When do stratocumulus clouds drizzle?

High-albedo marine stratocumulus clouds cool the top-of-atmosphere and surface radiation budgets over the eastern subtropical and tropical Oceans. Poor cloud representations lead to heat budget errors in numerical models. To better understand clouds and improve them in models, the VAMOS Ocean Cloud Atmosphere Land Study (VOCALS) investigated clouds in the southeastern tropical Pacific. The VOCALS Regional Experiment (REx) provides unique observations of clouds that help determine to what degree aerosols, cloud microphysics, and drizzle control cloud extent.

Seven years of ship deployments under the stratocumulus cloud deck culminated in 2008 with VOCALS REx. These observations provide a climatology of cloud height, thickness, liquid water path, and radiative forcing. In 2008 a millimeter-wavelength W-band Doppler cloud radar and a centimeter-wavelength C-band Doppler precipitation radar, were deployed on the NOAA ship Ronald H. Brown to observe drizzle-cloud interactions. The W-band radar pointed vertically and sampled the overlying column 3 times per second at 25-m vertical resolution. Sensitivity of the W-band radar was enhanced for the second VOCALS cruise leg so that it could detect cloud droplets.

Cloud and precipitation images from satellite were classified into closed-cell conditions (unbroken clouds) or open-cell conditions (broken clouds). The C-band radar detects precipitation over a 60-km volume, so images can be categorized as drizzling or non-drizzling. Some open-cell scenes were observed that had drizzle, and some open-cell scenes had no drizzle that could be detected by the C-band radar. Closed-cells were also observed both with and without C-band radar detectable drizzle. These observations suggest that drizzle is neither a necessary nor sufficient condition for open-cell broken stratocumulus clouds.

W-band radar column maximum reflectivity (CMR) is an indicator of cloud intensity at low reflectivity and rain intensity at higher reflectivity. The largest drops present dominate the reflectivity. Contoured frequency altitude diagrams (CFADs) of the W-band reflectivity are composited on CMR. When the cloud is not precipitating, the highest reflectivity occurs at the cloud top where cloud liquid water content is highest. When the cloud is precipitating, CMR occurs near the cloud base due to settling precipitation drops. Of the clouds detected by the W-band radar, 30% appear to be drizzling. As shown in previous studies, most drizzle evaporates below cloud base before reaching the ocean surface.