UNIVERSITY-OF-WISCONSIN HIGH SPECTRAL RESOLUTION LIDAR

The University of Wisconsin High Spectral Resolution Lidar (UW HSRL) produces direct measurements of cloud and aerosol optical depth, extinction cross section, backscatter cross section, and backscatter phase function. The HSRL uses a multietalon interferometer to separate the backscatter return into a component due to particle scattering and a component due to scattering from air molecules. The molecular backscatter component is affected by extinction but not by particle backscatter. Because the molecular backscatter cross section is determined by the known atmospheric density, the atmospheric extinction can be directly calculated from the measured decrease in molecular backscatter signal with range. The separation of aerosol from molecular scattering is possible because the backscatter component from air is Doppler-broadened by the thermal velocities of the molecules, while the backscatter from more massive, slower moving particles remains spectrally unbroadened. Although the HSRL was originally designed for airborne nadir observation of boundary layer aerosol optical properties, increases in transmitted power, receiver improvements, and modified calibration techniques have allowed it to measure cirrus cloud optical properties. A continuously pumped, Q-switched, 4 kHz pulse repetition frequency, injection seeded, frequency doubled Nd:YAG laser, still under development, has recently been installed and has reduced cirrus cloud measurement averaging times by a factor of approximately 10.