Long-term observations of marine clouds reveal how in-cloud motions shape rain

Research conducted by atmospheric scientists at the U.S. Department of Energy's Brookhaven National Laboratory has provided new insights into the role of in-cloud motions in shaping rain formation within marine clouds. These findings, published in Proceedings of the National Academy of Sciences, challenge the prevailing notion that clouds must first accumulate significant amounts of liquid water before rain can occur.

The study analyzed over 700,000 cloud samples collected over three decades by the Atmospheric Radiation Measurement (ARM) User Facility, a global network of observation sites. The researchers found that marine stratocumulus clouds, which are prevalent over the world's oceans, can readily produce drizzle even when they contain relatively little liquid water. This finding challenges the assumption that clouds need abundant water content to generate precipitation.

The team focused on the importance of in-cloud air motion in determining rain formation. While this hypothesis has been previously proposed, evidence supporting its significance was limited. By comparing shallow cumulus clouds, which exhibit vigorous air motions and varied moisture profiles, with stratocumulus clouds, the scientists found that precipitation formation occurred less frequently beneath shallow cumulus clouds.

However, when precipitation did occur, the drops falling from cumulus clouds were approximately twice as large as those beneath stratocumulus clouds, but they were tens to hundreds of times fewer in number. This difference in precipitation characteristics suggests that the journey of water droplets through a cloud plays a crucial role in determining the properties of the resulting rain.

The researchers emphasize that weather models typically do not track the vast number of individual drops within clouds. Instead, these models rely on simplified representations of how cloud droplets become precipitation and how precipitation grows. The new findings highlight the necessity of incorporating the varying pathways that drops follow through clouds into these models.

As computational power continues to grow, the study advocates for the exploration of more sophisticated approaches that follow drops as they move through clouds. By capturing their growth and development under different environmental conditions, such models could provide a more accurate representation of rain formation processes.

Furthermore, the research underscores the need for new observational strategies that can continuously monitor clouds and precipitation as they evolve, rather than merely capturing their state at a single moment. Brookhaven Lab, with its expertise in high-resolution observations and cloud chamber capabilities, is well-positioned to advance this line of inquiry through its experimental facilities.