عنوان مقاله [English]
There are wind belts or bands extending long distances over a region. According to the definition of World Meteorological Organization, if the speed of wind bands exceeds 30 m/s, jet streams emerge. However, based on the definition of Climate and Air Dictionary, jet streams are the highly intense horizontal winds with a speed more than 50 nodes or about 25 m/s blowing above planetary winds. In fact, jet streams are speed cores moving below short and high waves and, like them, have convergence and divergence areas. Windward core speed is also reduced from core center to the arounds; this reduction toward the pole is cyclicity or positive and toward the equator is anti-cyclicity or negative.
Materials and methods
This is an empirical research with a deductive approach. The geographical areas under study are the regions in western Iran. The research database, with an environmental approach to the circulation, includes two variable groups. First, daily precipitation data of 69 synoptic and climatological stations of western Iran (Hamedan, Kordestan, Kermanshah, Ilam, And Kurdistan provinces) in 1961-2010 were extracted from State Meteorological Organization. The second variable group involved orbit wind and meridian wind indices to define jet streams in 250, 300, 400, and 500 HPa. These were extracted from NOAA website. Moreover, the data were extracted using GrADS software. Given environmental incidence database, daily precipitation of the western Iran was interpolated using Kriging method. To do this, when interpolating daily precipitation, the area under study was converted to 1367 pixels with 2.5*2.5 dimensions. The result was the formation of an array of western Iran daily precipitation database with 18624*1367 dimensions. Performing calculations on these data was conducted by MATLAB software and the result has been illustrated as a map. ArcMap was used to draw the maps. There were 119 extreme precipitation days, which covers 30 percent of the stations under study. In spatial analysis, one of the most common parameters to assess the dispersion of the points around the average center is Standard Deviation Ellipse, because the point positions may have direct deviance regarding the incidences and Standard Deviational Ellipse. This can well show directed deviance of probability distribution; Moreover, the Standard Deviational Ellipse is used to indicate the deviance direction of probability distribution. It was also applied to windward incidence in each pixel.
Results and discussion
Windward frequency in 250 HPa level showed that the highest frequency of jet stream is from southern Red Sea to the southern Mediterranean; in other words, in more than 70% of the cases the establishment and passing of jet streamaffecting extreme precipitation of the western Iran are extended in this range. The same things cause the infusion of humidity from the Red Sea to western Iran precipitation systems. The frequency of jet stream in 300 HPa level indicated that during the research period, the highest frequency of jet stream extends from northern Red Sea to western and central Iran. In other words, the areas of this range have been present in 50% of the formation and establishment sites of the jet stream. It is evident that windward development and its influence are reduced by height reduction. Windward frequency in 500 Hpa level showed that during the research period, the highest frequency extends from northeastern Red Sea to western Iran regions. The areas of this range have been in 50% of formation and establishment sites of windward. This corresponds to the highest frequency of windward incidence. The windward frequency reduces when we go eastward. Jet stream frequency in 500 HPa level is not surprising because windward incidence in this atmosphere level is not basically so high.
The results indicated that jet stream in 250 HPa have a high frequency. The jet stream average velocity maps are corresponding to the incidence of jet stream maximum frequency in one hand, and in the other hand against the incidence of jet stream maximum speed in the area under study. This implies the situation of second quarter of jet stream core (which accompanies positive Vorticity as well as upper surfaces divergence and lower surfaces convergence) on western Iran.