Drizzle Characteristics from a Surface-Based 94 GHz Radar

The Drizzle and Entrainment Cloud Studies (DECS 99) experiment was conducted on the shore of Monterey Bay, California from the 14 June to the 9 July 1999. During DECS, the vertical structure of coastal stratocumuli associated with drizzle and non-drizzle episodes was examined using a surface-based vertically pointing 94 GHz Doppler radar collocated with a ceilometer. A 915 MHz wind profiler of the Naval Postgraduate School (NPS) and rawinsondes were used to obtain the thermodynamic structure of the boundary layer.

Nearly 100 hours of radar data were collected during the 4 weeks of the DECS experiment. The 94 GHz Doppler radar collected Doppler Spectra (512-points, +/- 8m/s) with a temporal resolution of 3 seconds and a height resolution of 30 m. The high-resolution radar data are used to map the vertical structure of the stratocumulus layer. The fields of the Doppler moments (reflectivity, mean Doppler velocity and spectrum width) are statistically analyzed for drizzle and non-drizzle periods. Cloud thickness varied from 100 m to 500 m and drizzle was observed in clouds less than 300 m thick. Using Doppler spectra, microphysical parameters are calculated for drizzle and non-drizzle conditions. The results show a clear picture of drizzle structure in the lower part of the cloud, which has a lower droplet concentration and bigger drops than in the upper part of the cloud. In addition, the temporal and spatial variability of the drizzle field is examined and its relationship to cloud updrafts and downdrafts is investigated. Finally the internal variability of drizzle and non-drizzle cloud modes is analyzed.