Demonstration of Combined High Spectral Resolution Lidar--Millimeter Radar Drizzle Measurements During MAGIC

Description

Measurements of drizzle at cloud base and drizzle evaporation rates in the sub-cloud layer are important for the evaluation of strato-cumulus models. Drizzle not only removes water from the cloud layer, but falling droplets evaporate and cool the sub-cloud layer, suppressing convection. Surface precipitation measurements are not adequate; we need to know both the water flux at cloud base and the evaporation rate as a function of altitude. High Spectral Resolution Lidar provides measurements of the optical scattering cross section. For drizzle droplets, the scattering cross section is equal to two times the number of particles per unit volume times the average geometric cross sectional area of the particles. Millimeter wavelength radar provides the radar backscatter cross section. When the droplets are small compared to the wavelength, this is proportional to the number of particles per unit volume times the average mass2 of the particles. The ratio of the radar backscatter cross section to the lidar scattering cross section eliminates the number density and leaves the ratio: (mass2) / (area). Assuming a modified gamma particle size distribution allows this to be converted to the ratio: (mass) / (area). Multiplying this by one-half the HSRL measured scattering cross section times the mass-weighted fall velocity derived from the combined lidar, radar and gamma distribution yields the precipitation rate through the entire sub-cloud layer. This poster demonstrates this measurement technique using HSRL and KAZR data from the MAGIC campaign. It also explains how the size distribution parameters are selected and examines the sensitivity of the measurement to the assumed parameters.