Ice Clouds and Asian Dust Studied with Lidar Measurements of Particle Extinction-to-Backscatter Ratio, Particle Depolarization, and Water-Vapor Mixing Ratio over Tsukuba

The tropospheric particle extinction-to-backscatter ratio, the depolarization ratio, and the water-vapor mixing ratio were measured by use of a Raman lidar and a polarization lidar during the Asian dust seasons in 2001 and 2002 in Tsukuba, Japan. The apparent (not corrected for multiple-scattering effects) extinction-to-backscatter ratios (Sp ) showed a dependence on the relative humidity with respect to ice (RHice ) obtained from the lidar-derived water-vapor mixing ratio and radiosondederived temperature; they were mostly higher than 30 sr in dry air (RHice < 50%), whereas they were mostly lower than 30 sr in ice-supersaturated air (RHice = 100%), where the apparent extinction coefficients were larger than 0.036 km-1 . Both regions showed mean particle depolarization ratios of 20% -22%. Comparisons with theoretical calculations and the previous experiments suggest that the observed dependence of Sp on RHice is attributed to the difference in the predominant particles: nonspherical aerosols (mainly the Asian dust) in dry air and cloud particles in ice-supersaturated air.