Regional Difference in the Characteristic of Dust Event in East Asia: Relationship among Dust Outbreak, Surface Wind, and Land Surface Condition

摘要

The regional differences in the characteristics of dust events (i.e., all phenomena related with dust; dust in the production and suspension phases) in East Asia were investigated by use of surface meteorological data from March 1988 to May 2004. Referring to the topography and land cover type (e.g., Bare Desert, Semi Desert Shrubs, Grassland, Cultivated Land), East Asia was divided into 11 regions. Dust outbreaks (i.e., dust in the production phase) frequently occur in the Gobi Desert, the western part of the Loess Plateau, and the Taklimakan Desert, whose land cover types are Bare Desert and Semi Desert Shrubs (i.e., Desert). Although the result of frequent dust outbreaks is common among these regions, the frequency of floating dusts (i.e., dust in the suspension phase) differs greatly. Floating dusts are frequently observed in the Taklimakan Desert because most of the dust particles are blocked by the steep topographical surroundings such as the Tibetan Plateau, Pamirs, and Tian Shan Mountains. On the other hand, floating dusts are seldom observed in the Gobi Desert and Loess Plateau. In these regions, although dusts are produced by synoptic disturbances, they are immediately swept away by these same synoptic disturbances.We clarified the regional differences in the relationships among dust outbreak, surface wind, and land surface condition by analyzing the correlation between dust outbreak frequency and strong wind frequency at a threshold wind speed ofa constant, 6.5 m sec−1. Our analysis showed that the surface wind is primarily responsible for dust outbreaks in Desert regions in March and April. On the other hand, land surface conditions are the determining factor for dust outbreaks in Grassland regions. Land surface conditions affect dust outbreaks in May even in Desert regions, but almost no impact ofwind is seen. An examination of the effect of snow cover on the threshold wind speed, using the parameterization of Kurosaki and Mikami (2004), shows snow cover to have marked effects on dust outbreaks in Inner Mongolia and the Hexi Corridor in March, and in Northern Mongolia in April. The seasonal and year-to-year variations in dust outbreaks, and the mean threshold wind speed of dust outbreak are also compared among regions.