Lidar observation and numerical simulation of a Kosa (Asian Dust) over Tsukuba, Japan during the spring of 1986

Abstract

A number of duststorms and/or sandstorms occurred in the deserts and loesslands of the Asian Continent in early March of 1986. After a few days the dust laden air was transported over the Yellow Sea to Japan by westerly winds. On 12-13 March, a number of Japanese meteorological observatories reported a "Kosa (Asian dust)" phenomenon. The lidar observation of the Kosa was made at Tsukuba, Japan from 15 JST to 21 JST on 13 March 1986. The vertical structure and time change of the Kosa layer observed by the lidar are presented. At 15 JST, two Kosa layers existed at 4km and 2km, respectively. The upper layer had a thickness of about 1km and a scattering ratio of 3.2. The lower layer had a scattering ratio of 2.6 and appeared to be mixed with background aerosols. Subsequently, the Kosa layer at 4km increased in thickness and scattering intensity, with a thickness of 1.5km and a scattering ratio of 5.7. At 18 JST the Kosa layer at 4km separated into two sublayers at 4.5km and 3.5km. The total thickness of the upper and lower sublayers was 2.3km. The lidar derived optical thickness was 0.086 (wavelength 694.3 nm). From 18 to 20 JST, the Kosa layer gradually lowered 0.5km. At 20 JST the Kosa layer separated into three sublayers at 4.0km, 3.2km and 2.7km. Analysis of concurrent radiosonde data showed that the upper and lower sublayers were dry, while the middle sublayer was humid. A numerical simulation was carried out to investigate the long range transport of the Kosa particles. Simulated horizontal and vertical distributions of the tracers were in good agreement with the lidar observation at Tsukuba and the routine meteorological observations in Japan and China. In particular, the observed structure of the two Kosa layers was well simulated. The two Kosa layers were found to originate from different altitudes over the source regions. The numerical simulation reveals the Loess Plateau and its neighboring deserts as important sources for the Kosa. Another possibility includes the Takla Makan Desert. Travel time of the Kosa particles to reach Japan was two to three days from the Loess Plateau and its neighboring deserts, and five to six days from the Takla Makan Desert.