Investigating the impact of overlying vegetation canopy structures on fire radiative power (FRP) retrieval through simulation and measurement
Investigating the impact of overlying vegetation canopy structures on fire radiative power (FRP) retrieval through simulation and measurement
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通过模拟和测量研究上覆植被冠层结构对火灾辐射功率 (FRP) 反演的影响
DOI：
10.1016/j.rse.2018.08.015
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发表时间：
2018
影响因子：
13.5
通讯作者：
Roberts G
中科院分区：
文献类型：
--
作者：
Roberts G
现在通常通过极地和对地静止仪器进行火辐射功率（FRP）反演，这提供了一种直接根据遥感观测来估计燃料消耗并追踪气体和气溶胶排放的方法。本研究基于 3D 辐射传输模型模拟，首次研究了植被冠层结构（冠层覆盖百分比和叶面积指数，LAI）对 FRP 反演的影响。离散各向异性辐射传输 (DART) 模型用于模拟通过具有不同结构排列的 3D 植被冠层进行的冠层上方观测，在冠层下，中心位置均匀的景观火焰正在燃烧。植被冠层以两种方式建模，作为不透明结构和混合浑浊介质。每个模拟场景中的树冠覆盖百分比在 5% 到 95% 之间变化，并且发现当树冠不透明时，在树冠上方检索到的 FRP 与树冠覆盖百分比成比例减少，这一发现与为评估模拟的真实性而进行的一系列小规模户外测量一致。然而，当冠层被建模为浑浊介质时，这在某种程度上是真实“有间隙”植被冠层的更真实的表示，在任何特定冠层覆盖处发生的 FRP 拦截程度会降低约 14%，因为一些火发出的热能通过冠层间隙传递。模拟还揭示了冠层 LAI 对冠层上方 FRP 检索的影响，当冠层覆盖率和 LAI 都很低（分别为 5% 和 <1.0）时，这些影响降低了 6%，但当冠层覆盖率和场景 LAI 很高（分别为 95% 和 ~8）时，影响高达 92%。我们利用 FRP 拦截和树冠结构之间的导出关系，以及 MODIS LAI 和树木覆盖率数据，调整根据对地静止气象卫星旋转增强可见光和红外成像仪 (SEVIRI) 仪器收集的 FRP 数据计算出的 2004-2012 年火灾辐射能 (FRE) 估计值。调整后的年度 FRE 平均比估计值高 15%，其特点是年际变化较小，因为大多数火灾活动发生在树木覆盖率仍低于 40% 的地区。景观燃烧发生在南半球树木覆盖率较高的地区，而不是北半球非洲，导致年度 FRE 调整幅度更大（18.5% 比 16.3%）。这项研究首次在卫星尺度上说明了冠层拦截对 FRP 的影响，并且在非洲上空表现出较大但时间上一致的低估，在估计 FRP 检索的燃料消耗和大气排放时，可以使用 LAI 和树木覆盖百分比指标来解释这一情况。
Fire radiative power (FRP) retrievals are now routinely made from polar and geostationary instruments, providing a means to estimate fuel consumption and trace gas and aerosol emissions directly from remotely sensed observations. This study presents the first investigation of the impact of vegetation canopy structure (percentage canopy cover and leaf area index, LAI) on FRP retrievals, based on 3D radiative transfer model simulations. The Discrete Anisotropic Radiative Transfer (DART) model is used to simulate above-canopy observations made through 3D vegetation canopies with different structural arrangements, under which a centrally positioned uniform landscape fire is burning. The vegetation canopy is modelled in two ways, as an opaque structure and as a hybrid turbid medium. The percentage canopy cover in each simulated scene is varied between 5 and 95%, and the FRP retrieved above the canopy is found to decrease in proportion to percentage canopy cover when the canopy is opaque, a finding that is in agreement with a series of small scale outdoor measurements conducted to evaluate the realism of the simulations. However, when the canopy is modelled as a turbid medium, which is in some ways a more realistic representation of a real ‘gappy’ vegetation canopy, the degree of FRP interception occurring at any particular canopy cover decreases by ~ 14%, due to some fire emitted thermal energy being transmitted through the canopy gaps. The simulations also reveal the impact of canopy LAI on above-canopy FRP retrievals, reducing these by 6% when both canopy cover and LAI are low (5% and < 1.0 respectively), but by up to 92% when canopy cover and scene LAI are high (95% and ~8 respectively). We use the derived relationships between FRP interception and canopy structure, along with MODIS LAI and percentage tree cover data, to adjust 2004–2012 fire radiative energy (FRE) estimates calculated from FRP data collected by the geostationary Meteosat Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument. The adjusted annual FRE is on average 15% greater than estimated, and is characterized by low inter-annual variability as result of the majority of fire activity occurring in areas where percentage tree cover remains below 40%. Landscape burning occurs more frequently in areas of higher tree cover in southern hemisphere rather than northern hemisphere Africa, leading to a larger annual FRE adjustment (18.5% compared to 16.3%). This study illustrates the impact that canopy interception has on FRP for the first time at the satellite scale, and over Africa demonstrates a large but temporally consistent underestimation which can be accounted for using LAI and percentage tree cover metrics when estimating fuel consumption and atmospheric emissions from the FRP retrievals.
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