Drizzle and Turbulence Below Closed Cellular Marine Stratocumulus Clouds

Abstract

Drizzle is ubiquitous in marine boundary layer stratocumulus clouds with much of it evaporating before reaching the surface. Ten days of observations made at the Atmospheric Radiation Measurement's Eastern North Atlantic site during closed cellular stratocumulus cloud conditions are used to characterize drizzle below the cloud base and its impact on the boundary layer turbulence. Cloud and drizzle microphysical and macrophysical properties were retrieved by combining the data from vertically pointing Doppler cloud radar, ceilometer, and microwave radiometer. On average, the drizzle shafts were 28.14 km wide, with cloud base rain rate and modal diameter of 0.98 mm/day and 138.62 μm, respectively. The rain rate at the surface was negligible yielding an average diabatic cooling of −28.68 W/m2 in the subcloud layer. The liquid water path and turbulence within the boundary layer increased with an increase in the cloud top radiative cooling; however, none of these variables exhibited any relationship with cloud base rain rate. For a similar amount of radiative cooling at the cloud top, the average variance of vertical velocity in the subcloud layer was about 16% lower during strongly precipitating conditions as compared to lightly precipitating conditions. The reduction in the variance of vertical velocity due to drizzle evaporation was primarily confined to the upper half of the subcloud layer and was due to reduction in the strengths of the downdrafts. Collectively, our results show substantial impact of drizzle evaporation on turbulence below stratocumulus clouds, necessitating its accurate representation in the Earth system models.