تحليل ديناميكي‌– سينوپتيكي سازوكار بارش‏هاي تابستانه جنوب شرق ايران

Dynamical- synoptical Analysis of Summer precipitation Process in Southeast of Iran

در اين پژوهش 78 مورد از بارش‏هاي تابستانه جنوب ‏شرق ايران انتخاب شد؛ پس از اجراي آزمون خوشه‏بندي، چهار الگوي سينوپتيكيِ مرتبط با اين بارش‏ها تشخيص داده شد. نتايج پژوهش بيانگر آن است كه اين بارش‏ها همزمان با گسترش زبانه‏اي از كم‏فشار گنگ بر روي شرق ايران از سطح زمين تا تراز 850 ه.پ صورت مي‏گيرد و در سطوح مياني و بالايي آتمسفر ناوه‏اي از موج بادهاي غربي بر روي منطقه مستقر نمي‏شود. در همه الگوها سه هسته واگرايي شار رطوبت بر روي شمال درياي عرب، غرب درياي عرب، و خليج‏ فارس شكل گرفت: شمال درياي عرب و پس از آن غرب درياي عرب منابعِ مهمِ تامين رطوبت جنوب ‏شرق ايران در اين الگوها بودند. گردش پادساعتگرد هواي مرطوب مونسون در دامنه‏هاي جنوبي هيماليا به شكلي ضعيف فقط در دو الگو مشاهده شد، اما نقشي از آن در تامين رطوبتِ اين الگوها مشاهده نشد، زيرا همزمان هسته‏اي قوي از همگرايي شار رطوبت بر روي مركز و شمال پاكستان قرار مي‏گيرد كه جريان پادساعتگرد هواي مرطوب دامنه جنوبي هيماليا و حتي بخشي از شار همگراشده رطوبت در جنوب ‏شرق ايران را به درون خود مي‏كشد. نيمرخ قايم آتمسفر منطقه نيز حاكي از همگراشدن رطوبت در سطوح زيرين آتمسفر (750 ـ 1000 ه.پ) و واگراشدن رطوبت در سطوح مياني و بالايي آتمسفر منطقه است.

Introduction In the Iran’s plateau, with move of the subtropical high pressure to low latitudes in cold period of year, the emigrant systems of westerly waves are dominant atmospheric phenomena in this regions and control atmospheric phenomena in this period of year. But in the warm period of year the Sub-Tropical high-pressured is dominate atmospheric pattern in higher level of atmosphere in the Iran plateau, is the main factor that control the weather and climate over this region in the middle and lower troposphere is a local to regional scale thermal forcing. Focusing on the source and the path of providing necessary humidity for summer precipitations in this region, Parand (1991), Alijani (1995) and Najarsaliqe (1998) believed that the humidity of Indian Ocean and Arab Sea in low pressure cyclonic circulation of Pakistan move parallel through southern feet of Himalaya Mountain and penetrate southeast of Iran with east- west direction from Pakistan. In case of existing necessary factors, they ascend and make summer precipitations of the region. Materials and methods The period used in this study was the 78 days in 22 years (2010-1982) from May until end of September. The atmospheric circulation types were extracted using daily mean of the 850 hPa geopotential height .Then the agglomerative hierarchical cluster analysis with the ward algorithm and Euclidean distance used to identify atmospheric circulation types over Iran in mentioned period of years .Then calculated the within-group correlation to identify representative days. The day with highest within-group correlation was representative day of atmospheric circulation types. Finally, 4 atmospheric circulation type were identified for this summertime precipitation. humidity flux divergence of the region was calculated by the relationship bellow called horizontal flux divergence, in which in x,y directions (longitude and latitude), (q) stands for small changes of specific humidity, (u) for U component wind, (v) for V component wind. Equation 1 HFD_ij=-[u_(ij ) (q_(i+lj)-q_(i lj))/(x_(i+lj)-x_(i lj) )+v_(ij ) (q_(ij+l)-q_(ij l))/(y_(ij+l)-y_(ij l) )+q_(ij ) (u_(i+lj)-u_(i lj))/(x_(i+lj)-x_(i lj) )+q_(ij ) (v_(ij+l)-v_(ij l))/(y_(ij+l)-y_(ij l) )] HFD is horizontal flux divergence, but ?x and ?y stand for the distance in longitude and latitude respectively. Besides, positive values mark humidity flux convergence, while negative values show humidity flux divergence. In fact calculated values are dedicated to each level considering the values for special level, as a result to find the real values of humidity flux vertical sum should be calculated. This value should be calculated for the distance of the height level of atmosphere. Following relationship is used in which (vq) stands for HFD, (p) for the level of atmosphere geo-potential height, and Qvi for vertical sum of humidity flux. Equation 2 Q_vi=1?(g.) ?_(p^1)^(p^2)??vq.dp? Since the used data are 6-hour, these calculation are done for a 6-hour period. To do the aforesaid calculations for a longer period (2 day) and the distance between several atmospheric levels following equation should be used. In this equation (t1) and (t2) stand for the beginning and end times of calculating respectively . Equation 3 Q_vi=?_t1^t2?.(1?(g.) ?_p1^p2??vq.dp?).dt Results and discussion Focusing on these precipitations, 4 patterns were recognized. Their humidity flux through 3 levels of atmosphere (lower levels, higher levels and vertical profile of atmosphere) were studied. There is a core of humidity flux convergence of southeast of Iran in lower levels of atmosphere(1000- 750 hPa). Besides, cores of humidity flux divergence on north of Arab Sea, west of Arab Sea and Persian Gulf are responsible to inject humidity to surrounding regions.A core of humidity flux divergence is also formed in southeast of Iran in middle and upper levels of atmosphere. Therefore, the injection of these precipitations happened in lower levels of atmosphere. Vertical profile of atmosphere revealed that, among the patterns, there is humidity flux convergence from the surface to 750 hPa level and humidity flux divergence in upper levels. Conclusion Studying summer precipitations of southeast of Iran revealed that a tongue from Gang low pressure in l000 and 850 hPa levels penetrate Iran Plateau and Arabian Peninsula, while there is no tangible trace of this low pressure on the region in 700 hPa level. The calculation of humidity flux function is in accordance with the findings of Karimi et al. (2007). Besides, north of Arab Sea is obviously considered as the most important source of the humidity in the region, as a core of humidity flux convergence is made in all patterns from the earth surface to 750 hPa level, while 3 cores of humidity flux divergence on north of Arab Sea, west of Arab Sea and Persian Gulf transfer humidity to surrounding regions. The divergent core of north of Arab Sea and west of this sea are considered to be the most important source to provide humidity for the region, while the core of Persian Gulf is minor. This phenomenon is mainly the result of central and southern Zagros Mountain chains which limit transformation of humidity from Persian Gulf to Iran Plateau. Moreover, there is no core of humidity flux divergence on Oman Sea to transfer humidity to surrounding regions. On the other hand, counter clockwise circulation of air through southern feet of Himalaya Mountain Chain in lower levels of atmosphere can only be seen in pattern 4 (Fig. 4) and slightly in pattern 3 (Fig. 3). Such synoptic order has no role in providing needed precipitation humidity of southeast of Iran, for it is too far from southeast of Iran to provide enough humidity. Moreover, a strong core of humidity flux convergence is made in north and center of Pakistan in which the aforesaid circulation in southern feet of Himalaya penetrates. The atmosphere of the studied region can be divided in to lower part (1000-750 hPa level) and upper part (700- 300 hPa level) in which humidity flux convergence and humidity flux divergence are made respectively.