Modeling dust as component minerals in the Community Atmosphere Model: development of framework and impact on radiative forcing

R A Scanza,S Ghan,C S Zender,S Albani,J F Kok,N Mahowald,X Liu,Y Zhang

doi:10.5194/acp-15-537-2015

Abstract

Abstract. The mineralogy of desert dust is important due to its effect on radiation, clouds and biogeochemical cycling of trace nutrients. This study presents the simulation of dust radiative forcing as a function of both mineral composition and size at the global scale, using mineral soil maps for estimating emissions. Externally mixed mineral aerosols in the bulk aerosol module in the Community Atmosphere Model version 4 (CAM4) and internally mixed mineral aerosols in the modal aerosol module in the Community Atmosphere Model version 5.1 (CAM5) embedded in the Community Earth System Model version 1.0.5 (CESM) are speciated into common mineral components in place of total dust. The simulations with mineralogy are compared to available observations of mineral atmospheric distribution and deposition along with observations of clear-sky radiative forcing efficiency. Based on these simulations, we estimate the all-sky direct radiative forcing at the top of the atmosphere as + 0.05 Wm−2 for both CAM4 and CAM5 simulations with mineralogy. We compare this to the radiative forcing from simulations of dust in release versions of CAM4 and CAM5 (+0.08 and +0.17 Wm−2) and of dust with optimized optical properties, wet scavenging and particle size distribution in CAM4 and CAM5, −0.05 and −0.17 Wm−2, respectively. The ability to correctly include the mineralogy of dust in climate models is hindered by its spatial and temporal variability as well as insufficient global in situ observations, incomplete and uncertain source mineralogies and the uncertainties associated with data retrieved from remote sensing methods.

Highlights

In order to discuss the significance of the spatial distribution of mineralogy and to give credibility to the simulations, the modeled distributions are evaluated with available observational data (Table 5)
For four of the seven minerals considered from Kandler et al (2009) – illite (Fig. 4a), kaolinite (Fig. 4b), quartz (Fig. 4c) and feldspar (Fig. 4f) – the simulations for both Community Atmosphere Model version 4 (CAM4) and Community Atmosphere Model version 5.1 (CAM5) simulate dynamic range in mineral mass fraction with particle size, while the mass fractions observed are relatively constant with size
The mineral distributions simulated in CAM4 and CAM5 lack the range of variability that the few available observations indicate, this is improved when daily averaged values are compared instead of monthly means

Summary

Introduction

Dust aerosols are soil particles suspended in the atmosphere, and they impact the climate system by influencing the radiation budget, cloud processes (Miller and Tegen, 1998; Mahowald and Kiehl, 2003; Karydis et al, 2011; DeMott et al., 2003; Levin et al, 2005) and various biogeochemical cycles (Swap et al, 1992; Martin et al, 1991; Jickells et al, 2005). The radiation balance of the Earth system is affected by the scattering and absorption of solar and infrared radiation by mineral aerosols (Miller and Tegen, 1998; Sokolik and Toon, 1999). Previous and ongoing modeling efforts address the importance of determining the mineral composition of dust and its impact on the radiation budget (Sokolik and Toon, 1999; Claquin et al, 1999; Balkanski et al, 2007).

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Conclusion