Shape-induced gravitational sorting of Saharan dust during transatlantic voyage: Evidence from CALIOP lidar depolarization measurements

Authors

  • Weidong Yang,

    Corresponding author
    1. Goddard Earth Sciences Technology and Research, Universities Space Research Association, Columbia, Maryland, USA
    2. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    • Corresponding author: W. Yang, Goddard Earth Sciences Technology and Research, Universities Space Research Association, Columbia, MD 21044, USA. (Weidong.Yang@nasa.gov)

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  • Alexander Marshak,

    1. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
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  • Alexander B. Kostinski,

    1. Department of Physics, Michigan Technological University, Houghton, Michigan, USA
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  • Tamás Várnai

    1. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    2. Joint Center for Earth System Technology, University of Maryland at Baltimore County, Baltimore, Maryland, USA
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Abstract

[1] 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.

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