High Spectral Resolution lidar measurements of atmospheric extinction: Progress and challenges

High Spectral Resolution lidar (HSRL) systems have proven capable of providing nearly continuous calibrated profiling of atmospheric backscatter cross-section and depolarization. These are combined to provide detailed images of aerosol and cloud structure. They also provide calibrated measurements of atmospheric extinction. Atmospheric values of the 1/e extinction length range from less than 100 m in water clouds to greater than 100 km in clear weather. While backscatter cross-section and depolarization are derived from ratios of HSRL signals that cancel out range sensitive instrumental artifacts, extinction measurements are derived from the molecular backscatter return alone. Furthermore, extinction is derived from the slope of the molecular signal. This amplifies noise due to photon counting statistics. Signal strengths vary over nearly 6-decades within the ~20 km useful clear air range of the lidar or within a few hundred meters in a dense water cloud. These factors make extinction measurements more demanding than the backscatter cross-section measurements and often preclude useful measurements in conditions of interest.