Do we really know by how much ellipsoidal shapes enhance the extinction of solar radiation by dust?

Description

Abstract

The climate impact of dust is still uncertain, partially due to poorly constrained dust physical and optical properties. Natural dust particles are known to have highly irregular shapes, but many models assume spheres when calculating the direct radiative effect (DRE). While the superior performance of non-spherical shapes in remote sensing applications has been widely recognized, there has been no consensus about the importance of dust non-sphericity in climate models. We assess the extent of the shape effect upon the dust optical properties and DRE at shortwave wavelengths within the NASA Goddard Institute for Space Studies ModelE2.1. We assume tri-axial ellipsoids as an approximation to natural dust shapes that is suitable for model applications, and combine a widely used database of ellipsoidal single-scattering properties with a recent shape distribution constructed from a comprehensive compilation of measurements. We find a shape-induced enhancement of global dust extinction of +20 % (±1 %), resulting in a global cooling increase of +24 % (±3 %) at the top of atmosphere and +12 % (±2 %) at the surface. The ellipsoidal shapes increase the total dust extinction per unit mass, improving our model representation of the dust optical depth compared to a semi-observational constraint and ground-based measurements. However, our analysis shows that the database of ellipsoidal scattering properties covers only one-third of the observed distribution of shapes. This represents a major uncertainty in evaluating the dust shape effect in model applications.