ميانگين الگوهاي گردشي توأم با بارش‌هاي سنگين در شمال غرب ايران

Circulation Patterns of Heavy Rains in the North West of Iran

بارش سنگين براي هر روز به بارشي است كه فراتر از بارش هاي معمول در يك محل و براي همان روز باشد. براي اين مقدار بارش ها دو آستانه ي مطلق و نسبي معرفي شده است. در تحقيق حاضر، بارش سنگين با استفاده از نمايه ي نسبي و بر اساس فراسنج صدك ها تعريف شده است. ميانگين الگوهاي گردشي تراز دريا و تراز 500 هكتوپاسكال توأم با بارش هاي مذكور بررسي شد. بدين ترتيب، دو گروه داده: يكي، زميني و ديگري، جوي براي تحقيق حاضر استفاده شده است. داده هاي زميني شامل: اندازه گيري بارش در ايستگاه هاي همديد، اقليم شناسي و باران سنجي سازمان هواشناسي كشور و ايستگاه هاي باران سنجي وزارت نيرو براي دوره ي آماري 1967-2007 است. روش كريجينگ روش بهينه ي ميان يابي و نيز اندازه ي شبكه ي ميان يابي، براي تهيه ي نقشه 14975 مورد استفاده شد. اندازه شبكه ميان يابي با ابعاد حدود 33*33 كيلومتر (تقريباً 116 ياخته) اختيار شد. بنابراين، داده هاي شبكه اي بارش شمال غرب به ابعاد 116*14975 با آرايش گاه جاي مرتب شد. داده هاي جوي شامل فشار تراز دريا و ارتفاع تراز 500 هكتوپاسكال از پايگاه داده هاي جوي مركز پيش يابي اقليم NCEP/NCARوابسته به سازمان ملي جو و اقيانوس شناسي ايالات متحده برداشت شد. گستره هاي 10-20 الي 60-70 درصد تحت پوشش بارش سنگين بررسي شد. نقشه ي ميانگين و گرانيگاه بارش براي تمامي اين حالات برآورد گرديد. بررسي اين وضعيت ها گواهي بر تصادفي بودن توزيع مكاني بارش هاي سنگين است. متوسط الگوهاي فشار تراز دريا حضور پرفشار سيبري با زبانه ي شرقي ـ غربي و نيز ناهنجاري مثبت را در محدوده ي ظهور آن نشان مي دهد. كشيدگي زبانه ي پرفشار سيبري به سمت اروپا، موجب ظهور ناهنجاري كمابيش قوي و مثبت در اين ناحيه شده است. ظهور يك كم فشار در ناحيه درياي سرخ و كشيدگي زبانه آن به مديترانه شرقي، شامات و شمال عربستان و نيز امتداد آن تا شمال غربي و بعضاً سرتاسر غرب ايران ضمن اين كه موجب تكوين يك ناحيه با ناهنجاري منفي فشار شده است، در مجاورت پرفشار و ناهنجاري مثبت اروپايي موجب شكل گيري شيو شديد فشار شده است. در تراز 500 هكتوپاسكال، شمال غرب ايران در جلو محور فرود (ناوه) حاكم بر روي مديترانه ي شرقي قرار دارد. با افزايش پهنه ي زير پوشش بارش سنگين، عمق فرود افزايش يافته و محور آن از حالت نسبتاً عمود به سمت حالت افقي و اريب ميل مي كند. حضور اين ناوه موجب شكل گيري ناهنجاري منفي در ناحيه تحت تأثير بوده است. در تمامي حالات، بلافاصله در غرب و شرق ناوه مورد بحث يك پشته قرار گرفته است. حضور پشته ي غربي ضمن ريزش هواي سرد به داخل ناوه، شرايط برخورد هواي سرد شمال اروپا و هواي گرم ترِ مديترانه شرقي و شكل گيري جبهه ها را ميسر ساخته است. با جابه جايي شرق سوي پشته ي شرقي، پهنه ي زير پوششِ بارشِ سنگين، زياد مي شود.

Climatic extremes are the special status (high or low) of climatic elements. In spite of the unique definition, there are a lot of thresholds which have been illustrated for extremes. For exle, Bonneted (2006) has defined the climatic extremes as intensive and abnormal events that include the lowest and highest values in a time series. Becker et al. (2007) have referred to the extremes of climate as events in every given point that exceeds a special threshold in that place. High extremes and the upper tail of precipitation distribution of frequency have attracted a lot of attention of experts. The thresholds of extremes have been chosen based on geographic situations. The Joint World Meteorological Organization Commission (CCL) for Climatology on Climate Variability and Predictability (CLIVAR) Expert Team on Climatic Change Detection, Monitoring and Indices (ETCCDMI) have been established in 1998 in order to study and determine the indices of climatic extremes. They have introduced quintile indices. Due to consequences of extreme precipitation characters e.g. frequency, duration and intensity, the precipitation extremes have been in the center of attentions of many branches of science. Some experts call these events as social challenges that can determine economic sustainable development. Extremes analyses are based on investigating the tails of statistical distribution of daily observations, because the longest time scale couldnrsquo t show what it should have shown for extremes. Heavy precipitation for each day is defined as precipitation which is more than normal precipitation of that day in every given place. For this amount of precipitation absolute and relative thresholds have been defined. In present research, heavy precipitation is defined based on relative index and percentile parameter. By using 90th percentile, some characters of tempospecial distribution of extreme precipitation in Northwest of Iran are analyzed based on 729 stations. Northwest of Iran includes four province East and west Azerbaijan, Ardabil and Zanjan. This part of country has 126544.4 and occupied 7.2% of the entire country mainland. Geographic location of Northwest of Iran is located between the following coordinates: The averages of sea level pressure (SLP) and 500 hp level patterns have been examined. Therefore, two data groups, station based and atmospheric based, have been used. Station based data include precipitation measurements during 19682007 synoptic , climatology and rain gauge stations related to Islamic Republic of Iran Meteorology Organization (IRIMO), rain gauge stations of Ministry of Power. Kriging Method is used as optimum interpolation method in order to provide maps of 14975 days. The pixel size of interpolation is chosen with 33times 33 kilometers dimension (approximately 116 pixel). Thus, data set of northwest precipitation with 14975 times 116 dimension and Smode have been arranged. Atmospheric data include SLP and 500 hp data have been derived from NCEP/NCAR. The area experiencing heavy precipitation from 1020 to 6070 percent have been investigated. The map average and precipitation and precipitation center for all of these cases have been estimated. Some characters, for instance tempospatial presentation of heavy precipitation has determined by using Geostatistics Methods. A 14975 116 pixel data network was defined. According to 6 categories of extreme precipitation have been recognized. These categories are based on the extent of the area under extreme precipitation. It has been discovered 6 categories 1020 percent to 6070 percent of space under investigation. With the average increase of extreme precipitation amounts, the central mean of precipitation has centralized and the isohyets have become irregular. The small change in central mean of precipitation, a serious change had happened in precipitation distribution. Mean of SLP pattern showed Siberian high pressure system that extended from east to west and indicated positives anomalies. low pressure system in the Red sea which is extended to Europe region and its extension to eastern of Mediterranean sea, south and north of Saudi Arabia as well as its extension to northwest and sometimes the whole west parts of Iran, formed an area with negative anomalies. The low pressure system which was close to European high, formed extreme pressure gradients. In the 500 hp level, the northwest of Iran is in front of the trough which is located in the east of Mediterranean Sea. As the depth of the trough increased, the area where experiencing heavy precipitation increased and the axis of the trough changed from vertical into horizontal shape. The occurrence of the trough formed negative anomalies in the area. In all cases, there are two ridges immediately in west and east of the trough of the Mediterranean Sea. The occurrence of the western ridge caused cold air mass flowing in the trough where the Mediterraneanrsquo s warmer air mass exists and made the frontrsquo s formation possible. As the eastern ridge moved eastward, the area where experiencing heavy precipitation increased.