Stepwise Assessment of Different Saltation Theories in Comparison with Field Observation Data

Abstract

Wind-blown dust models use input data, including soil conditions and meteorology, to interpret the multi-step wind erosion process and predict the quantity of dust emission. Therefore, the accuracy of the wind-blown dust models is dependent on the accuracy of each input condition and the robustness of the model schemes for each elemental step of wind erosion. A thorough evaluation of a wind-blown model thus requires validation of the input conditions and the elemental model schemes. However, most model evaluations and intercomparisons have focused on the final output of the models, i.e., the vertical dust emission. Recently, a delicate set of measurement data for saltation flux and friction velocity was reported from the Japan-Australia Dust Experiment (JADE) Project, which enabled the step-by-step evaluation of wind-blown dust models up to the saltation step. When all the input parameters were provided from the observations, both the two widely used saltation schemes showed very good agreement with measurements, with the correlation coefficient and the agreement of index both being larger than 0.9, which demonstrated the strong robustness of the physical schemes for saltation. However, using the meteorology model to estimate the input conditions such as weather and soil conditions, considerably degraded the models' performance. The critical reason for the model failure was determined to be the inaccuracy in the estimation of the threshold friction velocity (representing soil condition), followed by inaccurate estimation of surface wind speed. It was not possible to determine which of the two saltation schemes was superior, based on the present study results. Such differentiation will require further evaluation studies using more measurements of saltation flux and vertical dust emissions.Atmosphere 2020, 11, 10 2 of 18 erosion [9], based on the soil characteristics, including land-use type and soil texture [10]. In this step, the drag partitioning effect due to nonerodible elements [11][12][13] and the soil moisture effect [14] are also calculated. By combining this information, the threshold friction velocity, which quantifies the minimum wind speed to trigger the wind erosion, is determined as the final output of the first step [15]. When the actual friction velocity exceeds the threshold friction velocity, saltation (horizontal leapfrogging movement of soil grains) takes place [16,17]. When the leaping soil grains return to the ground, they release fine dust particles attached to them or are broken into pieces, resulting in the vertical emission of dust particles, which is called sandblasting [18,19]. These steps have been interpreted in various different ways in several modeling studies [20].Several studies on wind-blown dust in East Asia have been reported. Kang et al.[21] compared the performances of three vertical dust flux schemes suggested by Marticorena and Bergametti [17] and Lu and Shao [22] for an intense dust storm episode that broke out in the Gobi desert in winter. Reportedly, all three schemes ...

Effect of stones on the sand saltation threshold during natural sand and dust storms in a stony desert in Tsogt-Ovoo in the Gobi Desert, Mongolia

“…It is currently difficult to account for the effects of stones on the threshold friction velocity in a three-dimensional dust model because no reliable data on the spatial distribution of stones are available. However, several studies (e.g., Darmenova et al, 2009;Xi and Sokolik, 2015;Foroutan et al, 2017;Lee and Park, 2020) have conducted a three-dimensional dust simulation by employing a double shear stress partition scheme. These studies prepared a lookup table that contains representative roughness density values of solid elements (i.e., stones) and vegetation for each land cover type.…”

“…These studies prepared a lookup table that contains representative roughness density values of solid elements (i.e., stones) and vegetation for each land cover type. The studies used roughness density values of 0.002 (Darmenova et al, 2009;Foroutan et al, 2017) and 0.020 (Xi and Sokolik, 2015;Lee and Park, 2020) for barren and sparsely vegetated land cover types, respectively. In this study, all of the sites were considered barren or sparsely vegetated, the threshold friction velocity simulated with roughness density of 0.002 was 0.21 m/s, and that simulated with roughness density of 0.020 was 0.28 m/s (horizontal dashed lines in Fig.…”