Abstract
Abstract. This study examines the uncertainties in simulating mass balance and radiative forcing of mineral dust due to biases in the dust size parameterization. Simulations are conducted quasi-globally (180° W–180° E and 60° S–70° N) using the WRF-Chem model with three different approaches to represent dust size distribution (8-bin, 4-bin, and 3-mode). The biases in the 3-mode or 4-bin approaches against a relatively more accurate 8-bin approach in simulating dust mass balance and radiative forcing are identified. Compared to the 8-bin approach, the 4-bin approach simulates similar but coarser size distributions of dust particles in the atmosphere, while the 3-mode approach retains more fine dust particles but fewer coarse dust particles due to its prescribed σg of each mode. Although the 3-mode approach yields up to 10 days of longer dust mass lifetime over the remote oceanic regions than the 8-bin approach, the three size approaches produce a similar dust mass lifetime (3.2 days to 3.5 days) on quasi-global average, reflecting that the global dust mass lifetime is mainly determined by the dust mass lifetime near the dust source regions. With the same global dust emission (~4600 Tg yr−1), the 8-bin approach produces a dust mass loading of 39 Tg, while the 4-bin and 3-mode approaches produce 3% (40.2 Tg) and 25% (49.1 Tg) higher dust mass loading, respectively. The difference in dust mass loading between the 8-bin approach and the 4-bin or 3-mode approaches has large spatial variations, with generally smaller relative difference (<10%) near the surface over the dust source regions. The three size approaches also result in significantly different dry and wet deposition fluxes and number concentrations of dust. The difference in dust aerosol optical depth (AOD) (a factor of 3) among the three size approaches is much larger than their difference (25%) in dust mass loading. Compared to the 8-bin approach, the 4-bin approach yields stronger dust absorptivity, while the 3-mode approach yields weaker dust absorptivity. Overall, on quasi-global average, the three size parameterizations result in a significant difference of a factor of 2~3 in dust surface cooling (−1.02~−2.87 W m−2) and atmospheric warming (0.39~0.96 W m−2) and in a tremendous difference of a factor of ~10 in dust TOA (top of atmosphere) cooling (−0.24~−2.20 W m−2). The impact of different size representations on dust radiative forcing efficiency is smaller. An uncertainty of a factor of 2 is quantified in dust emission estimation due to the different size parameterizations. This study also highlights the uncertainties in modeling dust mass and number loading, deposition fluxes, and radiative forcing resulting from different size parameterizations, and motivates further investigation of the impact of size parameterizations on modeling dust impacts on air quality, climate, and ecosystems.
Highlights
Desert dust has been suggested as the main contributor to the global aerosol burden (Forster et al, 2007)
In order to examine the uncertainties in modeling dust mass balance and radiative forcing resulting from size representation, Weather Research and Forecasting (WRF)-Chem simulations are conducted for four cases: BIN8, BIN4, MOD3, and MOD3_tuned
This value is about 50 % higher than 3157 Tg yr−1 estimated by the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model for year 2000 (Huneeus et al, 2011) and ∼3000 Tg yr−1 estimated by Mian Chin et al (2009) and and Kim et al (2013, this issue) for 2000–2007 using the GOCART model
Summary
Desert dust has been suggested as the main contributor to the global aerosol burden (Forster et al, 2007). Mann et al (2012) compared the modal and sectional approach in modeling aerosol microphysics using a 3-D global offline chemistry transport model They found differences in annual mean surface-level masses of sulfate, sea salt, black carbon (BC), and organic carbon (OC) within 25 % in most regions. These studies have demonstrated some differences in modeling aerosols between modal and sectional approaches, none of them focused on dust, and the impact of different size representations on simulating dust and its radiative forcing has not yet been investigated.
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