Effects of Soil and Land Surface Conditions in Summer on Dust Outbreaks in the Following Spring in a Mongolian Grassland

We propose an index of soil and land surface conditions for wind erosion to investigate their effects on dust outbreaks. The index is the normalized dust outbreak frequency (Nf(DO)), which is the ratio of dust outbreak frequency to strong wind frequency. Nf(DO) for April was always low in Mandalgobi, Mongolia, when the accumulated precipitation amount for June to August (Prec(Jun-Aug)), soil moisture averaged for June to August (SMJun-Aug), and above-ground biomass for August (AGB(Aug)) of the previous year exceeded their thresholds (100 mm, 13 mm, and 2.2*10(-2) kg m(-2), respectively). This suggests that dead leaves of grasses in spring, which are the residues of vegetation from the preceding summer, suppress dust outbreaks. However, when Prec(Jun-Aug), SMJun-Aug, and AGB(Aug) are lower than the thresholds, Nf(DO) varies over a wide range. This implies that when there are few dead leaves in spring, other possible factors after summer such as liquid precipitation leading to soil freezing, snow cover, melted water, and grazing, affect erodibility in spring. These results suggest that changes in soil and land surface conditions, rather than in wind conditions, chiefly affect the increased frequency of dust outbreaks. This dead-leaf hypothesis can be used as an early warning of dust-storm hazards.