HSRL backscatter, depolarization, optical depth and lidar ratio measured while scanning near zenith during snowfall on 28-Dec-2017.
Aerodynamic forces can create preferred orientations in falling ice crystals--they tend orient in a maximum drag configuration. It is well known that specular reflection from these oriented crystals can produce large enhancements of the zenith-pointing lidar backscatter signal along with small depolarization values (Platt, 1977: J. Appl. Met. 16: 339-345)
This paper reports High Spectral Resolution Lidar (HSRL) measurements of backscatter cross-section, extinction cross-section, and depolarization made while scanning within 20 degrees of the zenith in ice clouds and snowfall. These observations are sensitive to particle orientation and provide direct measurements of the lidar ratio. The HSRL is particularly suited to this task because it provides robustly calibrated values of particulate backscatter and extinction cross-sections.
Observations show up to a factor of 100 increase in backscatter cross-section at zenith. The enhanced backscatter typically shows a rapid decrease within 1 to 3 degrees of zenith, but is sometimes coupled with a continued slow decrease all the way to 20 degrees. The extinction cross section shows little zenith-angle variation. As a result, the lidar ratio can vary from less than 5 at zenith to nearly 100 at 20 degrees. Measured depolarization can drop to less than 5% at zenith and only increase to typical values of ~35% near 20 degrees.
Lidar discrimination between water and ice clouds is typically based on high values of depolarization in ice clouds contrasted with the low values of depolarization in liquid clouds. Extinction cross-sections estimations are often based on an assumed value of the lidar ratio. To avoid misidentification of cloud phase and spurious extinction determinations, lidars are often pointed ~4 degrees from the zenith. We show some cases where this avoids the specular peak. However, in many cases, the angular variation of lidar ratio and depolarization extends across the entire 20-degree scan. In these cases, backscatter cross-sections at 4 degrees can be twice as large as the 20-degree value, while the 4-degree particulate depolarization is one-half as large as the 20-degree value.
Particle orientation is not only important for remote sensing; it plays an important role in cloud physics. Slowly falling crystal spend a longer time in the cloud forming regions with potentially large influence on particle size and morphology.
