تحلیل آماری - همدیدی آتش سوزی جنگل در استان گلستان (مطالعه موردی :روزهای 25 آذر و 18 بهمن سال 1384)

آتش‌سوزی جنگل‌ها یکی از مخاطراتی می‌باشد که با شرایط جوی مرتبط است. با مطالعه شرایط جوی در مواقع رخداد آتش‌سوزی می‌توان به این ارتباط پی برد. داده‌های استفاده شده در این تحقیق شامل داده‌های هواشناسی در دوره آماری 1386-1377 به همراه گزارش‌های وقوع آتش‌سوزی جنگل‌ها در همین دوره می‌باشد. در این تحقیق تعداد و مساحت آتش‌سوزی ها به همراه روند و پراکندگی زمانی و مکانی آنها مورد بررسی قرار گرفت. سپس داده‌های هواشناسی و نقشه‌های سطح زمین تا 500 هکتوپاسکال به همـراه نقشه‌های دمای سطوح بالا و نقشه‌های ضخامت و بردار باد تحلیل شد. بررسی‌ها نشان می‌دهد که در دوره آماری تعداد و وسعت آتش‌سـوزی‌ها روند افزایشی دارد. از نظر پراکندگی زمانی، بیشترین فراوانی آتش‌سوزی مربوط به دوره‌ی سرد سال است. از نظر مکانی نیز بیشتر آتش‌سوزی‌ها در جنوب‌شرق استان متمرکز است. آرایش همدیدی نقشه‌های هوا نشان می‌دهد که در سطح زمین با شکل گیری یک مرکز کم فشار بر روی خزر و یک مرکز پر‌فشار بر روی زاگرس بین مرکز و شمال ایران گرادیان فشار ایجاد می‌شود.قرارگیری پشته بر روی ایران و در جلوی ناوه عمیق شرق اروپا در سطح 500 هکتوپاسکال، به همراه جهت و سرعت باد در سطوح مختلف جو باعث ایجاد جریانات جنوب‌غرب به شمال‌شرق و فرارفت هوای گرم عرض‌های پایین‌تر (به ویژه شمال آفریقا و شبه‌جزیره عربستان) به نواحی مرکزی و شمال کشور می‌شود. نقشه‌های ضخامت نیز موید فرارفت گرم در روزهای وقوع آتش‌سوزی می‌باشد. علاوه بر این کوهستان البرز باعث رخداد گرمباد در دامنه بادپناه خود می‌شود.

Statistical and Synoptic Analysis of Forest Fire in Golestan Province (Case Study: 16 December 2005 and 9 March 2006)

IntroductionWhen 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 methodsData 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 DiscussionIn 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.ConclusionHigh 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.