Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling
Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling
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DOI：
10.1007/s00382-015-2522-4
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发表时间：
2014-12
期刊：
影响因子：
4.6
通讯作者：
F. de Sales;Y. Xue;G. Okin
中科院分区：
文献类型：
--
作者：
F. de Sales;Y. Xue;G. Okin
这项研究调查的影响，烧毁的地区的表面能量平衡和每月降水量在北方非洲国家的最先进的区域模式模拟。在一组为期1年的WRF-NMM/SSiB 2模式模拟中，利用MODIS数据得到的燃烧产物的平均燃烧面积分数。植被覆盖率和叶面积指数每天退化的基础上平均烧毁面积分数和每种植被土地覆盖类型的存活率。此外，与野火引起的灰烬和木炭沉积相关联的地面变暗是通过燃烧后的一段时间内较低的地面反射来实现的。在一般情况下，野火引起的植被和地面条件恶化增加了平均表面辐射，暴露了更明亮的裸露地面，这反过来又导致每月的表面净辐射减少。平均而言，萨赫勒地区野火季节的林火增加约为6.3%。在冬季和早春的干燥月份，地表可用能量的相关减少导致了地表感热通量向大气的下降，这逐渐过渡到4月和5月地表蒸散量的大幅减少，整个雨季都在减少。总体而言，火灾后土地状况恶化导致撒哈拉以南非洲降水减少，这与西非季风在该地区的传播减弱有关。在燃烧区模拟中观察到大气水汽通量辐合的减少，这在减少该地区的降水方面发挥了主导作用，特别是在季风爆发前的几个月。受降水影响最大的地区是热带稀树草原和热带雨林覆盖的地区，年降水量分别减少了3.8%和3.3%。由此造成的降水量减少和植被退化导致该区域的总初级生产力下降，这种下降在冬末和早春最为严重。研究表明，火烧区引起的大气冷却和干燥，使季风爆发前的沉降加强，季风爆发和成熟期的大气上升运动减弱，导致对流不稳定和降水减弱。冬季和春季对流层中层垂直风的下降趋势增强，雨季上升趋势减弱。此外，降水能量分析表明，降水减少的主要原因是对流事件，这支持了野火减少对流不稳定性的假设。
This study investigates the impact of burned areas on the surface energy balance and monthly precipitation in northern Africa as simulated by a state-of-the-art regional model. Mean burned area fraction derived from MODIS date of burning product was implemented in a set of 1-year long WRF-NMM/SSiB2 model simulations. Vegetation cover fraction and LAI were degraded daily based on mean burned area fraction and on the survival rate for each vegetation land cover type. Additionally, ground darkening associated with wildfire-induced ash and charcoal deposition was imposed through lower ground albedo for a period after burning. In general, wildfire-induced vegetation and ground condition deterioration increased mean surface albedo by exposing the brighter bare ground, which in turn caused a decrease in monthly surface net radiation. On average, the wildfire-season albedo increase was approximately 6.3 % over the Sahel. The associated decrease in surface available energy caused a drop in surface sensible heat flux to the atmosphere during the dry months of winter and early spring, which gradually transitioned to a more substantial decrease in surface evapotranspiration in April and May that lessened throughout the rainy season. Overall, post-fire land condition deterioration resulted in a decrease in precipitation over sub-Saharan Africa, associated with the weakening of the West African monsoon progression through the region. A decrease in atmospheric moisture flux convergence was observed in the burned area simulations, which played a dominant role in reducing precipitation in the area, especially in the months preceding the monsoon onset. The areas with the largest precipitation impact were those covered by savannas and rainforests, where annual precipitation decreased by 3.8 and 3.3 %, respectively. The resulting precipitation decrease and vegetation deterioration caused a drop in gross primary productivity in the region, which was strongest in late winter and early spring. This study suggests the cooling and drying of atmosphere induced by burned areas caused the strengthening of subsidence during pre-onset and weakening of upward atmospheric motion during onset and mature stages of the monsoon leading to a waning of convective instability and precipitation. Monthly mid-tropospheric vertical wind showed a strengthening of downward motion in winter and spring seasons, and weakening of upward movement during the rainy months. Furthermore, precipitation energy analysis revealed that most of precipitation decrease originated from convective events, which supports the hypothesis of reduced convective instability due to wildfires.