Effects of nonsphericity on mineral dust modeling

[1] The dependency of nonsphericity on gravitational settling of mineral dust particles is parameterized for prolate ellipsoids and Reynolds number lower than 2. The settling speed is numerically solved from the momentum equation as a function of particle diameter and aspect ratio. The reduction of settling speed due to nonsphericity is included in the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model to simulate dust size distribution for April 2001. Two numerical schemes for solving sedimentation are compared. For particles of diameter greater than 5 mum, the simulated size distribution is sensitive to the numerical sedimentation scheme. Changing the particle shape from spherical to nonspherical with lambda = 2 makes little difference to the simulated surface concentration and size distribution except at the periphery of the dust sources. However, when very elongated particles ( lambda = 5) are simulated, the differences between nonspherical and spherical particles are significant. With limited in situ measurements reporting most frequent lambda around 1.5, the overall effects on global modeling is rather negligible and the essential benefit is to relax the CFL condition of Eulerian settling schemes.