موقعیت رودباد جبهه‌ قطبی در ارتباط با بارش ‌های سنگین و شار رطوبت ترازهای پایین غرب ایران

هدف از این مطالعه شناسایی و تحلیل موقعیت رودباد جبهه‌قطبی و شار رطوبت در ارتباط با بارش‌های سنگین غرب ایران با استفاده از روش همدید است، که چهار الگو در این زمینه شناسایی شد. در همه الگوها در روز رخداد بارش، ناحیه چپ خروجی (ربع دوم) هسته رودباد جبهه‌قطبی (که بیشترین واگرایی سطوح بالا و همگرایی سطوح زیرین جو را فراهم می‌کند) بر فراز آسمان غرب ایران قرار گرفته است. نتایج نشان می‌دهد که در روز رخداد بارش، محور هسته رودباد در منطقه مورد مطالعه دچار انحنای نصف‌النهاری شده است که این موضوع از دو جنبه به صعود شدیدتر هوا کمک می‌کند. این آرایش هستة رودباد سبب وزش دمایی گرم به مناطق تحت تسلط نیمه خروجی رودباد می‌شود و این شرایط صعود و همرفت بیشتر هوا را سبب می‌شود و از دیگر سوی متمایل‌شدن هسته رودباد به وزش نصف‌النهاری موجب می‌شود تا جریان هوا دچار چرخندگی انحنایی شود و صعود حاصل از این چرخندگی انحنایی به چرخندگی برشی ناشی از شیب سرعت در ربع دوم هسته رودباد افزوده شود و صعود هوا را تشدید کند. منابع تغذیه‌کننده رطوبتی این بارش‌ها نیز در مرتبه اول دریای عرب سپس دریای سرخ، خلیج‌فارس و دریای عمان است.

An Analysis of polar Jest Stream (PJS) location Associated with Low Level Moisture Flux and Heavy Rainfalls in West of Iran

IntroductionMany aspect strongly influence regional climate includes localized surface processes, largescale patterns, especially mid and upper level tropospheric circulation and many factors control precipitation in the west of Iran includes location of emigrate westerly winds systems, Jet streams location, Humidity Flux and topography. Upper tropospheric jet stream constitutes a significant factor influencing physical processes, includes ascend or descend movements in the lower atmosphere in both synoptical and climatological time scales. Polar jet stream and subtropical jet stream are two main Upper tropospheric jet streams affects climatology mid latitude atmosphere. In this regard, formation of polar jet stream is related to thermal contrast in the polar front because of its proximity to the ground has greater role in providing ascendant atmospheric condition and precipitation. But subtropical jet stream locate at upper atmospheric level in tropopause and such condition can’t have a prominent role as polar jet stream in ascendant condition, especially in precipitation. In addition because the subtropical jet stream does not have other factors in polar jet stream such as polar front, dynamic properties of this does not have affect climate feature of earth surface. Typically Jet stream potential can affect divergence and convergence, develop and steer the pressure systems (Farajzadeh et al., 2008) and change control the weather patterns and climate. The Polar jet stream is one of main factors that affect climatology if western Iran in winter. In addition to jet stream location, the atmospheric moisture budget plays an important role in precipitation and hydrology. Existence of polar jet stream the companied with adequate moisture can lead to heavy rainfall in each region. The objectives of the present study are understand the location of polar Jest Stream during the heavy rains in Western Iran and characteristics of moisture flux from each region of moisture source and their contribution to the rainfall during the mentioned period.Material and MethodsThis study focus on western Iran, that extends between the latitudes 33° N and 36° N and the longitudes 46° E and 48° E (Fig 1). 1. The study areaFor determine the days with heavy rainfall the Mofidi et al (2007) method have been used. Hence the heavy rainfall was the amount of rainfall during the day with equal to or greater than 5% of the average annual precipitation and also over 50 percent of western Iran must receive heavy rainfall. So the 48 days with heavy rainfall for current study have been extracted. The location of PJS were analyzed according to atmospheric circulation types in 75 days with heavy rainfall using daily mean of the 500 hPa geopotential height data for these days between 10°–80°E, 10°–60°N, with a 2.5° (lat) × 2.5° (lon) spatial resolution that’s includes 609 grids. So a 48 × 609 matrix was created. For determine the atmospheric circulation types and location of PJS during heavy rainfall over western Iran an agglomerative hierarchical cluster analysis was applied to the 48×609 matrix using the ward algorithm with Euclidean distance to identify atmospheric circulation types. Then calculated the withingroup correlation to identify representative days. In continues, the convergence and divergence of moisture flux from two days prior to representative days was investigated. The convergence and divergence of moisture flux was calculated in 1000 hPa to 500 hPa levels for determine the main source of moisture flux in various atmospheric circulation types and arrangement of polar jet stream in time of heavy rainfall in western Iran. For this purpose, four daily NCEP/NCAR reanalysis data includes specific humidity, zonal and meridional wind speed components (U; V components) for 1000 to 500 hPa have been used.Results and DiscussionAccording to results of hierarchical cluster analysis, 4 groups of atmospheric circulation types affecting the location of polar jet stream in both level of 300 and 500 hPa, associated with heavy rainfall in western Iran was detected. Usually the annual rainfall in western Iran are associated with location of polar jet stream and emigrated systems of westerly winds. In pattern 1, the heaviest rainfall occurred at 00Z. Formation of deep trough in westerly winds on the East Mediterranean, lead to the formation of cutoflow in both of 500 and 300 hPa levels. Existence of this trough in company with polar jet stream lead to intensification of the unstable condition in the atmosphere of western Iran. So that in both level of 300 and 500 hPa, the west of Iran is located in the second quarter zone of polar jet stream. In this pattern the polar jet stream core in the west of Iran has taken a meridian curve and this issue provides more favorable conditions for convection and ascendant condition in atmosphere. In this pattern the main sources of moisture are West Arabian Sea, Persian Gulf, Red Sea and East Mediterranean is the secondary source that is less important.In the second pattern, formation of a deep trough on the Red Sea convergence zone provides more favorable conditions for ascendant condition in the west of Iran. The location of polar jet stream in this pattern is in lower latitude in comparison with first pattern and intensify of polar jet stream is less than previous pattern. The main sources of moisture in second pattern are Arabian Sea and Red Sea and also Persian Gulf and Oman Sea are secondary sources for this pattern. In the third pattern, the expanding of trough in waves of westerly winds was wide, from the Mediterranean Sea to the West of Iran. This issue, decrees of deep and increase of expansion in westerly wind waves, cause decrease in intensify of unstable condition in atmosphere of western Iran. Expansion the core of polar jet stream from east of Africa to west of Iran that located in the second quarter zone of polar jet stream has caused increase of unstable condition in this region. The main sources of moisture in this pattern are Arabian Sea, Gulf of Aden, Red Sea Persian Gulf and Oman Sea. In the fourth pattern, existence of a deep trough on westerly wind waves over the Red Sea lead to increase in intensity of polar jet stream that located in Middle East. So, this condition caused increase of unstable atmospheric condition in western Iran and according to moisture flux lad to heavy rainfall in this region. The main sources of moisture in this pattern are Arabian Sea, Gulf of Aden, Persian Gulf and Oman Sea.ConclusionResults of this study show that in all atmospheric circulation type that lead to heavy rainfall in western Iran, this region was located under the second quarter zone of polar jet stream. Therefore this condition provides more favorable conditions for convection and ascendant condition in atmosphere. And also the main sources of moisture in heavy rainfall cases were Arabian Sea, Gulf of Aden, Read Sea, Persian Gulf and Oman Sea.