Azores Virga Depth: analytical expression as a function of cloud depth evaluated with observations

Poster PDF

Description

Drizzle is frequently observed in marine stratocumulus clouds and plays a crucial role in radiation and cloud lifetime. Most drizzling stratocumulus clouds form drizzle virga below cloud base, where sub-cloud scavenging and evaporative cooling are important. Here we use ground-based cloud radar observations (1) to examine the statistical properties of drizzle frequency and virga depth and (2) to test a simple relationship derived between drizzle virga thickness (Hv) and cloud thickness (Hc). Observations show that 83% of marine stratocumulus clouds are drizzling although only 31% generate surface precipitation. An analytical expression for drizzle virga thickness is derived as a function of cloud thickness and sub-cloud relative humidity considering in-cloud accretion and sub-cloud evaporation of drizzle drops. The derived third-order power law relationship between Hv and Hc (Hv ~ Hc^3) shows good agreement with long-term remote sensing data. Our formula provides a simple parameterization for drizzle virga of stratocumulus clouds suitable for use in models.