Mineral dust aerosols impact Earth's radiation budget through interactions with clouds, ecosystems, and radiation, which constitutes a substantial uncertainty in understanding past and predicting ...future climate changes. One of the causes of this large uncertainty is that the size distribution of emitted dust aerosols is poorly understood. The present study shows that regional and global circulation models (GCMs) overestimate the emitted fraction of clay aerosols (< 2 μm diameter) by a factor of ~2-8 relative to measurements. This discrepancy is resolved by deriving a simple theoretical expression of the emitted dust size distribution that is in excellent agreement with measurements. This expression is based on the physics of the scale-invariant fragmentation of brittle materials, which is shown to be applicable to dust emission. Because clay aerosols produce a strong radiative cooling, the overestimation of the clay fraction causes GCMs to also overestimate the radiative cooling of a given quantity of emitted dust. On local and regional scales, this affects the magnitude and possibly the sign of the dust radiative forcing, with implications for numerical weather forecasting and regional climate predictions in dusty regions. On a global scale, the dust cycle in most GCMs is tuned to match radiative measurements, such that the overestimation of the radiative cooling of a given quantity of emitted dust has likely caused GCMs to underestimate the global dust emission rate. This implies that the deposition flux of dust and its fertilizing effects on ecosystems may be substantially larger than thought.
The wind‐blown flux of sand generates dunes, wind erosion, and mineral dust aerosols. Existing models predict sand flux using the wind friction velocity that characterizes near‐surface turbulent ...momentum fluxes. However, these models struggle to accurately predict sand fluxes. Here we analyze root causes of these model discrepancies using high‐frequency field measurements of winds and sand fluxes. We find that friction velocity is only predictive of sand fluxes on long timescales, when it correlates with horizontal wind speed. On shorter timescales, and for non‐ideal surface conditions, friction velocity is much less predictive, likely because the near‐surface wind momentum budget is dominated by other, less predictable terms. We furthermore find that variability in 30‐min averaged sand fluxes at a given friction velocity is not driven by changes in turbulence but by changes in surface conditions, raising a challenge for models. These findings can improve sand flux models and clarify their limitations.
Plain Language Summary
The wind‐blown transport of sand on beaches and in deserts creates sand dunes, causes wind erosion, and generates dust storms. Current theoretical models show large discrepancies in comparisons with measurements. We investigate the fundamental reasons for these discrepancies using high‐frequency measurements of sand transport and turbulent winds. We find that the downward transport of horizontal fluid momentum, which models use to predict the wind‐blown sand flux, is only predictive when it correlates strongly with the horizontal wind speed itself. This occurs for long (∼30 min) averaging timescales and idealized surface conditions. But for shorter timescales and realistic field conditions, other processes impact momentum fluxes. These processes are difficult to predict and are not accounted for in current models. Moreover, changes in surface conditions can also drive variability that models do not account for. Our findings help clarify the limitations of existing sand transport models and inform how future models can be improved.
Key Points
Saltation flux is correlated to, and likely determined by, the fluctuating wind velocities
Turbulent wind momentum flux is only predictive of sand flux on longer time scales when the former is correlated with the wind velocities
Predictability of sand flux under realistic field conditions is limited by non‐stationary flow and evolving/heterogeneous surface conditions
Feedbacks between the global dust cycle and the climate system might have amplified past climate changes. Yet, it remains unclear what role the dust-climate feedback will play in future anthropogenic ...climate change. Here, we estimate the direct dust-climate feedback, arising from changes in the dust direct radiative effect (DRE), using a simple theoretical framework that combines constraints on the dust DRE with a series of climate model results. We find that the direct dust-climate feedback is likely in the range of -0.04 to +0.02 Wm
K
, such that it could account for a substantial fraction of the total aerosol feedbacks in the climate system. On a regional scale, the direct dust-climate feedback is enhanced by approximately an order of magnitude close to major source regions. This suggests that it could play an important role in shaping the future climates of Northern Africa, the Sahel, the Mediterranean region, the Middle East, and Central Asia.
Coarse mineral dust (diameter, ≥5 μm) is an important component of the Earth system that affects clouds, ocean ecosystems, and climate. Despite their significance, climate models consistently ...underestimate the amount of coarse dust in the atmosphere when compared to measurements. Here, we estimate the global load of coarse dust using a framework that leverages dozens of measurements of atmospheric dust size distributions. We find that the atmosphere contains 17 Tg of coarse dust, which is four times more than current climate models simulate. Our findings indicate that models deposit coarse dust out of the atmosphere too quickly. Accounting for this missing coarse dust adds a warming effect of 0.15 W·m
and increases the likelihood that dust net warms the climate system. We conclude that to properly represent the impact of dust on the Earth system, climate models must include an accurate treatment of coarse dust in the atmosphere.
Atmospheric deposition of dust aerosols is a significant source of exogenous iron (Fe) in marine ecosystems and is critical in setting primary marine productivity during summer. This dust‐borne input ...of Fe is particularly important to the Southern Ocean, which is arguably the most biogeochemically important ocean because of its large spatial extent and its considerable influence on the global carbon cycle. However, there is large uncertainty in estimates of dust emissions in the Southern Hemisphere and thus of the deposition of Fe‐containing aerosols onto oceans. Here we hypothesize that sparsely vegetated surfaces in arid and semiarid regions are important sources of Fe‐containing aerosols to the Southern Ocean. We test this hypothesis using an improved dust emission scheme in conjunction with satellite products of vegetation cover and soil moisture in an atmospheric chemistry transport model. Our improved model shows a twofold increase of Fe input into the Southern Ocean in austral summer with respect to spring and estimates that the Fe input is more than double that simulated using a conventional dust emission scheme in summer. Our model results suggest that dust emissions from open shrublands contribute over 90% of total Fe deposition into the Southern Ocean. These findings have important implications for the projection of the Southern Ocean's carbon uptake.
Key Points
Treatments of soil moisture, texture, and vegetation cover are improved for physically based dust emission scheme
Dust Fe input to the Southern Ocean is elevated in austral summer
Majority of atmospheric Fe input into the Southern Ocean comes from sparsely vegetated regions
Plain Language Summary
Atmospheric deposition of dust aerosols is a significant source of exogenous iron (Fe) in marine ecosystems and is critical in setting primary marine productivity during summer. This dust‐borne input of Fe is particularly important to the Southern Ocean, which is arguably the most biogeochemically important ocean because of its large spatial extent and its considerable influence on the global carbon cycle. However, there is large uncertainty in estimates of dust emissions in the Southern Hemisphere and thus of the deposition of Fe‐containing aerosols onto oceans. Here we hypothesize that sparsely vegetated surfaces in arid and semiarid regions are important sources of Fe‐containing aerosols to the Southern Ocean. We found that open shrubland could be a key contributor to atmospheric soluble Fe input to the Southern Ocean, especially in austral summer. These findings have important implications for the projection of the Southern Ocean's carbon uptake.
Saltation, the wind‐driven hopping motion of sand grains, forms dunes and ripples, and ejects fine dust particles into the atmosphere on Mars. Although the wind speed at which saltation is initiated, ...the “fluid threshold,” has been studied extensively, the wind speed at which saltation is halted, the “impact threshold,” has been poorly quantified for Mars conditions. I present an analytical model of the impact threshold, which is in agreement with measurements and numerical simulations for Earth conditions. For Mars conditions, the impact threshold is approximately an order of magnitude below the fluid threshold, in agreement with previous studies. Saltation on Mars can thus be sustained at wind speeds an order of magnitude less than required to initiate it, leading to the occurrence of hysteresis. I include the effect of hysteresis into an improved parameterization of sand transport on Mars.
Loess deposits are recorders of aeolian activity during past glaciations. Since the size distribution of loess deposits depends on distance to the dust source, and environmental conditions at the ...source, during transport, and at deposition, loess particle size distributions and derived statistical measures are widely used proxies in Quaternary paleoenvironmental studies. However, the interpretation of these proxies often only considers dust transport processes. To move beyond such overly simplistic proxy interpretations, and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition. Furthermore, using high-resolution bulk loess and quartz grain size datasets from a last glacial/interglacial sequence, we show that, because grain size distributions are affected by multiple, often stochastic processes, changes in these distributions over time allow multiple interpretations for the driving processes. Consequently, simplistic interpretations of proxy variations in terms of only one factor (e.g. wind speed) are likely to be inaccurate. Nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires (i) a careful site selection, to minimize the effects of topography and source distance, and (ii) the joint use of bulk and quartz grain size proxies, together with high resolution mass accumulation rate calculations if possible.
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.
Key Points
A compilation of measurements shows that current climate models and remote sensing retrievals substantially underestimate dust asphericity
Measurements suggest that North African dust becomes more aspherical during transport, whereas Asian dust might become less aspherical
Dust asphericity increases gravitational settling lifetime by ~20%, which helps explain the underestimation of coarse dust transport by models
The blowing of sand by wind, known as saltation, ejects dust aerosols into the atmosphere, creates sand dunes, and erodes geological features. We present a comprehensive numerical model of steady ...state saltation (COMSALT) that, in contrast to most previous studies, can reproduce a wide range of measurements and can simulate saltation over mixed soils. COMSALT calculates the motion of saltating particles due to gravity, fluid drag, particle spin, fluid shear, and turbulence and explicitly accounts for the retardation of the wind due to drag from saltating particles. Furthermore, we included a novel physically based parameterization of the ejection of surface particles by impacting saltating particles which matches experimental results. COMSALT is the first numerical saltation model to reproduce measurements of the wind shear velocity at the impact threshold (i.e., the lowest shear velocity for which saltation is possible) and of the aerodynamic roughness length in saltation. It also reproduces a range of other saltation processes, including profiles of the wind speed and particle mass flux, and the size distribution of saltating particles. As such, COMSALT is the first physically based numerical model to reproduce such a wide range of experimental data. Since we use a minimum of empirical relations, COMSALT can be easily adapted to study saltation under a variety of physical conditions, such as saltation on other planets, saltation under water, and saltating snow.
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