Dust Devil–Like Vortices in an Urban Area Detected by a 3D Scanning Doppler Lidar

Abstract

Atmospheric boundary layer (ABL) observations were conducted in an urban area (Sapporo, Japan) from April 2005 to July 2007 using a three-dimensional scanning coherent Doppler lidar. During this period, 50 dust devil?like vortices were detected in the area; they occurred during the daytime and were located at vertices or in the branches of convective cells (?fishnet? patterns of wind field). The diameters of the vortex cores ranged from 30 to 120 m, and maximum vorticity ranged from 0.15 to 0.26 s?1. More than 60% of the vortices were cyclonic; the rest were anticyclonic. The tangential velocity component of the strongest vortex varied from ?5.4 to +1.4 m s?1, and the signal-to-noise ratio was weak in the core. Temporal changes were observed in the three-dimensional structures of two vortices from 1330 to 1354 (Japan standard time) 14 April 2005, and the temporal evolution of the stronger vortex was studied. The vortex initially formed along a low-level convergence line in a fishnet and developed vertically. Its vorticity increased with time in association with shrinkage in the core diameter.