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#1 Review

An improved dust emission model - Part 1: Model description and comparison against measurements ., Kok et al. 2014 (ACP)

Abstract. Simulations of the dust cycle and its interactions with the changing Earth system are hindered by the empirical nature of dust emission parameterizations in weather and climate models.

Here we take a step towards improving dust cycle simulations by using a combination of theory and numerical simulations to derive a physically based dust emission parameterization. Our parameterization is straightforward to implement into large-scale models, as it depends only on the wind friction velocity and the soil’s threshold friction velocity. Moreoever, it accounts for two processes missing from existing parameterizations: a soil’s increased ability to produce dust under saltation bombardment as it becomes more erodible, and the increased scaling of the dust flux with wind speed as a soil becomes less erodible. Our treatment of both these processes is supported by a compilation of quality-controlled vertical dust flux measurements. Furthermore, our scheme reproduces this measurement compilation with substantially less error than the existing dust flux parameterizations we were able to compare against. A critical insight from both our theory and the measurement compilation is that dust fluxes are substantially more sensitive to the soil’s threshold friction velocity than most current schemes account for.

  • Study problem. hinderance by the empirical nature of dust emission parameterizations in the climate models. Two processes: 1) a soil’s increased ability to produce dust under saltation bombardment as it becomes more erodible, and 2) the increased scaling of the dust flux with wind speed as a soil becomes less erodible.
  • Study solutions/results. Our parameterization is straightforward to implement into large-scale models, as it depends only on the wind friction velocity and the soil’s threshold friction velocity. Confirmed that dust fluxes are substantially more sensitive to the soil’s threshold friction velocity than most current schemes account for.
  • BASIC What-s known/unknown. Because of their small size, dust particles in soils ( <62.5 μm diameter; Shao, 2008) experience cohesive forces that are large compared to aerodynamic and gravitational forces. Consequently, dust aerosols are usually not lifted directly by wind (Gillette et al., 1974; Shao et al., 1993; Sow et al., 2009) and instead are emitted through saltation, in which larger sand-sized particles (∼70–500 μm) move in ballistic trajectories (Bagnold, 1941; Shao, 2008; Kok et al., 2012). Upon impact, these saltating particles can eject dust particles from the soil, a process known as sandblasting. Moreover, some saltating particles are actually aggregates containing dust particles. Upon impact, these aggregates can also emit dust aerosols (Shao et al., 1996). Examples of dust emission processes other than aggregate fragmentation that are controlled by a normally distributed threshold could include dust emission from crusted soils (Rice et al., 1996) and from sand particles with clay coatings (Bullard et al., 2004). Since we do not know what the relative contribution of different dust emission processes is to each of the dust flux data sets used to calibrate the dimensionless coefficients in Eq. (18), it is likely that the obtained values of these coefficients represents some weighted average of the relative contribution of each dust emission process.
  • Agreement&Conflicts.
  • Statistics (location, data).
  • Methods. used a combination of theory and numerical simulations
  • Specifics.
  • What’s left/limitation. We derived the dust emission parameterization of Eq. (18) for dust emission occurring primarily through the fragmen tation of either soil dust aggregates or saltating aggregates by the energetic impacts of saltators. Furthermore, as mentioned in Sect. 2.2.1, our theory applies only to soils for which the saltation flux is limited by the availability of wind momentum, and are thus transport limited (e.g., Nickling and McKenna Neuman, 2009). The present theory is thus not valid for soils for which the horizontal saltation flux at a given point in time is limited by the availability of sand-sized sediment. Such supply-limited soils are inherently inefficient sources of dust aerosols (Rice et al., 1996), and are thus probably less important in the global dust budget.
  • Note&Take back
    • Does the research complete?
    • Could it have been conducted more soundly?
    • What further questions does it raise?
July 17, 2020

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