Robert A. Stillwell

Polarization lidar at summit, Greenland, for the detection of cloud phase and particle orientation

Accurate measurements of cloud properties are necessary to document the full range of cloud conditions and characteristics. The Cloud, Aerosol Polarization and Backscatter Lidar (CAPABL) has been developed to address this need by measuring depolarization, particle orientation, and the backscatter of cloudsandaerosols.Thelidar islocated at Summit,Greenland(72.68N, 38.58W; 3200 m MSL), as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit Project and NOAA’s Earth System Research Laboratory’s Global Monitoring Division’s lidar network. Here, the instrument is described with particular emphasis placed upon the implementation of new polarization methods developed to measure particle orientation and improve the overall accuracy of lidar depolarization measurements. Initial results from the lidar are also shown to demonstrate the ability of the lidar to observe cloud properties.

Monte Carlo method for the analysis of laser safety for a high-powered lidar system under different atmospheric conditions

A major concern of high-powered atmospheric lidar systems is eye safety. Atmospheric lidars are often run unattended in adverse weather conditions where scattering redirects laser energy from the main beam. These naturally varying “soft targets” (such as fog and precipitation) are not accounted for in American National Standards Institute (ANSI) standards but, through multiple scattering events, can potentially create adverse viewing conditions. This paper introduces a Monte Carlo method that uses scattering phase functions for fog and snow and applies multiple scattering analysis to map the energy density within a scattering volume around the primary beam. Careful attention is given to accurately describing the forward scattering portion of the phase function as it scatters a significant amount of the beam energy. This method is compared to ANSI standard hazard zone calculations to determine what effect scattering has on the size of the hazard zone. For direct beam viewing, hazard zone size estimates are within about 3% of the ANSI defined Nominal Ocular Hazard Distance (NOHD) for clear air but are approximately 56% smaller than the NOHD as optical density increases for scattering in fog and approximately 33% smaller for scattering in blowing snow. For indirect beam exposure, clear air gives the worst approximation to the ANSI defined Nominal Hazard Zone (NHZ), in error by approximately 93%; fog approaches the ANSI results, within 30% error, whereas blowing snow shows 70% error. Finally, scattering enhancement mechanisms are considered which relate to the definition of the scattering layer of interest and increase scattered energy density observed by approximately 4%. In all cases, the ANSI calculated NOHD and NHZ are larger than the hazard zones that include scattering but the size of the zones is inextricably linked to the type of scattering ignored in the standard NOHD and NHZ calculations.

Characterizing ice particles using two-dimensional reflections of a lidar beam

We report a phenomenon manifesting itself as brief flashes of light on the snows surface near a lidar beam. The flashes are imaged and interpreted as specular reflection patterns from individual ice particles. Such patterns have a two-dimensional structure and are similar to those previously observed in forward scattering. Patterns are easiest to capture from particles with well-defined horizontal facets, such as near-horizontally aligned plates. The patterns and their position can be used to determine properties such as ice particle shape, size, roughness, alignment, and altitude. Data obtained at Summit in Greenland show the presence of regular hexagonal and scalene plates, columns, and rounded plates of various sizes, among others.