واكاوي بسامد رخداد رودبادها هنگام بارش‌هاي سيل‌آساي استان كردستان

Analysis of Jet Streams Frequency Occurrence during Heavy Precipitation over Kurdistan, Province

برای انجام این پژوهش، داده‌های بارش روزانه 8 ایستگاه همدید بر روی استان كردستان طی بازه زمانی 11/10/1339 تا 12/10/1389(18263 روز) از سازمان هواشناسی كشور اخذ شد. برای شناسایی رخداد بارش‌های سنگین منجر به سیلاب بزرگ در سطح استان دو شرط گذاشته شد: حداقل نیمی از ایستگاه‌ها بارش دریافت كرده باشند و میانگین بارش ایستگاه‌ها بیش از میانگین صدك 98 اُم بارش طی دوره مورد پژوهش باشد. به كمك دو آستانه یاد شده 107 رویداد بارشی برگزیده شد. برای شناخت بارش‌های سیل‌آسای فصل گرم (خرداد تا مهر) میانگین بارش برای استان در هركدام از روزها محاسبه شد و برحسب صدك 99 اُم بارش 32 روز نخستین كه استان كردستان شدیدترین بارش را دریافت كرده بود، برگزیده شد. در روزهای انتخاب شده طی فصل مرطوب و خشك (گرم) بر روی كرنل 10 تا 100 درجه طول شرقی و 0 تا 70 درجه عرض شمالی برای هركدام از یاخته‌ها (1073) جداگانه فراوانی رخنمود و سرعت رودبادها در چهار دیده‌بانی 00، 06، 12 و 18 به‌وقت گرینویچ محاسبه شد. فراوانی رخداد رودبادها نشان داد كه به هنگام رخداد بارش‌های سنگین استان در فصل مرطوب رودبادها تا تراز 700 هكتوپاسكال كشیده شده‌اند. هسته بیشینه فراوانی رخداد رودباد بر روی شمال و شمال‌شرق عربستان قرار دارد. در بین 4 دیده‌بانی روزانه، بیشترین فراوانی رخداد و بیشترین سرعت رودباد مربوط به دیده‌بانی ساعت 18 است. در تمام دیده‌بانی‌ها استان كردستان در قطاع چپ خروجی رودباد، جایی كه واگرایی و ناپایداری در جو رخ می‌دهد، قرار دارد و محور خروجی به‌صورت مایل به سمت نیمه غربی كشور كشیده شده است. شرایط و مكان شكل‌گیری هسته‌های بیشینه بسامد رخداد رودبادها طی بارش‌های سنگین فصل خشك و مرطوب مشابه است، ولی در مقایسه با رخنمود آن‌ها در فصل مرطوب، از شدت و بسامد كمتری برخوردارند و در ترازهای زیر 500 هكتوپاسكال مشاهده نمی‌شوند.

Introduction Jet stream is strong narrow current concentrated by strong vertical and lateral wind shears (Vasantha et al, 2002). According to World Meteorological Organization nomenclature, jet stream formed when the speed of wind greater than 60 knots or 30 m/s. The major characteristics of the troposphere and lower stratosphere in the vicinity of jet streams were pointed out by a group of meteorologist at the University of Chicago (Rossby et al,1974; Rossby, 1974). These characteristics are the tropopause discontinuities, the intense horizontal temperature gradients considerably below the jet stream core, and the associated frontal systems(Endlich,1953). Jet streams possess a considerable amount of kinetic energy with velocities comparable to horizontal phase velocities of atmospheric waves (Vasantha et al., 2002). Study Area The study area is the Kurdistan Province in the west of Iran. The land area of the Province is approximately 29,600 km2. The topographical elevation varies between 712 m in the southwest and northwest and 3219 m in western and central parts of the study area. The differences in elevation and mountains configuration resulted in differences in precipitation reception. The long-term mean annual precipitation of the study area varies between 300 mm in the east and over 900 mm in the west with decreasing trend from west to east. The aim of this study is the analysis of jet streams frequency occurrence during heavy precipitation over Kurdistan province during 1/1/1961 to 31/12/2010(18263days). To doing this study daily precipitation data of 8 synoptic stations have been extracted from Iranian meteorological organization. The frame of 10° to 100° eastern longitude and 0° to 70° northern latitude has been selected to recognition of jet streams frequency occurrence. Figure1. The spatial distribution of synoptic stations (black dots) and elevation (color background) over Kurdistan Province Data and Methods In this study, environment to circulation approach has been selected. By using two thresholds of extensively and intensity of precipitation occurrence, 107 days had been recognized. In temporal view, the selected days occurrence in wet seasons of year, start from October to the June. The u and v wind components data on the 2.5×2.5 pixels in 10° to 100° eastern longitude and from 0° to 70° northern latitude frame from National Oceanic and Atmospheric Administration has been extracted. One matrix with dimension 107×1073 created that days located on the rows and pixels on the columns. The heavy precipitation in Kurdistan province not only observed in the wet season but also occur in the arid season. These kind of the precipitation occurred in the arid season with lower intensity and frequency. In order to recognize heavy precipitation, the area mean precipitation for the province had been calculated. According to the 99 percentile threshold and extensively of precipitation the heavy precipitation recognized. By two thresholds 32 days selected and same as to the wet season precipitation, u and v wind components data from NOAA extracted. Figure2. The selected frame in order to analysis of jet stream frequency Results and discussion The results of this study showed that in the wet season, during the occurrence of heavy precipitation, the jet streams descend to the 700 hPa level (about 3 km above earth). The highest frequency of jet streams are located on the north and northeastern parts of Arabian. The highest frequency and speed of jet streams observed in the 18 UTC. The Kurdistan province located on the left output part of jet streams where atmospheric divergence and instability occurred in the whole of observations. The output axis of jet stream is toward to semi western parts of Iran. The heavy precipitation during arid season is accompany with jet streams in the upper levels. The formation location of the maximum frequency core of jet stream is same as jet streams in wet season but the frequency of jet streams is lower and don’t observed in the levels below 500 hPa.. Conclusion According to the results of this study we can say that jet streams have important role in amplification and intensification of air instabilities and prone to occurrence of the heavy precipitation in Kurdistan province. This circumstances are present during heavy precipitations both in the wet season and arid season. Jet streams frequency occurrence in the wet season are more frequent and their speed are faster. Also in comparison to the arid season, jet streams are shown in the low levels.