Vertical Motions of Drops of Different Sizes and the Growth of Drizzle in Marine Stratus

Swarndeep Gill

University of Wyoming

Marine stratiform clouds have been an important topic of study in recent years due to their impact on the global energy balance. These shallow layer clouds, only a few hundred meters in thickness, usually persist for long periods of time. Therefore they have a much greater effect on shortwave reflection, while contributing little to the longwave radiation budget of the earth since they are similar in temperature to the ocean they lie over. Modeling these clouds accurately is essential to predicting future climate change. However, this has been complicated by the fact that the processes that lead to their formation, persistence and even break up occur on scales too small for Global Climate Models (GCM’s).

The Wyoming King Air and the Wyoming Cloud Radar investigated the microphysical processes and structure of stratus clouds off the Coast of Oregon in both 1995 and 1999. Vertical motions are observed to be closely tied to the evolution of the drop spectra. In-situ data reveal that concentrations of drops of different sizes vary with vertical motions and that this is necessary for the development of drizzle in marine stratus. The WCR provides additional information about vertical velocity structure, drizzle concentrations, and the variations of inhomogeneities in these clouds. Traditional models of droplet growth do not apply to marine stratus. Key factors for cloud droplet evolution in marine stratus will be discussed, as well as cautions to modelers.