Characteristics of Drizzle under Stratocumulus using Cloud Doppler Radars

by Ghate, Virendra

GHATE, VIRENDRA (M.S., Meteorology and Physical Oceanography) Characteristics of Drizzle under (May 2006) Stratocumulus Using Cloud Doppler radars Abstract of a thesis at the University of Miami. Thesis supervised by Professor Bruce A. Albrecht. No. of pages in text. (93) Marine stratocumulus clouds cover extensive areas of the subtropical oceans and greatly influence Earths radiation by strongly reflecting the incoming solar radiation. The most climatologically pronounced stratus regime is located in the South-East Pacific. Drizzle is one of the several physical processes that affects the lifecycle and evolution of marine stratus by depleting the cloud liquid water and by stabilizing the marine boundary layer through evaporative cooling. In this study we use ship-borne radar observations from two innovative research cloud radars a Millimeter Cloud Radar (MMCR) (=8 mm) and Frequency Modulated Continuous Wave (FMCW) radar (=3 mm) to study the fallout of drizzle in the sub-cloud layer. Radar inter-comparison is used to perform calibration and quality control of the FMCW radar. The FMCW observations suffer no saturation and provide profiles of radar Doppler moments from the ship level to the cloud base. A lognormal drizzle drop size distribution is assumed and the parameters (N0, r0 and x) are retrieved using the observed radar reflectivity and mean Doppler velocity profiles. The retrieved parameters are used to extract bulk parameters of the drizzle size distribution such as liquid water content, total number of droplets and rainfall rates at various heights within the sub-cloud layer (typically from 50-500 m). It is demonstrated that a simple evaporation model can be used to constrain the inversion from radar observables to drizzle size distribution parameters. The model output showed that the drizzle DSD is truncated at lower end due to the rapid evaporation of smaller drops and the logarithmic width of drizzle DSD is lower than the typically prescribed value of 0.35.