Climate Models and Remote Sensing Retrievals Neglect Substantial Desert Dust Asphericity

Abstract

Climate models and remote sensing retrievals generally assume that dust aerosols are spherical or spheroidal. However, measurements show that dust aerosols deviate substantially from spherical and spheroidal shapes, as ratios of particle length to width (the aspect ratio) and height to width (height‐to‐width ratio) deviate substantially from unity. Here, we quantify dust asphericity by compiling dozens of measurements of aspect ratio and height‐to‐width ratio across the globe. We find that the length is on average 5 times larger than the height and that climate models and remote sensing retrievals underestimate this asphericity by a factor of ~3–5. Compiled measurements further suggest that North African dust becomes more aspherical during transport, whereas Asian dust might become less aspherical. We obtain globally‐averaged shape distributions, from which we find that accounting for dust asphericity increases gravitational settling lifetime by ~20%. This increased lifetime helps explain the underestimation of coarse dust transport by models.

Apparent dust size discrepancy in aerosol reanalysis in north African dust after long-range transport

“…Africa contributes almost 50 % of the world's dust emissions (Huneeus et al, 2011), with easterly winds transporting the dust over the Atlantic subtropical belt throughout the full seasonal cycle. The impacts of dust span from climate-relevant timescales, which primarily focus on dust's radiative effects, to the shorter timescales important for human endeavors.…”

Capability of Superspheroids for Modeling PARASOL Observations Under Dusty‐Sky Conditions

“…In particular, mineral dust aerosols with aspect ratios close to unity (Veghte & Freedman, 2014) are unnecessarily to be excluded. The aspect ratios larger than 2.0 or smaller than 0.5 that are most useful for spheroidal model are not essential when the superspheroidal model is employed, although realistic dust aerosol aspect ratio could be out of this range (0.5–2.0; Y. Huang, Kok et al., 2020). To conclude, the superspheroidal model with large roundness parameter appears to be a superior dust aerosol model for polarized radiative transfer simulations.…”

Less atmospheric radiative heating due to aspherical dust with coarser size

“…The Soil Moisture Active Passive (SMAP) Level-4 Soil Moisture data product addressed these limitations by merging the satellite observations into a numerical model of the land surface water and energy balance while considering the uncertainty of the observations and model estimates (Reichle et al, 2019). In this work, we utilized the 3-hourly data of soil moisture derived from the SMAP for barren and open shrublands, separately (Reichle et al, 2018 Compared to the assumption on spherical shapes of aerosols, the dust asphericity increased aerodynamic drag at a given volume and mass, and thus increased gravitational settling lifetime by about 20% (Huang et al, 2020a). Here, we implemented globally averaged asphericity factor of 0.87 (Huang et al, 2020a) to the gravitational settling scheme for mineral dust.…”

“…In this work, we utilized the 3-hourly data of soil moisture derived from the SMAP for barren and open shrublands, separately (Reichle et al, 2018 Compared to the assumption on spherical shapes of aerosols, the dust asphericity increased aerodynamic drag at a given volume and mass, and thus increased gravitational settling lifetime by about 20% (Huang et al, 2020a). Here, we implemented globally averaged asphericity factor of 0.87 (Huang et al, 2020a) to the gravitational settling scheme for mineral dust. Consequently, the lifetime of the dust aerosol for the largest-size bin in the IMPACT model even after accounting for asphericity (1.4 days for 5-20 µm of diameter) was significantly shorter than an ensemble of model results (2.1 ± 0.3 days for the mass mean diameter of 8.3 µm) (Kok et al, 2017).…”

“…(RRTMG) (Iacono et al, 2008) (i.e., default simulation). We improve the accuracy of these simulations by correcting the bias in size-resolved dust concentration with the Dust Constraints from joint Observational-Modelling-Experimental analysis (DustCOMM) data set ) (i.e., improved simulation), as well as by considering the aspherical shape (Huang et al, 2020a(Huang et al, , 2020b. We then explore the sensitivity to dust refractive index.…”