5.3. Dust emission

Dust emissions in arid and semi-arid regions have posed serious problems such as soil nutrition loss, crop and vegetation damage, and air quality deterioration. Dust emissions, for example, from Northeast Asia, influence not only individual arid or semi-arid regions but also regions across national boundaries and seas because some dust is raised by strong atmospheric convection and carried by strong westerly winds [53]. Dust emission from the Saharan Desert often harms the surrounding regions including regions far from Africa [53]. Wind erosion from agricultural land often causes local and regional problems according to tillage practices [54]. To predict these dust emissions in advance, monitoring and prediction of the surface soil moisture distribution over an area where dust emission occurrences are concentrated are important. Scheidt et al. [24] examined the spatial distribution of thermal inertia for five soil types in a desert of approximately 20 km using eight couples of the daytime and nighttime thermal-infrared surface temperatures observed by the Advanced Spaceborne Thermal Emission and Reflection (ASTER) and MODIS and then estimated the threshold wind speed based on the estimated thermal inertia values after Fécan [55]. The threshold wind speed or friction velocity for dust emissions according to surface soil moisture was examined using carefully designed wind tunnel experiments with multiple soil types, and the threshold friction velocity was found to be related to the soil matric potential and not aligned with the gravimetric soil moisture for the examined soil types [55–58]. The matric potential is not as readily available as the soil moisture, otherwise the function connecting the two variables is known in advance. Considering that it should be difficult to obtain the relationship between thermal inertia and matric potential, and practical relationships between thermal inertia-derived soil moisture and threshold wind speed with regard to individual soil types are required.

In the region where the Earth's surface and subsurface are seasonally frozen, dust emissions begin to occur (early spring) when only the surface is melted and dry but not the subsurface just beneath a thin surface layer. The thermal inertia at the representative depth is still affected by the frozen soil, but the surface radiative temperature is highly positive in degrees Celsius due to the dried surface, which is suitable for dust emission (in other words, high erodibility) if the wind is necessarily strong. The temperature difference was up to at most 20C (unpublished result). Dust emission is likely to occur in early spring when the meteorological conditions are likely to be windy; however, an empirical relationship between thermal inertiaderived soil moisture and threshold wind speed using observations during spring to early autumn [28] is difficult to apply because thermal inertia is likely to be underestimated in early spring possibly due to the large difference between the surface and the subsurface temperatures. There is no common formulation for thermal inertia-derived soil moisture with regard to the threshold wind speed in early spring or other seasons.
