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پژوهش های جغرافیای طبیعی، جلد ۵۱، شماره ۲، صفحات ۲۰۱-۲۲۱
عنوان فارسی اقلیم شناسی رودبادها در خاورمیانه
چکیده فارسی مقاله بسیاری از پدیده‏های اقلیمی سطح زمین در ارتباط با موقعیت رودبادها در سطوح فوقانی جو  هستند. این پژوهش با هدف شناخت موقعیت و فراوانی رودبادها در بخشی از نیمکره شمالی (خاورمیانه) انجام شد. رودبادها در محدوده‏ وسیعی از صفر تا 120 درجه طول شرقی و صفر تا 80 درجه عرض شمالی در 5 تراز 300، 400، 500، 600، و 700 هکتوپاسکال طی دوره آماری 1965-2014 به‏صورت ساعتی بررسی شدند. نتایج نشان داد که از نظر زمانی و مکانی تغییرات عمده‏ای در فراوانی و سرعت رودبادها در طول سال به‏وجود می‏آید. در دوره سرد سال فراوانی رودبادها و سرعت آن‏ها در عرض‏های جغرافیایی 20 تا 30 درجه شمالی بیشینه است. در دوره گرم سال فراوانی و سرعت رودبادها در مناطق یادشده کاهش می‏یابد و به عرض‏های 35 تا 45 درجه عرض شمالی منتقل می‏شود. در همه ترازهای جو اغلب می‏توان دو کمربند پیوسته از بادهای شدید را در عرض‏های جغرافیایی 20 تا 30 درجه شمالی و 50 تا 60 درجه شمالی مشاهده کرد. در میان این دو کمربند می‏توان چهار هسته رودباد اصلی (شرق چین و بخشی از خاورمیانه، همچنین غرب اروپا و بخشی از مناطق مرکزی و جنوب روسیه) را شناسایی کرد.
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عنوان انگلیسی Climatology of Jet streams in the Middle East
چکیده انگلیسی مقاله 1- Introduction One of the main elements of the general circulation in mid latitudes are the fast and narrow flow maxmia called jet streams whoes speed is usually more than 30 mters per second (Magata, 1950; Geer et al, 1996). They are one of the dominant features of upper level weather maps but changing through time, space and layers of atmosphere. The jetstreams cause vertical motion underneath through which produce stability and unstability over the earth surface; it should be mentioned that they are much known for their instability production (Magata, 1950). Jet cores are one of the main components of the general circulation and their location and displacement are controlled by the elements of the circulation such as the Arctic Oscillation (Strong and Davis, 2008). According to these researches the location of the jet streams are very important in climate events. Therefore, this research tried to identify and present the location and speed of jet streams in Middle East. Because a comprehensive study as this scale has not been carried out so far. 2- Materials and methods In order to study the jet streams, the six hourly (00, 06, 12 and 18 GMT) speed of U and V components of the winds at the 700, 600, 500, 400, and 300 hPa levels during 1965-2014 period were obtained for the window of 0 to 120E and 0 to 80N (Figure 1) from www.esrl.noaa.gov. In total 20 time series were produced from the combination of these hour and level scales for each pixel with the size of 2.5 by 2.5 degrees. In each time series the wind maxima of 30 m/s and higher were extracted. At the final stage, the mean monthly speed and monthly frequency of jet streams of all pixels were mapped for the study area. 3-Results The frequency and speed of jet cores were mapped and are described here in monthly, seasonal and annual scales. The annual frequency of jet cores are mapped in Figure 2. According to this figure, the highest speed maxima in 300 hPa level are located over the North Africa at 40 percent of times, depicting the main track of the jet cores. Considering the fact that jet cores enter the study region only in the cold period of year, this temporal frequency of 40 percent is not a low value. The location of speed maxima is the same in 500 hPa level but with a lower value of about 20 percent of the time. This low value is reasonable for this level. As we know, the synoptic systems of this level control most of the time the weather and climate of the surface and these surface systems are not very frequent during a normal year. In this level, the bifurcation of westerlies is obvious. The southern branch is very influencial in the Middle East. This bifurcation indicates the presence of a blocking high in the region, which most of the times prevent the entering of jet cores and hence an unstable conditions over the region. There are no speed maxima in the region in 700 hpa level. Since in the annual scale only the 500 and 300 hPa levels showed the jet cores, in the seasonal scale we look only for these levels. The frequency of jet cores in winter is more than the other seasons and show two separate belts. The jet core affecting the climate of Iran is passing over the Persian Gulf which extends from north of Africa towards China. Jet cores pass through this belt all of the winter season. However, their frequency decreases toward the north over Iran and from there increases toward the other maxima over the latituds 50N to 60. The jet frequencies have decreased in spring reaching to about 52 percent over the area (Figure 3B). This rate of decrease indidates the suden and rapid change from winter to the spring conditions over Iran. Its northward shift is also obvious over Iran. In this season, the jet maxima travel in a ridge like track over Iran. It indicates an unstable weather over the west of Iran while dominating the stable conditions over the east of the country. 4- Conclusion Jet streams are very important instability factors in the atmosphere. Their spatial location and speeds control the pressure systems tracks and surface climate. For this reason, this study tried to understand their speed and spatial variations in the Middle East. Their speed and frequencies were studied at the pressure levels of 700 hPa to 300 hPa levels in the monthly, seasonal and annual scales. The result showed that in all levels and scales two speed maxima tracks were established over the latitude belts of 20N – 30N and 50N – 60N which is confirmed by findings of Li et al (2004); Zhang et al ( 2006); Li and Wettstein( 2012) and Pena-Ortiz et al( 2013). The most important finding of the research is the coincidence of speed maxima with frequency maxima of jet cores in these latitude belts. During the warm period, both the frequency and speed of the jet cores decreased over the Middle East. The jet cores were absent at the levels lower than 500 hPa, and in 300 hPa all winds experienced speeds higher than jet threshold which is in accordance with Geer et al (1996). In brief we can conclude that through the fall season jet cores move southward to establish over the Middle, East including, Iran during winter and through the spring season they began backward movement to the northern latitudes so that in May no ject core could be found in the area.This research demonstrated the importance of the wind patterns of 500 hPa on the climate and weather conditions of the Middle East as well as Iran. Keywords: Jet cores, Middle East, climatology of jet streams, upper level jets, polar front and subtropical jets.
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نویسندگان مقاله بختیار محمدی |
استادیار اقلیم ‏شناسی، گروه اقلیم ‏شناسی، دانشکدة منابع طبیعی، دانشگاه کردستان

بهلول علیجانی |
استاد اقلیم ‏شناسی، دانشگاه خوارزمی

آرام عمر صالح |
کارشناس ارشد اقلیم شناسی، دانشگاه کردستان

نشانی اینترنتی https://jphgr.ut.ac.ir/article_73364_a547048461fd5f0336d310dfa38bf118.pdf
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