What is it about?
Two clouds contain the same amount of liquid water. One rains, the other doesn't. Why? Air motions matter! It shapes the journey of drops through clouds. Our new article, published in the Proceedings of the National Academy of Sciences (PNAS), compares precipitation properties beneath marine stratocumulus and shallow cumulus — two types of clouds with contrasting air motions and moisture field profiles. 700, 000 cloud observations from four oceanic regions were compared while controlling for the amount of liquid water to tease out the influence of cloud dynamics on warm rain formation and growth. Three key findings : - Marine stratocumulus drizzle readily. Even at liquid water paths (LWP) of 30–130 g/m², precipitation was detected beneath more than 56% of cloudy columns. At LWP above 130 g/m², precipitation probabilities exceeded 84%. These observations suggest that barriers to warm-rain formation in marine stratocumulus are weaker than commonly assumed. - Oceanic cumulus drizzle selectively. At the same low LWP of 30–130 g/m², precipitation was detected beneath only 5% of cumulus columns at Barbados and was nearly absent at Gan Island. When precipitation did emerge, the drops were approximately twice as large and tens to hundreds of times fewer than beneath stratocumulus containing the same amount of liquid water. - The journey of drops through clouds matters. A simple gravitational growth model could largely explain the precipitation drop sizes observed beneath stratocumulus, but could not explain the substantially larger drops beneath cumulus. The stronger updrafts and greater moisture heterogeneity of cumulus lead to drops experiencing more complex growth pathways. Some may repeatedly encounter liquid-water-rich regions favorable for growth, while others encounter drier regions where they partially or completely evaporate. Weather prediction models typically do not track the enormous number of individual drops in every cloud. Our findings provide extensive observational support for continued exploration of Lagrangian particle-tracking or Superdrop approaches in models as well as new observational strategies that can track drop and clouds over their lifecycle.
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Why is it important?
Warm rain in marine low clouds influences Earth’s radiation budget and boundary-layer energetics, yet its prevalence and growth mechanisms remain poorly constrained. Multiyear observations from four oceanic sites reveal that precipitation beneath stratocumulus is common even when clouds contain little liquid water. In contrast, cumulus with similar amounts of liquid water precipitate less frequently, but when they do, produce larger and fewer drops, which cannot be explained by gravitational growth alone. These contrasting rain properties accompany differences in updraft strength and degree of adiabaticity further suggesting the importance of cloud dynamics in shaping precipitation outcomes. This work motivates re-examining the treatment of precipitation formation in models and the further exploration of drop tracking approaches in rain prediction and observations.
Perspectives
“The finding of this study challenges the common picture that clouds must first accumulate substantial amounts of liquid water before rain forms. This puts into question where weather model development efforts should focus. For one thing, we should revisit model parameters that artificially prevent rain formation in marine stratocumulus.” “The results support a picture in which the history of a drop matters. Strong air motions can keep drops suspended longer, while the variable structure of cumulus means that different drops can follow very different pathways. Some may repeatedly encounter regions rich in liquid water that favor growth, while others encounter drier regions where they can partially or completely evaporate.” “As computing capabilities continue to advance, our results motivate further exploration of approaches that follow individual drops as they move through clouds, capturing how their growth depends on the different environments they encounter along the way,”
Katia Lamer
Brookhaven National Laboratory
Read the Original
This page is a summary of: In-cloud motions shape warm-rain production in marine clouds: Evidence from multiyear observations, Proceedings of the National Academy of Sciences, October 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2608656123.
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