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Characterizing Scattering Properties of Concave Particles

Characterizing Scattering Properties of Concave Particles. Kwo-Sen Kuo 1,2 , Eric A. Smith 2 , and Qingyuan Han 3 Goddard Earth Science and Technology Center (GEST), Caelum Research Corporation, Rockville, MD 20850

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Characterizing Scattering Properties of Concave Particles

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  1. Characterizing Scattering Properties of Concave Particles Kwo-Sen Kuo1,2, Eric A. Smith2 , and Qingyuan Han3 Goddard Earth Science and Technology Center (GEST), Caelum Research Corporation, Rockville, MD 20850 NASA/Goddard Space Flight Center, Laboratory for Atmospheres (Code 613.1), Greenbelt, MD 20771 Department of Atmospheric Science, Univ. of Alabama in Huntsville, Huntsville, AL 35899 NCU Seminar

  2. Outline Emphasis on Solid Hydrometeors • Complexity of ice crystal morphology • Particulate retrieval fundamentals • Hypothesis • Snowfake – numerical crystal growth model • Scattering – DDSCAT; orientation averaged • Conclusions • Applications Outline NCU Seminar

  3. Convex versus Concave • Convex: Connect a line segment between any two points inside a body. If no part of the line segment lies outside the body then the body is convex. Otherwise, it’s concave • Vouk (1948): for convex bodies, S = 4Ā, S: surface area Ā: mean projected area. • For concave ones, S > 4Ā. • Convex • Concave Complexity NCU Seminar

  4. Complexity of Snow Crystals Types of snow flakes • Simple prisms • Stellar plates • Sectored plates • Stellar dendrites • Fernlike stellar dendrites • Hollow columns • Needles • Capped columns • Double plates • (SnowCrystals.com) • Split plates and stars • Triangular crystals • 12-sided snow flakes • Bullet rossets • Radiating dendrites • Rimed crystals Complexity NCU Seminar

  5. Natural Ice Crystals Heymsfield et al. 2002, J. Atmos. Sci. • CPI images during Lagrange spiral descent on 9 March 2000 over Oklahoma ARM site. Complexity NCU Seminar

  6. Natural Snow Crystals Libbrecht 2005, Rep. Prog. Phys. • 1.4 mm • 2.1 mm • 3 mm • 0.45 mm long • 1.1 mm long (max.) • 0.6 mm long Complexity NCU Seminar

  7. Rimed Snow Crystal Physics Today, December 2007 • Supercooled water droplets accreted on caps • Length between two plate caps: ~500 μm • Low-temperature scanning electron micrograph. Complexity NCU Seminar

  8. Physics of Snow Crystals Snow Crystal Morphology Diagram Factors • Large scale: particle and heat transport • Microscopic dynamics: surface diffusion and chemistry [Libbrecht 2005] Complexity NCU Seminar

  9. Retrieval Fundamentals • Single-scattering properties of particle ensembles is at the heart of the atmospheric particulate constituent retrieval problems. • Scattering by volumes with particle size distributions is dealt with in a probabilistic manner – convolution with size distribution. • Independent observations available are seldom adequate to resolve size distributions – need to find the few parameters that actually matter. • All particulate retrieval problems use “matching” – matching with forward simulations with varying degrees of sophistication. Retrieval NCU Seminar

  10. “Matching” Example • Nakajima and King, 1990, JAS. NCU Seminar

  11. Single Scattering Calculation Methods • Lorenz-Mie (Mie) • sphere, spheroid, infinite cylinders • T-Matrix (EBCM) • a.k.a. Extended Boundary Condition Method • regular geometry with some symmetry • Finite-Difference Time Domain (FDTD) • general geometry • small size parameter • Geometric Optics (Ray Tracing) • general geometry • large size parameter • Discrete Dipole Approximation (DDA) - DDSCAT • general geometry • small size parameter (due to computational instability) Retrieval NCU Seminar

  12. Current Status of the Problem We understand sufficiently (though not completely) • Physics of snow • Scattering theory and method We are confounded by • Complexity • Effective parameterization with systematic investigation • Means to verify • Direct measurements commensurate with remote sensing sampling volume Retrieval NCU Seminar

  13. Retrieval of Water Cloud Microphysics Hansen and Travis (1974) show that single scattering properties, including the scattering phase functions, of ensembles of cloud (spherical) droplets can be accurately parameterized by 3 parameters: • liquid water content (proportional to number concentration) • effective radius • effective variance The specific form of the size distribution becomes immaterial. Retrieval NCU Seminar

  14. Characterizing Scattering Properties of Spherical Droplets • Hansen and Travis, 1974, Space Sci. Rev. NCU Seminar

  15. Re-examination of Spheres Retrieval NCU Seminar

  16. Hypothesis Instead of insisting on one, single illusive effective size — cross-section effective radius (rĀ) and surface effective radius (rS) together provide a more effective means to parameterize single scattering properties. Issues: How to • obtain realistic particles? • generate PSDs with same rĀ and rS? rs Hypothesis NCU Seminar

  17. Snowfake – a Numerical Crystal Growth Model Gravner and Griffeath 2008 (GG08) Snowfake NCU Seminar

  18. Snowfakes Examples from GG08 D N E F Snowfake NCU Seminar

  19. Snowfakes Generated using GG08 algorithm Dmax=100 Dmax=300 Dmax=150 Snowfake NCU Seminar

  20. Definitions of rĀ and rS For spheres, in fact for all convex bodies, rĀ/rS = 1! Snowfake NCU Seminar

  21. Snowfake Geometric Properties Snowfake NCU Seminar

  22. Snowfake ConcavitiyT = rĀ/rS Snowfake NCU Seminar

  23. Single-Scattering of Snowfakes 20-μm wavelength (n = 1.5015 + i 0.0670)with 1 μm inter-dipole distance N D F E Scattering NCU Seminar

  24. PSD with Three Habits (NDF)Volume Size Distribution : [ 0.95, 0.04, 0.01 ] Scattering NCU Seminar

  25. PSD with Three Habits (NFE)Volume Size Distribution : [ 0.80, 0.15, 0.05 ] Scattering NCU Seminar

  26. PSD with Four Habits (NDFE)Volume Size Distribution: [ 0.95, 0.04, 0.01 ] Scattering NCU Seminar

  27. Bulk Scattering PropertiesFor NDE with VSD = [0.95, 0.04, 0.01] 0–Single scattering albedo g–Asymmetry factor ext–Extinction coefficient for IWC = 4.6 mg m-3 Scattering NCU Seminar

  28. Bulk Scattering PropertiesImpact of rS Solid – NDE Dashed – NDF βext g 0 Scattering NCU Seminar

  29. Bulk Scattering PropertiesFor NDE with VSD = [0.95, 0.04, 0.01] 0–Single scattering albedo g–Asymmetry factor ext–Extinction coefficient for IWC = 4.6 mg m-3 Scattering NCU Seminar

  30. Bulk Scattering PropertiesImpact of rĀ Solid – NDFE Dashed – NDF Dotted – NDE βext g 0 Scattering NCU Seminar

  31. Scattering Phase FunctionFor NDFE with VSD = [0.95, 0.04, 0.01] Scattering NCU Seminar

  32. Scattering Phase FunctionFor NDF with VSD = [0.67, 0.22, 0.11] NCU Seminar

  33. Scattering Phase FunctionVSD = [0.95, 0.04, 0.01] NDFE NDE NDF Scattering NCU Seminar

  34. Conclusions • Effective surface radius, rS, is important for concave particles which are more prevalent. • IWC, rĀ, and rSare effective parameters for characterizing scattering properties of irregular particles. • When habit composition is known, IWC, rĀ, and rS characterize single-scattering properties with excellent accuracy. • When habit composition is not known exactly, it is better to include all possible habits. Problems NCU Seminar

  35. Applications • Retrievals involving irregular particles—with improved uncertainty estimates—including • Snow • Cirrus ice • Aerosol • Radiation parameterization involving irregular particles Applications NCU Seminar

  36. Thank you! NCU Seminar

  37. Wings3D • Creating a realistic crystal is easy! NCU Seminar

  38. Welding • Simple shapes can be “welded” together in Wings3D to create realistic, complicated aggregates. NCU Seminar

  39. Index of Refraction for H2O NCU Seminar

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