كليدواژه :
بيضي استاندارد , رودباد , غرب ايران , ناپايداري جوي , بارش فرين
چكيده لاتين :
Introduction:
In all aerial maps, there are wind belts or bands extending long distances. According to the definition of World Meteorological Organization, if the speed of these bands exceeds 30 m/s, windward emerges (Kaviani and Alijani, 2000: 291). However, based on the definition of Climate and Air Dictionary, jet stream are those highly intense horizontal winds with a speed more than 50 nodes or about 25 m/s blowing above planetary winds (Geer, 1961). In fact, jet stream are speed cores moving below short and high waves and, like them, have convergence and divergence areas (Alijani, 2002). Windward core speed also reduce from core center to the dies; this reduction toward the pole is cyclicity or positive, and toward the equator is anti-cyclicity or negative.
Material and methods:
This research is empirical with a deductive approach. The geographical areas under study are western Iran regions. The research database, having 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 period were extracted from State Meteorological Organization as the database of environmental incidence. The second variable group involved orbit wind and meridian wind indices for defining jet stream in 250, 300, 400, and 500 HPa were extracted from Noaa website. Moreover, the data were extracted using GrADS software. Given environmental incidence database, western Iran daily precipitation was interpolated to be 18624 days 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 of which 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. Arc Map software was used to draw the maps. There were 119 extreme precipitation days, which covers 30 percent of the stations under study, and they were analyzed. In spatial analysis, one of the most common parameters to assess the dispersion of points around the average center is Standard Deviational Ellipse. Because points positions may have directed deviance regarding the incidences and Standard Deviational Ellipse can well show directed deviance of probability distribution (Asgari, 2011: 89); moreover, Standard Deviational Ellipse is used to indicate the deviance direction of probability distribution. Standard Deviational Ellipse was 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 formation cases the establishment and passing of jet stream affecting western Iran extreme precipitation are in this range, the route of which is oer the Red Sea. 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 formation and establishment sites of jet stream. It is evident that windward development area and its influence reduces 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. To put differently, the areas of this range have been in 50% 50% of formation and establishment sites of windward. This corresponds to the highest frequency of windward incidence, and 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 that high.
Conclusion:
The results shows that jet stream in 250 HPa have a high frequency. 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, and imply 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.
