Drizzle and Turbulence in Open Cellular Marine Stratocumulus Clouds

Marine boundary layer stratocumulus clouds cover vast areas of eastern subtropical oceans and persist over longer periods. These clouds have a net cooling effect on the Earth’s surface as they reflect much greater amount of solar radiation back to space compared to the underlying ocean surface, making them a significant component of radiation budget. Marine stratocumulus clouds exist in two distinct mesoscale cellular organizations, open and closed cellular organization. In this study we have used data collected at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic (ENA) site during open cellular stratocumulus cloud conditions to characterize drizzle microphysical and boundary layer dynamic and thermodynamic properties.

The data from the vertically pointing Ka-band Doppler cloud radar known as Ka-band ARM Zenith Radar (KAZR) and the laser ceilometer were combined to retrieve profiles of drizzle microphysical properties at 50 m range and 1-minute temporal resolution. The data from the collocated Microwave Radiometer (MWR) and Doppler Lidar were used to calculate the Liquid Water Path (LWP) and boundary layer turbulence structure. Data from radiosondes that are launched twice a day and the Raman Lidar were used to characterize the boundary layer thermodynamic structure, while data from the surface meteorological instruments were used to identify thermodynamic and wind changes as a result of cold pools.

Ten cases of open cellular stratocumulus clouds were identified using satellite reported visible reflectance and liquid water path estimates together with the ground-based data. The averaged cloud base height and the cloud top heights were 1303 m and 1709 m respectively with an averaged LWP of 266.51 g/m2. The retrieved rain rates at the cloud base were much higher in open cellular stratocumuli compared to their closed cellular counterparts with a mean of 4.47 mm/day. We will simulate the radiative fluxes and heating rates using the Rapid Radiative Transfer Model (RRTM), characterize the evaporative cooling and assess its impact on the boundary layer turbulence and cold pools.