Measurement of Ice Water Content in Cirrrus with the HSRL

High Spectral Resolution Lidar Measurements of Ice Water Content: Approach and Initial Progress
E. W. Eloranta, R. E. Kuehn and R. E. Holz
University of Wisconsin

More accurate methods to measure the ice water content of cirrus clouds are needed. We propose to attempt this difficult measurement using the University of Wisconsin High Spectral Resolution Lidar (HSRL) and radar observations. The HSRL provides calibrated measurements of the scattering cross section. Because cirrus particles are large compared to the 532 nm wavelength transmitted by the lidar, the scattering cross section is just 2 times the total area of particles per unit volume. Because ice crystals are non-spherical and often oriented by the aerodynamic forces it is important to note that this is the particle area projected on a plane perpendicular to the lidar beam. We are currently working to measure the effective radius of the ice particles (again projected on a plane perpendicular to the lidar beam) by observing multiple scattering in the lidar beam. Theory shows that the multiple scattering is robustly related to the angular width of the diffraction peak in the scattering phase function and this width is a directly related to particle area. Together these measurements provide the cross sectional area per particle and the total number of particles. The fall velocity of ice crystals is dependent on the horizontal projection of the particle area and on the particle mass. Thus, Doppler fall velocities measured by radar coupled with HSRL measurements offer the potential to determine ice water mass. This poster describes progress on this recently initiated project.

UW Lidar // April 5, 2001 //