كليدواژه :
بارش‌هاي فوق سنگين , ديناميك جو , سنگين , سواحل جنوبي خزر , همرفت و غير همرفت
چكيده فارسي :
شاخص وضعيت ديناميكي جو كه ميزان انحراف از غير ايستا بودن، خشكي و ويسكوزيته جو را بيان مي‌كند، مي‌تواند به عنوان يك تيوري نوين، محل و زمان حضورسامانه‌هاي پرفشار و كم فشار و ميزان شدت آن را تعيين كند. اين تحقيق به بررسي رابطه بين بارش‌هاي سنگين سواحل جنوبي خزر و شاخص DSI مي‌پردازد. شاخص DSI در ترازهاي مختلف هم دماي پتانسيل و در زمان رخداد بارش‌هاي فوق سنگين و سنگين با منشا همرفت و غير همرفت با استفاده از داده‌هاي يك ساعته دما، ارتفاع ژيوپتانسيل و سرعت باد( استخراج شده ازپايگاه داده اروپايي( ار40) و با محاسبه پارامترهاي چگالي هوا، تاوايي پتانسيل، دماي پتانسيل و ضريب تابع جريان برنولي به دست آمد. ميانگين روزانه اين شاخص در ترازهاي مختلف محاسبه و براي هر گروه بارشي، ضريب همبستگي اسپيرمن با ميانگين بارش منطقه اي در سواحل جنوبي خزر به دست آمد. نتايج نشان داد كه براي همه گروه‌هاي بارشي سنگين و فوق سنگين در تراز 280 تا 310 درجه كلوين، شمال درياچه خزر داراي مقادير مثبت (سامانه پرفشار و فرونشيني هوا) و جنوب درياي خزر داراي مقادير منفي (سامانه كم فشار و صعود هوا) است. در گروه بارشي فوق سنگين با منشا همرفت، تنها در ناحيه كوچكي از شمال شرقي درياچه خزر، مقادير مثبت قوي اين شاخص و در نتيجه فرونشيني هوا ديده مي‌شود و در بخش وسيع باقي مانده صعود هوا روي مي‌دهد. اين شاخص در هر دو گروه بارش‌هاي همرفت و غير همرفت در گروه بارشي سنگين كوچك تر از فوق سنگين است تنها وسعت مناطق صعود و نزول هوا متفاوت است. با وجود هماهنگي خوب نقشه‌هاي DSI و نقشه‌هاي همديد جوي در شناسايي نواحي صعود و نزول هوا، ضرايب همبستگي بين بارش و مقادير مطلق DSI در اروپا قوي تري از اين رابطه در سواحل جنوبي خزر است.
چكيده لاتين :
Introduction
The Dynamic State Index (DSI) is being introduced on the basis of a newly developed theory termed the Energy-Vorticity Theory (EVT). As a weather index, the DSI can describe the temporal sequence and intensity of areas of high and low pressure; but as a climate index, it can also diagnose climate change based on the variation in solar radiation or the influences of friction. In physical terms, the index is a parameter that combines the retention of energy and the atmospheric vorticity with each other. If the index is at zero, the atmosphere is in a stable basic energy-vorticity state. The new idea is to determine the variability of the weather and the climate as deviations from this characteristic status. If the index is positive, the large-scale weather pattern is characterized by extended areas of high pressure. If, however, it takes on a negative value, atmospheric activity features mainly low-pressure vortices (cyclones) instead. Hot summers, such as that experienced in Europe in 1997, are characterized by especially low fluctuations in this index. This research investigates relationships between heavy precipitation events and DSI.
Study Area
The north of Iran including Gilan, Mazandaran and Golestan provinces is the study area for surface data analyses. Also a region between 0° to 80°N in latitude and -20° to 120°E in longitude is the study area for upper levels analyses. The Caspian Sea is located in this region. The Caspian Sea is the largest closed body of water on the surface of the Earth. The sea has a surface area of 371,000 square kilometers and a volume of 78,200 cubic kilometers. Its basin has no outflows and is bounded by northern Iran, southern Russia, western Kazakhstan and Turkmenistan, and eastern Azerbaijan.
Material and Methods
On the basis of daily precipitation events and with regard to 25 and 50 percent probability, precipitation events were divided into two groups of heavy and super heavy precipitation. The precipitation was grouped into two classes including convective and non-convective clouds based on clouds synoptic indexes. The DSI was computed in different levels and different precipitation groups including heavy, super-heavy, convective and non-convective events using density, potential vorticity, potential temperature and Bernoulli-Stream function. These atmospheric characteristics were calculated using temperature, Geopotential height and wind velocity (ERA40 database). Spearman’s correlation coefficient between mean daily of DSI and mean of precipitation was computed in different levels and precipitation groups. The mean SLP and mean DSI maps were compared in different precipitation groups.
Results and Discussion
investigation on correlation coefficient between DSI and mean precipitation in different groups (super-heavy and convective, heavy and convective, super-heavy and non-convective, heavy and non-convective) and different levels show that the super-heavy and convective precipitation events in 310° Kelvin level show the highest positive correlation coefficient (40%) in the north east of Caspian Sea and the highest negative correlation coefficient (-32%) in south west of Caspian Sea among the groups significantly. The north and the south of Caspian Sea have positive and negative DSI respectively in different levels and precipitation groups (figures 1-4).
Conclusion
The results reveal that the north of Caspian Sea have positive DSI(subsidence) and negative DSI(air lifting) in all precipitation groups and in 280 to 310 Kelvin. In super-heavy precipitation group when convective clouds are in the sky, there are just a small area with strong positive DSI (strong subsidence) in the north of Caspian Sea. The other parts of the Sea have negative DSI or air lifting. DSI in heavy precipitation group is less positive and less negative than super precipitation group both convective and non-convective events. Subsidence and airlifting area are different in each precipitation group. Despite the good role of the DSI for recognition of subsidence and airlifting area, correlation coefficient between precipitation and the absolute DSI is more in Europe than the north of Iran.
