Investigation of Drizzling Virga Depth and Vertical Velocity below Marine Stratocumulus Clouds Using Ground-Based Remote Sensing

Stratocumulus clouds are common in tropical and subtropical oceans and play a crucial role in the Earth’s hydrological cycle and energy balance. Marine stratocumulus clouds produce drizzle readily and form drizzle virga below cloud base, where sub-cloud scavenging and evaporative occurs. In addition, sub-cloud convection due to either cloud top radiative cooling or surface heating can affect cloud structure and lifetime. Here we use ground-based cloud radar and lidar observations to examine (1) the statistical properties of drizzle frequency and virga depth and (2) the statistical properties of vertical velocity below cloud base. The motivation is to investigate the relationship between variables in the sub-cloud region (such as drizzle virga depth and vertical velocity) and the cloud properties (such as cloud thickness, cloud reflectivity). Results show that drizzle virga with varying thickness is frequently observed below marine stratocumulus clouds. An analytical expression for drizzle virga thickness (H_v) is derived as a function of cloud thickness (H_c) and sub-cloud relative humidity considering in-cloud accretion and sub-cloud evaporation of drizzle drops. The derived third-order power law relationship between H_v and H_c (H_v ~ H_c³) shows good agreement with independent, long-term remote sensing data. Our formula provides a simple parameterization for drizzle virga of stratocumulus clouds suitable for use in models. The correlation between vertical velocity and several cloud properties will also be presented.