پديد آورندگان :
محمدي، حسين نويسنده دانشكده جغرافيا,دانشگاه تهران,ايران Mohammadi, H. , يلمه، اسماعيل نويسنده ايران Yolmeh, I.
كليدواژه :
آتش‌سوزي جنگل , تحليل همديد , فرارفت , گرمباد , گلستان
چكيده فارسي :
آتش‌سوزي جنگل‌ها يكي از مخاطراتي مي‌باشد كه با شرايط جوي مرتبط است. با مطالعه شرايط جوي در مواقع رخداد آتش‌سوزي مي‌توان به اين ارتباط پي برد. داده‌هاي استفاده شده در اين تحقيق شامل داده‌هاي هواشناسي در دوره آماري 1386-1377 به همراه گزارش‌هاي وقوع آتش‌سوزي جنگل‌ها در همين دوره مي‌باشد. در اين تحقيق تعداد و مساحت آتش‌سوزي ها به همراه روند و پراكندگي زماني و مكاني آنها مورد بررسي قرار گرفت. سپس داده‌هاي هواشناسي و نقشه‌هاي سطح زمين تا 500 هكتوپاسكال به همـراه نقشه‌هاي دماي سطوح بالا و نقشه‌هاي ضخامت و بردار باد تحليل شد. بررسي‌ها نشان مي‌دهد كه در دوره آماري تعداد و وسعت آتش‌سـوزي‌ها روند افزايشي دارد. از نظر پراكندگي زماني، بيشترين فراواني آتش‌سوزي مربوط به دوره‌ي سرد سال است. از نظر مكاني نيز بيشتر آتش‌سوزي‌ها در جنوب‌شرق استان متمركز است. آرايش همديدي نقشه‌هاي هوا نشان مي‌دهد كه در سطح زمين با شكل گيري يك مركز كم فشار بر روي خزر و يك مركز پر‌فشار بر روي زاگرس بين مركز و شمال ايران گراديان فشار ايجاد مي‌شود.قرارگيري پشته بر روي ايران و در جلوي ناوه عميق شرق اروپا در سطح 500 هكتوپاسكال، به همراه جهت و سرعت باد در سطوح مختلف جو باعث ايجاد جريانات جنوب‌غرب به شمال‌شرق و فرارفت هواي گرم عرض‌هاي پايين‌تر (به ويژه شمال آفريقا و شبه‌جزيره عربستان) به نواحي مركزي و شمال كشور مي‌شود. نقشه‌هاي ضخامت نيز مؤيد فرارفت گرم در روزهاي وقوع آتش‌سوزي مي‌باشد. علاوه بر اين كوهستان البرز باعث رخداد گرمباد در دامنه بادپناه خود مي‌شود.
چكيده لاتين :
Introduction
When temperature increases and relative humidity decreases, especially if this condition is associated with the warm wind, the condition is prepared for forest fires. In seasons that branches and leaves have less moisture, fire events will happen more likely. Forest fires takes a lot of damage to the country#039 s natural resources each year. In recent years effects of temperature is rising and changing of precipitation pattern on forest fires risk had been different on various regions of Alps (wastl et al, 2012). At Australian Alps the foehn winds suddenly changed climatic parameters and increased risk of forest fires (sharples et al, 2010). Climate change is an important factor in causing forest fires in northern Europe and Asia (Groisman et al, 2007). During the period 19802000 the mean burnt area by wildfires in Portugal was higher than 90000 ha per year (Pereira et al, 2005). Large forest fires in Portugal occur when the atmospheric circulation forms a prominent ridge over the Iberian Peninsula with the flow being dominated by a strong meridional component. These days are associated with southeasterly winds that led to warm advection from North Africa to the Iberian Peninsula (Pereira et al, 2005). Several forest fires also occur each year in Iran, especially in the northern forests. During the 19982005 periods, 1258 forest fires occurred in the northern forest area, where about 7623.29 hectares were burned (azizi yusefi, 2009). In golestan province averagely occur 82 forest fires every year and forests with area of 740 hectares were destroyed (Department of Natural Resources, Golestan Province, 2006). Given the importance of forests and the role of Atmospheric Condition in causing forest fires, the relationship between fire event and Atmospheric Condition should be considered.
Materials and methods
Data used in this study includes meteorological and forest fire data. First, spatial and temporal distributions of forest fires were determined. Then in the fire occurrence days, meteorological data that consist of temperature, relative humidity, wind and pressure were surveyed. Precipitation and cloudiness were investigated in south slope of Alborz Mountains to ensure the formation of foehn wind. Finally synoptic patterns during fire event using sea level and upper level maps, along with vector wind and thickness maps were analyzed.
Results and Discussion
In temporal distributions, most of fire cases occurred in cold seasons. Considering the spatial distributions, most fire cases were in the southeast of province. In fire occurrence time the temperature had a remarkable rise and the relative humidity had a remarkable decrease. Also there were high speed winds in all stations. Moreover rainfall had been occurred over southern slopes of Alborz Mountain. The synoptic pattern of weather maps showed that a low pressure was formed over Caspian Sea and a high pressure over Zagros mountain at sea level map. At 500 geopotential height, there was a deep trough over Eastern Europe and was formed a ridge over Iran. Vector wind maps shows that, wind direction is south west North East at upper levels. In fire occurrence days, thickness of atmosphere was high in studying region.
Conclusion
High number of fire occurrence in cold seasons showed that, there was no relationship between warm season and fire occurrence. Remarkable increase of temperature and remarkable decrease of relative humidity during the fire times along with high speed wind showed that, foehn had been occurred. In addition, foehn occurrence was confirmed by existence of precipitation and cloudiness in Alborz southern slopes. At sea level map, a low pressure tongue over Caspian Sea is stretched. If this tongue accompanied with deep trough at 500 geopotential height, a low pressure over Caspian Sea is formed. With the formation of a high pressure over the Zagros Mountains, the pressure gradient is created between the center of Iran and the Caspian Sea. At 500 geopotential height, placement of a ridge over Iran, along with southwestern wind in upper level, forms warm advection from low latitudes to northern area of Iran. Also high thickness of atmosphere in studying region confirms warm advection from Arabian Peninsula and north of Africa. Existence of Alborz Mountains in front of lowlevel air makes up the foehn in northern coast of Iran.
