Shape‐induced gravitational sorting of Saharan dust during transatlantic voyage: Evidence from CALIOP lidar depolarization measurements
Shape‐induced gravitational sorting of Saharan dust during transatlantic voyage: Evidence from CALIOP lidar depolarization measurements
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跨大西洋航行期间撒哈拉沙尘的形状诱导重力分选：来自 CALIOP 激光雷达去极化测量的证据
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发表时间：
2013
期刊：
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通讯作者：
T. Várnai
中科院分区：
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作者：
Weidong Yang;A. Marshak;A. Kostinski;T. Várnai
受形状相关空气阻力的物理图景以及由此引起的尘埃粒子的差异沉积的启发，我们寻找并发现了尘埃粒子的非球面性影响尘埃散射光退偏比(δ)演化和分布的证据。具体地说，我们研究了2007年6月至8月使用正交偏振(CALIOP)观测撒哈拉沙尘的云-气溶胶激光雷达的大型数据集。沿典型的跨大西洋沙尘路径观察发现：(1)当扬尘沙尘离开非洲西海岸时，中位数δ在2~5 公里高度上均匀分布，表明粒子形状与高度的随机混合；(2)中值δ的垂直均匀性在向西输送过程中破坏：在2~5 公里范围内，δ随海拔升高而增大，且随向西推进而增大；(3)在向西输送过程中，δ在较高海拔(>；4 km)处增大，在较低海拔(<；4 km)处减小。所有这些特征都被一个最小模式(只有两个形状)定性地捕捉到，这表明形状诱导的差异沉降和随后的分类确实对观测到的尘埃属性的时间演化和垂直分层有重要贡献。由于粒子形状可能是重力分选的原因，这些结果将影响对撒哈拉尘埃层辐射传输的估计。
Motivated by the physical picture of shape‐dependent air resistance and, consequently, shape‐induced differential sedimentation of dust particles, we searched for and found evidence of dust particle asphericity affecting the evolution and distribution of dust‐scattered light depolarization ratio (δ). Specifically, we examined a large data set of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations of Saharan dust from June to August 2007. Observing along a typical transatlantic dust track, we find that (1) median δ is uniformly distributed between 2 and 5 km altitudes as the elevated dust leaves the west coast of Africa, thereby indicating uniformly random mixing of particle shapes with height; (2) vertical homogeneity of median δ breaks down during the westward transport: between 2 and 5 km δ increases with altitude and this increase becomes more pronounced with westward progress; (3) δ tends to increase at higher altitude (>4 km) and decrease at lower altitude (<4 km) during the westward transport. All these features are captured qualitatively by a minimal model (two shapes only), suggesting that shape‐induced differential settling and consequent sorting indeed contribute significantly to the observed temporal evolution and vertical stratification of dust properties. By implicating particle shape as a likely cause of gravitational sorting, these results will affect the estimates of radiative transfer through Saharan dust layers.