مطالعه بارش‌هاي فرين فصل بهار استان آذربايجان غربي(2008– 2003)

Spring Extreme Precipitation in the Province of West Azarbaijan (2003 – 2008)

اهداف: هدف اصلي اين پژوهش، شناخت منشا شكل‌ گيري و ويژگي الگو هاي جوي و كميت‌هاي هوا-شناسي موثر بر بارش‌ هاي فرين بهاره استان آذربايجان غربي است. روش: براساس نقشه ‌ها و داده ‌هاي هواشناسي در تراز هاي مختلف جو، بارش ‌هاي فرين رخ‌داده درفصل بهار، طي دوره آماري 2003 تا 2008 بررسي شده است. سپس ويژگي الگو هاي جوي موثر در ايجاد اين بارش ‌ها، بي ‌هنجاري ‌هاي ميدان‌ هاي فشاري سطح دريا، ارتفاع تراز 500 هكتوپاسكال ، تاوايي‌نسبي و فرارفت آن در سطح 500 هكتوپاسكال، مطالعه شده است. يافته ها/ نتايج: نتايج نشان داد سامانه‌ هاي فشاري كه موجب بارش ‌هاي فرين بهاره در منطقه مي‌ شوند، اغلب از كم فشار هاي بريده مديترانه‌ اي بوده اند كه با ماندگاري در حدود سه روز در نواحي شرق مديترانه، رطوبت كافي براي ايجاد اين بارش ‌ها را كسب مي‌ كنند، همچنين در روز رخداد بارش ‌هاي فرين، بي ‌هنجاري‌ هاي ارتفاع تراز 500هكتوپاسكال، نسبت به ميانگين بلند مدت در منطقه شاخص بوده است و كاهش زيادي را نشان مي دهد. نتيجه گيري: در همه موردهاي مطالعه شده، تاوايي نسبي و فرارفت آن نسبت به روز قبل از بارش، افزايش قابل ملاحظه‌اي داشته است، به-طوري كه مقدار بيشينه تاوايي حدودs-15-10×4 تا s-15-10× 6و پيشينه فرارفت تاواييs-29-10×6 تا s-29-10×12 است و مي ‌تواند به عنوان يكي از پيش‌ نشانگر هاي بارش فرين در شمال غرب كشور استفاده شود.

1. INTRODUCTION West Azarbaijan province is located in the Northwest (NW) of Iran. This area is characterized by specific topography (mountainous) and geographical complexity, which plays a major role in meteorological parameters (especially on precipitation). A large part of Northwest rainfalls is in spring. In general, heavy rainfalls are mainly dependent on atmospheric circulations, geographical position and topography, which can lead to flash floods and pose serious threats to people and their properties. Therefore, the study of pressure pattern characteristics and meteorological quantities affecting heavy rainfalls and extreme events can help enhance the accuracy of predictions and provide early warnings which can significantly decrease the probable damages. Over the past few years, extensive research has been carried out on extreme precipitation events. Therefore, given the important role of atmospheric circulation patterns in the temporal and spatial distribution of extreme precipitations, they have been used here to investigate the relationship between the pressure patterns and extreme precipitations over Province of West Azarbaijan. 2. THEORETICAL FRAMEWORK Extremes are the infrequent maximum and minimum events at the range of values of a particular climate or weather variable. In other words, extreme weather events refer to rare events s at a particular place and time of year. Various indices have been developed for monitoring of precipitation extremes. In many studies, percentile index is used to determine extreme precipitation. Also, many climatic research and extreme precipitation trends draw on percentile index in their studies. This paper employs base percentile index, which out of the total amount of precipitation per year in a specific location of the distribution of daily rainfall, only uses days in which the amount of rainfall is equal or greater than 1 with days in which the amount of rainfall is equal or greater than 95th and 99th of percentile treated as heavy and extreme participations. Given the compel mechanism of extreme precipitation and the inaccuracy of numerical weather forecasts in prediction of extreme rainfall, other accurate meteorological quantities such as 500-hPa relative vorticity have been used. The results of this research can enhance the awareness of extreme precipitation and their important role in the northwest of Iran. To this end, in this paper Vertical Vorticity and Vorticity advection on 500hPa level are investigated. 3. METHODOLOGY In this study, weather patterns inducing heavy precipitations were studied in West Azarbaijan province for six years during the spring (2003-2008). For this purpose, meteorological data, weather charts at standard pressure levels and total precipitation of meteorological stations derived from Iran meteorological organization and gridded sea level pressure (SLP), 500-hpa geopotential height fields and their anomalies data derived from National Center for Environmental (NCEP) were used. In addition, relative vorticity and its advection were computed on 500-hPa level for rainfall episodes. 4. RESULTS & DISCUSSION In this study, the extreme precipitation measured in seven meteorological stations in West Azarbaijan province and their relation with atmospheric circulation patterns were examined using synoptic and dynamic analysis. The results showed that: 1- At the best conditions, during extreme precipitation events, the average pressure in the studied area was between 1000 and 1005hPa and elevation level of 5000hPa in the area was between 5550 and 5600m.. 2-The combination of low pressure systems (Sudan thermal low pressure and Mediterranean dynamic low pressure), provides the best condition for deepening of Mediterranean trough and strengthening of low pressure dynamic pressure in the Eastern Mediterranean, the results of which can produce extreme precipitation in Western Azarbaijan. 3-The Mediterranean low pressure systems which produce extreme precipitation in the area are frequently characterized by cut-off low pressure systems that last at least for three days in the East Mediterranean 4- In all cases, upper anticyclones have expanded from south of Iran to north latitude, thud playing an important role in of the stay of low pressure system in the eastern regions of the Mediterranean and provide appropriate conditions to obtain greater moisture and enough energy to produce extreme events in the area. 5. CONCLUSIONS & SUGGESTIONS The results indicate that extreme rainfalls in spring are not typical of spring regime in West Azarbaijan. These precipitations have been examined synoptically and dynamically by combining sea level pressure, 500mb height level and their anomalous data. All events were controlled by the strong cut-off low-pressure systems that are located in the east of Mediterranean Sea and remain stagnant for 3-7 days in this area. Also, in the days leading to the extreme rainfall periods, relative vorticity and its advection on mid-troposphere(500-hPa) level changed respectively from 4× 10-5 s-1 to 6× 10-5 s-1 and from 6× 10-5 s-2 to 12× 10-5 s-2). One day before the occurrence of heavy rainfalls, vorticity and its advection reached their maximum values, which can be used as a predictor for intense rainfall episodes in spring in the northwest of Iran.