly negative gradient in the northern region. The explanation of a range for Δ leads to a larger number of detected blocking events.
According to this definition, it is possible to find blocking structures with a maximum between the center and southern region. If, for example, only the southernmost latitude in the mid-latitude region has higher geopotential height than one of the more northern latitudes of the southern region, GHGS > 0 and blocking will be detected. Without this modification, such a combination is not possible and blocking is not identified, because the high geopotential is too far south and the lower geopotential too far north, associated with a relatively small meridional extent of the block. The increase of the blocking frequency by the variable gradient ranges in the basic setup amounts to 20% in regions with high blocking frequency and to 50% in regions with low blocking activity compared to the original TM-Index. 3.1. Location Blocking system:Most of the blocking systems in the northern hemisphere are in all seasons and their continuity in the Atlantic and Pacific. The analysis of the frequency of blocking systems in (2012 -1951) finding that most of the blocking systems occurred in the Atlas region 11.2%, the 4.8% traffic, Europe 3.9%, the United States 3.7%, and Asia 0 .75%. The highest frequency of blocking systems was observed in the central part of the Atlantic on the W ° 30, then the Pacific in the western and central part on W ° 140, in the west of the United States on W ° 125, west Europe on the W ° 10 and in Asia between the region (E ° 85 - E60) (Figure1 and 2).   The location of the blocking systems in the northern hemisphere.   3.2. Duration of Blocking System:Between 1951-2012, the Atlantic was 20.8% in 1954 and 19.5% in 1998, with the highest annual rates in 1968 and 4.6% in 1987, and 5.5% in 1987, with the lowest incidence of 5.5% Block diagrams (Figure 3) and Pacific Ocean in 1985 with 12.9% and 1998 in 10.7% were highest in 1957 with 3.6% and in 1968 and 1997 with 3 .8% of the days of the year had the lowest incidence (Fig. 4).
Fig (3) Blocking System Frequency at 1954
Fig (4) Blocking System Frequency at 1985 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. Moreover, finding show that Most of the blocking systems in the northern hemisphere are in all seasons and their continuity in the Atlantic and Pacific. The analysis of the frequency of blocking systems in (2012 -1951) finding that most of the blocking systems occurred in the Atlas region 11.2%, the 4.8% traffic, Europe 3.9%, the United States 3.7%, and Asia 0 .75%. Over the years (1951-2012) in Iran, 11 years without a blocking system, 1975, the highest frequency of blocking systems has been observed in the east of the country. Also, seasonal distribution of systems showed that the highest frequency of occurrence of blockade systems is in summer, autumn, winter and spring