Synoptic Analysis of Blockings Causing Heavy and Continuous Rains in Iran

The effect of blocking on Iran’s precipitation in numeral way and long-term has not been done yet. However، in some researches based on weak indexes like one-dimension has been implied to Iran that in studying many cases we observe lack of blocking event and or their continuation that has been for less than 5 days. This phenomenon in some cases occurs in our country directly and most of its parts are under its predominance، but according to its placement in proximity of zone، its maximum in the world، Atlantic-Europe zone is influenced by its effects repeatedly and probably may be affected more by recent cases. Given the fact that in each zone، precipitation is considered as a permanent development base، therefore the knowledge of effective models on it has an extraordinary importance in order to predict and knowing from them. On the other hand، with respect to this that most studies have been carried out in sample way، the knowledge of these patterns can have a high carefulness and accuracy in quantity way. Study Area: In order to investigate the blocking systems affecting the climate of Iran، at first the study area was chosen in the within the 0– 90N and 90W – 100E. At the second stage the origin and location of the blocking systems were determined and study area was decreased to the 20N – 80N and 40W – 100E in order to study only those systems in close contact with the climate of Iran. In order to better understand the effect of blocking patterns of precipitation and temperature the study area was limited to Iran.

Data related to geo-potential height of 500 hPa level in regard to two dimensions index of visible blocking، daily for cold season in year for a 65-year period from 1953 to 2012 in a 2/5 *2/5 network that is appropriate for studying the large-scale occurrences like the blocking was extracted from site NCEP-NCAR for an area between 0 to 90N latitudes and 90W to100E longitudes. Then the investigated zone was bounded according to the knowledge of the limit and the origin of the effective blocking on Iran’ weather from 40 W to 100 E longitude Using a quantity way of two-dimensional revelation and synoptic filters along with it، all blocking events were revealed. The effective happenings on rainfall were separated then these events with use of a factor analysis way were analyzed and based on classification way of K-means were categorized and finally the blocking prevailing events that are effective on Iran’s precipitation were identified. Four prevailing patterns for rainfall were extracted that the first two models، the precipitation pattern resulted from the left and right half were totally considered as omega kind model. Two other models were including dipole and Rex Patterns of blocking. Synoptic conditions of above levels of atmosphere and ground level were analyzed for each pattern

In omega pattern، right trough of Iran is located in front of the trough. Lifetime duration of this blocking from forming until puberty and death has been in total the days and the system with regard to the feature of its being quasi-stationary، has had a moving toward east with a very low speed and this matter has been caused that Iran’s different parts from west to east is located respectively in front of trough and under trough axis several continuous days and have instable conditions and necessary rise for raining. In omega model، left trough of west and western north is situated in front of the left trough and conditions for instability and continuous precipitation has been ready and in most stations in addition to the permanence of rainfall for several days، it was also shown heavy rainfall. In dipole pattern of west and western north، the country is located in front of below low-rise trough and conditions are appropriate for rising، instability and continuous and heavy precipitation. In Rex Pattern، ridge is placed in high latitudes and trough is situated in lower latitudes and Iran is influenced by in front of below trough. In this model، in Iran’s western half، conditions are available for constant and heavy precipitation

Blocking is one of the atmospheric large-scale phenomenon which have numerous synoptic phenomenon synoptic scales. The effective blockings on Iran’s weather among atmospheric parameters have the most effect on rainfall. Blocking event in some cases can on account of severity and continuous precipitation in a zone lead to flood and damage، therefore the knowledge of its synoptic prevailing pattern in order to predict and issuing the necessary per-awareness’s and taking the essential measures in the affected area in terms of risk management have considerable importance. Among the blocking patterns leaded to rainfall، the omega model has more abundance than other models. In omega pattern، two marginal troughs on ground level have a low pressure and under and in front of trough، conditions for raining are available. In terms of the continuity of more precipitation، the omega model can has more permanence and other kinds have less severity and permanence. Statistical survey of the precipitations resulted from blocking showed that blocking can lead to continual and severe rainfalls، but every continual and severe rainfall is not resulted from blocking.