Measurement of ice crystal precipitation at cloud base

Description

Thin mixed-phase clouds are frequently observed in the Arctic. These often persist for days with nearly continuous, light precipitation. Models have difficulty maintaining these clouds and are very sensitive to micro-physical assumptions. Changes in microphysics yield variations of the removal of cloud water by ice crystal precipitation. This poster will describe the use of High Spectral Resolution Lidar (HSRL) and millimeter wavelength cloud radar (MMCR) data to estimate ice water and ice particle number fluxes from cloud base. Sample data will be shown. A ratio formed from HSRL and MMCR backscatter cross sections provides a robust measurement proportional to the fourth root of the average mass-squared over the average area of the ice crystals. Using these ratios and Doppler velocities with an equivalent spheroid model for ice particles, we compute the ice water content in the precipitating ice. Multiplying the ice water content by the Doppler velocity generates the precipitation rate. The measured velocity is a sum of the particle fall velocity and the vertical air motion. Because these are both order 1 m/s, in the past, time averaging was necessary to suppress the air motion. However, slowly varying vertical motions, often caused by gravity waves, could not be removed by while maintaining structure in the ice fall streaks. Following the lead of previous investigators, we assume that the lowest frequency contributions to the MMCR Doppler spectra are produced by particles with negligible fall velocities so that they trace air motion. Time average profiles of the vertical air motion show limitations of this approach. In regions of high turbulence, the Doppler spectrum is broadened by velocity variations within a single radar range bin. This produces a small apparent mean upward vertical velocity. In regions where the radar return is very small, the vertical velocity shows a small mean downward motion. This indicates the absence of small particle radar returns. To reduce these errors, we use a 1-hour mean vertical air motion profile to correct individual air motions. The derived air motions are subtracted from the Doppler velocities to derive the corrected fall velocity. This correction eliminates the need for time averaging and improves the capture of structure in ice fall streaks.