Simulated polarization diversity lidar returns from water and precipitating mixed phase clouds

The dependence of polarization lidar returns on basic microphysical and thermodynamic variables is assessed by using a cloud model to simulate the growth of water and mixed (water and ice) phase clouds. Cloud contents that evolve with height in updrafts are converted, by using Mie theory, into cloud droplet single and double backscattering and attenuation coefficients. The lidar equation includes forward multiple scattering attenuation corrections based on diffraction theory for droplets and ice crystals, whose relative scattering contributions are treated empirically. Lidar depolarization is computed from droplet and crystal single scattering and an analytical treatment of droplet double scattering. Water cloud results reveal the expected increases in linear depolarization ratios (delta) with increasing lidar field of view and distance to cloud but also show that depolarization is a function of cloud liquid water content, which depends primarily on temperature. Ice crystals modulate mixed phase cloud liquid water contents through water vapor competition effects, thereby affecting multiple scattering delta values as functions of updraft velocity, temperature, and crystal size and concentration. Although the minimum delta at cloud base increases with increasing ice content, the peak measurable delta in the cloud decreases. Comparison with field data demonstrate that this modeling approach is a valuable supplement to cloud measurements.