Ubiquitous Drizzle from Marine Stratocumulus Clouds

Marine stratocumulus clouds are widespread over the tropical and subtropical oceans, cooling the climate with their high solar albedo. Drizzle is expected to affect clouds by scouring cloud condensation nuclei; in addition the dynamical effect of drizzling cells have been implicated in pockets of open-cell clouds (POCs) and lower fractional cloud coverage. Very clean marine stratocumulus clouds were observed for the first time by a motion-compensated Doppler cloud radar aboard the NOAA ship Ronald H. Brown during VOCALS in 2008. The 20-day radar time series shows these clouds to be drizzling at cloud base almost all the time they can be detected by the radar, even though drizzle usually evaporates before reaching the surface. Empirical relations between fall velocity and reflectivity show most Doppler fall velocities are more than 1 m/s at heights lower than 400 m below cloud top. We compare this result with fall velocities predicted by drop size distributions consistent with Doppler moments. Subtracting fall velocity, we also retrieve profiles of turbulent velocity and dissipation in the cloud. Joint distribution of Doppler fall velocity and radar reflectivity at cloud top (left) and 800 m below cloud top during a rainy period.