تعيين زمان آغاز، خاتمه و طول مدت وزش باد سيستان با بهره‌گيري از روش‌هاي تخمين نقطه تغيير

Determination of the Onset, Withdrawal and Duration of Sistan wind using a Change Point Approach

باد سيستان به عنوان برجسته‌ترين جريان ترازهاي زيرين جو در فلات ايران، بنا به ماهيت، گستره‌اي محلي تا منطقه‌اي را در شرق اين فلات اشغال مي‌نمايد. اگرچه محلي و يا منطقه‌اي بودن باد سيستان موضوعي باز تلقي مي‌گردد، اما آغاز و خاتمه ناگهاني وزش باد سيستان در شرق ايران و تغييرات سال به سال قابل ملاحظه در زمان آغاز و خاتمه اين باد، نقش برجسته‌ عوامل بزرگ مقياس و مقياس منطقه‌اي را در شكل‌گيري باد سيستان متذكر مي‌گردد. پژوهش حاضر سعي دارد با بهره‌گيري از روش‌هايي موسوم به «نقطه تغيير»، زمان آغازگري (Onset)، زمان خاتمه و طول مدت وزش باد سيستان را مورد بررسي قرار دهد. مطالعه با بهره‌گيري از داده‌هاي روزانه شدت و جهت باد در ايستگاه زابل، به عنوان ايستگاه معرف باد سيستان، براي يك دوره 41 ساله (2012-1972) به انجام رسيد و جهت تعيين زمان آغازگري و خاتمه باد سيستان از سه روش Pettitt، SNHT و Buishand استفاده شد. در اين تحقيق با بهره‌گيري از سري زماني روزانه شدت و جهت وزش باد، زمان آغاز، خاتمه و طول مدت وزش باد سيستان تعيين گرديد. يافته‌ها بيانگر آن است كه از ميان سه روش تخمين نقطه تغيير، روش پتيت در تعيين زمان آغاز و خاتمه باد سيستان از كارايي مناسب‌تري برخوردار است. بر اساس روش پتيت باد سيستان بطور متوسط در روز 122 ام (2 مي) از سال ميلادي آغاز شده و تا روز 287 ام (14 اكتبر) تداوم مي‌يابد. بر اين اساس، متوسط طول دوره وزش باد سيستان 165 روز در سال تعيين گرديد. يافته‌ها همچنين بيانگر آن است كه زمان آغازگري، زمان خاتمه، شدت و طول مدت وزش باد سيستان تغييرات قابل ملاحظه‌اي را از سالي به سال ديگر نشان مي‌دهد. از ديگر نتايج‌ اين مطالعه، نبود يك روند معني‌دار در رفتار تمامي ويژگي‌هاي باد سيستان است.

1. Introduction Wind is one of the important meteorological parameters which have an extensive application in environmental studies. The winds have various spatial and temporal scales regarding to their characteristics and structures. Sistan wind as an outstanding low level flow in eastern Iran is playing an important role to creation of local and regional climatic features. The Sistan wind initiates and ends with an abrupt change in wind speed and wind direction which mostly related to a regional scale atmospheric circulation pattern. 2. Study Area The Sistan area is located at the tail end of a large closed inland basin (Fig. 1), in one of the driest regions of the world (Smith, 1974). The area has very varied climatic conditions. The annual precipitation in the lower Sistan basin is about 50 mm. The Helmand river plays a major role in the Sistan region by draining the snowmelt waters from the mountains of the southern Hindu Kush. The basin has no outlet and receives drainage from an area of about 335,000 sq km. Its floor occupies about a third of this area and lies at elevations that range from 500 to 900 m., the west end of the Hindu Kush, with peaks more than 4000 m a.s.l., borders the basin on the north. Mountains as much as 2000 m high border the basin on the west, south and east. The climate of this area is arid. Mean monthly low temperatures are near 5°C, and mean high monthly temperatures are near (35°C). 3. Material and Methods This research aims to clarify the onset and the withdrawal dates of Sistan wind by using a change point approach. The meteorological dataset, i.e. surface wind at 10 m above ground level (agl), was obtained from the Zabol meteorological station, located well within Sistan basin and in close proximity (~10 km) from the Hamoun lakes. The dataset corresponds to 3-hrs and/or daily-averaged observations covering the whole period 1972-2012 with specific emphasis on wind velocity and wind direction. Three most popular change point techniques including Pettitt, SNHT and Buishand were applied to estimate the onset and withdrawal dates of Sistan wind during the whole period of the time. All three techniques are used wind velocity time series to clarify the specific time for each year. Also, the wind direction time series were used to confirm the results which obtained by change point techniques. 4. Results and Discussion The result indicates that the wind over the Sistan basin experiences two types of abrupt changes during the warm period of the year. The first type of abrupt change is related to the eastward movement of synoptic systems originated from extra-tropics which can affect the region for a short term (i.e. for a couple of days). In contrast, the second type of the abrupt change can recognized as a sign of the onset and withdrawal of Sistan wind over eastern Iran. In the beginning of summer, when the wind over Sistan basin suddenly changes from an irregular and low intensity wind to a stable and strong northwesterly wind, one can suppose that the onset of Sistan wind is occurred. On the other hand, the withdrawal of Sistan wind is related to an abrupt change in direction and the intensity of wind where the stable and strong northwesterly wind replaces by an irregular and low intensity wind over the Sistan basin only during a few days at the end of warm period of the year. 5. Conclusion The results indicate that the Pettitt is the most reliable method among three change point techniques, regarding to estimation of onset and withdrawal dates of the Sistan wind. The Sistan wind is initiated on 122nd Julian day (May 2), and is ended on 287th Julian day (October 14), on the base of Pettitt method. Therefore, the average duration of Sistan wind blowing is about 165 days in the year which is apparently longer than the duration which former researchers has been estimated. The results also indicate that the inter-annual variations of Sistan wind intensity are significantly high. Also, it should note that, there is no any significant trend in the behavior of Sistan wind, (i.e. onset, withdrawal and wind blowing duration), when the wind velocity and direction of Sistan wind is analyzed. It should also conclude that the inter-annual variations of wind velocity and its onset and withdrawal times over Sistan basin are mostly related to the variation of regional to large scale atmospheric circulation over southwest Asia.