基于WRF-Fire模式的2009年“5·21”蒙古国入境大火数值模拟研究

Simulation of the evolution of the 21 May 2009 wildfire from Mongolia to China using WRF-Fire Model

利用WRF-Fire模式对2009年“5·21”蒙古国草原入境大火进行火场发展数值模拟研究,并通过敏感性试验分析了防火隔离带对火势的阻挡作用。结果表明,模式可以较准确地再现环境风场对本次大火蔓延过程的影响,模拟火场面积、蔓延方向与实况基本一致。第一阶段(21日12—22时,世界时,下同)在蒙古国境内起火点附近,地面为裸露地表,可燃物负载量小,近地面为东北风,火线向西南方缓慢蔓延;第二阶段(21日22时—22日11时)风向转为西北,火线经平缓均一草地向东南方发展,在到达中蒙边境前地面风速增大到8~10 m/s,且风向与坡向一致,火线迅速移向中蒙边境防火隔离带;第三阶段(22日11时—23日20时)隔离带和人工灭火有效阻止了火线继续向中国境内蔓延,此阶段风向转为南风,火线向北扩展;第四阶段(23日20时—24日12时)风速达到16 m/s以上且风向转为西北风。受东面隔离带和湖泊的阻碍,火线缓慢向南蔓延。当火线到达南面的隔离带后,受隔离带阻挡及人工扑火作业的共同作用,本次大火于26日08时结束。敏感性试验表明,若不存在隔离带,火线将在风场的驱动下继续向东南、东北方向蔓延,所经区域主要地面可燃物类型从草地过渡到针叶林,火场的蔓延将造成大面积的森林资源损失。

The evolution of the“5·21”Mongolia wildfire in 2009 and the effect of fire-prevention isolation barrier along the China-Mongolia border were investigated by using WRF-Fire.The results showed that the model could reproduce the impacts of boundary winds on fire propagation.The simulation for fire area and propagation are shown to be broadly consistent with the observations:The first stage(1200 UTC 21st to 2200 UTC 21st),fire spread southwest slowly in the Mongolia because of low fuel loading on the near-bare surface with northeast wind.The second stage(2200 UTC 21 to 1100 UTC 22),wind direction turned to northwest,and fire spread southeast across flat and uniform grassland.Before the fire reached the border,the fire spread rapidly towards the isolation barrier because wind speed increased to reach around 8~10 m/s and wind direction was consistent with slope direction.The third stage(1100 UTC 22 to 2000 UTC 23),the barrier and firefighting was effective in preventing the fire entering Chinese territory.Then,the fire line spread north after wind direction turned to south.The fourth stage(2000 UTC 23 to 1200 UTC 24),wind speed reached more than 16 m/s and direction turned to northwest.However,fire spread south slowly due to the eastern isolation barrier and lake.When fire line reached the southern barrier,because of barrier and firefighting,the wildfire event ended at 0800 UTC 26.In the case that there was no barrier,sensitivity test showed that the fire would across the border and spread southeast and northeast following wind direction.In this situation,the main fuel type changed from grassland to needle forest,and the spreading fire would lead to a great loss of forest.