Soil Crust Cover Impacts Threshold Friction Velocity

Abstract. It is widely recognized that saltation is a turbulent process, similar to other transport processes in the atmospheric boundary layer. But due to the lack of high frequency observations, the statistic behavior of saltation is so far not well understood. In this study, we use the data from the Japan-Australian Dust Experiment (JADE) to investigate turbulent saltation by analyzing the probability density function, energy spectrum and intermittency of saltation fluxes. Threshold friction velocity, u*t, and saltation coefficient, c0, are two important parameters in saltation models, often assumed to be deterministic. But as saltation is turbulent, we argue that it is more reasonable to consider them as parameters obeying certain probability distributions. The JADE saltation fluxes are used to estimate the u*t and c0 probability distributions. The stochasticity of these parameters is attributed to the randomness in friction velocity and threshold friction velocity as well as soil particle size.

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An improved dust emission model – Part 2: Evaluation in the Community Earth System Model, with implications for the use of dust source functions

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-13043-2014 ◽

2014 ◽

Vol 14 (23) ◽

pp. 13043-13061 ◽

Cited By ~ 43

Author(s):

J. F. Kok ◽

S. Albani ◽

N. M. Mahowald ◽

D. S. Ward

Keyword(s):

Large Scale ◽

Friction Velocity ◽

Source Function ◽

Dust Emission ◽

Companion Paper ◽

System Model ◽

Dust Source ◽

Threshold Friction Velocity ◽

Scale Models ◽

Community Earth System Model

<p><strong>Abstract.</strong> The complex nature of mineral dust aerosol emission makes it a difficult process to represent accurately in weather and climate models. Indeed, results in the companion paper indicate that many large-scale models underestimate the dust flux's sensitivity to the soil's threshold friction velocity for erosion. We hypothesize that this finding explains why many dust cycle simulations are improved by using an empirical dust source function that shifts emissions towards the world's most erodible regions. Here, we both test this hypothesis and evaluate the performance of the new dust emission parameterization presented in the companion paper. We do so by implementing the new emission scheme into the Community Earth System Model (CESM) and comparing the resulting dust cycle simulations against an array of measurements. We find that the new scheme shifts emissions towards the world's most erodible regions in a manner that is strikingly similar to the effect of implementing a widely used source function based on satellite observations of dust source regions. Furthermore, model comparisons against aerosol optical depth measurements show that the new physically based scheme produces a statistically significant improvement in CESM's representation of dust emission, which exceeds the improvement produced by implementing a source function. These results indicate that the need to use an empirical source function is eliminated, at least in CESM, by the additional physics in the new scheme, and in particular by its increased sensitivity to the soil's threshold friction velocity. Since the threshold friction velocity is affected by climate changes, our results further suggest that many large-scale models underestimate the global dust cycle's climate sensitivity.</p>

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