چكيده لاتين :
1. Introduction
Forest fire is considered as a natural disaster and a major potential threat in many parts of the world. It also has a huge impact on the form and compounds of different species and the spatial pattern of plant coverage. Fires in forests and pastures are one of the world 's most important issues, not only environmentally, but also from an economic and social aspect. Forest fires usually result in the destruction of the forest or its renewal which is of different importance in different regions regarding its destruction and the amount of damages it brings about, because its severity depends on the climate of the region in which it occurs. Forest fire is a primary process which also affects botanicals and their spatial structure. The emitted smoke from forest fires is amongst the most worrying consequences of this phenomenon. Fires have a large impact on the quality of air and human health, because they release large amounts of compounds such as carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nitrous oxide (NOX), ammonia ( NH3), particulate matter (PM), non-methane hydrocarbons (NMHC) and other chemicals in the atmosphere. On the other hand, as a result of forest fires, the soil loses a large degree of its organic materials, the risk of soil drifting increases and it also has significant negative impacts on the regeneration of previous species and the environmental conditions. Many physical, chemical, mineralogical and biological features of the soil are changed as a result of the forest fires. Due to the fact that Iran is at a critical condition with regard to its forest ecosystem and this ecosystem has experienced huge fires on an annual basis, the necessary measures should be taken to identify the causes of forest fires and prevent them if possible.
2. Study area
The city of Dezful is located at the plain areas of Khuzestan province, Iran, with an area of 7844 km. This city has expanded at the geographical longitude of 48 ° and 24 minutes east and latitude of 32 ° and 22 minutes north and its height is 140 meters above the sea level. Like most cities in Khuzestan, Dezful has a warm and humid climate with hot summers and Mediterranean winters. The annual average precipitation in Dezful is 250 mm and its average temperature is 3° C in winter and 49 ° C in summer.
3. Material and methods
In this study, two sets of data were used to analyze the fire at Dezful’s forests. First, the statistics regarding the fires were received from the department of forest and rangelands and in order to synoptically analyze this phenomenon, the data from the upper levels of atmosphere were taken into consideration. Based on a report submitted on 12.05.2012 and 08.01.2012 (23rd Ordibehesht and 11th Mordad 1391 in the Iranian calendar), two of the fiercest forest fires of 2012 in Iran have occurred in Dez and Dezful national parks which have resulted in 700 and 270 million Tomans damage to the forests and natural rangelands of Khuzestan Province, respectively. In order to analyze this damaging risk from an atmospheric point of view, the induced data of the components of sea level pressure, geopotential height, orbital and meridional wind at 2 meters above the ground and the upper levels of atmosphere, the temperature of different levels of the atmosphere and 2 meters above the ground and the relative humidity which were taken from the website of the National Center for Environmental Prediction (NCEP) were used. The maps which have been drawn and analyzed for the atmospheric analysis of forest fires in Dezful include the maps of sea level pressure, geopotential height, the thickness of the atmosphere, wind direction, atmospheric temperature, vorticity, thermal wind and relative humidity. The maps were analyzed at the following levels: temperature maps at the levels of 850, 925, 1000 HP and maps at 2 meters above the ground, maps of geopotential heights and wind currents at different levels (500, 600, 700 and 850 HP), vorticity maps at the levels of 1000, 925, 850, 700, 600 and 500 HP, thermal wind map at the levels of 1000, 925, 850 HP, wind and thermal wind at 2 meters above the ground, and relative humidity map at the levels of 1000, 925, 850, 700, 600 and 500 HP. All the maps were developed at the latitude of 15-70 and longitude of 10-75 and the time for drawing the maps was selected to be 12 Zulu time which is equivalent to 3:30 P.M. local time because of the higher visibility of the phenomenon under study.
4. Discussion and Findings
The analysis of atmospheric maps indicated that at the time of occurrence of fires in thermal forests, the blocking phenomenon associated with a stack on the study area on the 12th of May and very high contours on the 1st of August were observed.The thickness of the atmosphere which is a sign of the establishment of a hot system, hot thermal wind above the region with the dominance of thermal conditions and temperature of above 40 ° C on the 12th of May and the temperature above 48 ° C on the 1st of August are amongst the other dominant atmospheric patterns these days. Atmosphere stability and negative vorticity flow which has been followed by the subsidence of the warm air flow of southern latitudes from upper levels to the ground level together with very low relative humidity (10%), which has imposed a dry atmosphere on the climate of Dezful, have prepared the necessary conditions for the occurrence of fire at the forests of Dezful.
5.Conclusion
Based on what has been stated so far, the establishment of thermal units at ground level, occurrence of blocking at the upper levels of the atmosphere and the deployment of high contours and stacks on the studied area, thickness of the atmosphere and hot thermal wind above the region with the dominance of thermal conditions and a high temperature alongside atmosphere stability and negative vorticity flow which has resulted in the subsidence of the warm air flow of southern latitudes from upper levels to the ground level together with very low relative humidity (10%) which has imposed a dry atmosphere on the climate of Dezful, have provided the necessary conditions for the occurrence of fire at the forests of Dezful. Since no study has to date investigated the thermodynamic causes of fire at the forests of the southwest of Iran and researchers have just examined the causes of this phenomenon in the northern forests of Iran, the results obtained from previous studies have similarities and conflicts with the findings of the current study. Regarding the fires which occur at the northern forests of Iran, Foehn wind is very important and flows at the high pressure ground level, while at the time of fire in the forests of Dezful, this type of wind was very important at the low pressure ground with a high pressure of central core. However, the commonality of both studies is high temperature,the relative humidity of less than 20%, the position in height, and the stack at the upper levels of the atmosphere which is observed in both the environments. In this study, some parameters have been investigated which have not been considered at all in other related studies.
