The structure of the dust layer over the Taklimakan desert during the dust storm in April 2002 as observed using a depolarization lidar

In April 2002, a severe dust storm occurred in the Taklimakan Desert. A large amount of the dust was lifted up by the dust storm and gradually removed in the following few days. The whole event of the dust storm was observed by the Mie-scattering depolarization lidar at Aksu, Xinjiang, China (40.62 degrees N, 80.83 degrees E, 1028 m above mean sea level). This paper describes the dust event and the removal process that was observed by the lidar. During the dust storm (April 13-16), a dense dust layer developed from the ground up to 5.5 km. The backscattering ratio was 20 or more, and the depolarization ratio was 15-25%. Due to the absorption of the laser beam by the heavy dust, a normal lidar observation was impossible for several hours. In this study, we estimated the backscattering ratio at the lowest height during the dust storm by solving the lidar equation directly. After the dust storm (April 17-20), a clear diurnal variation of the top of the dust layer was found by the lidar. An investigation of the lidar signals at different heights shows that there were two types of the removal process of the dust. The lidar signals at lower heights (less than the 2 km) gradually decreased during the post-dust storm period. This result indicates that the gravitational settling of the relatively large sized dust (coarse particles with a diameter of 10 mu m or more) occurs near the ground. On the other hand, lidar signals at 2-4 km had a clear diurnal variation with spike-like peaks from evening to midnight. These peaks suggest that the advection of the relatively small sized dust picked up in other location is due to the local circulation that occurs in the Tianshan Mountains and Tarim Basin