This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Research led by atmospheric scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory sheds new light on one of the fundamental challenges of weather forecasting: determining the conditions under which a cloud will produce rain—and how much rain it will produce.
DOE's Atmospheric Radiation Measurement (ARM) User Facility has been collecting observations of clouds at different sites around the world for more than 30 years. In a study published in Proceedings of the National Academy of Sciences, an analysis of over 700,000 cloud samples showed that marine stratocumulus, low-level clouds that blanket large areas of the world's oceans, can readily produce drizzle, even when they contain relatively little liquid water. "This finding challenges the common picture that clouds must first accumulate substantial amounts of liquid water before rain forms," said Katia Lamer, the paper's first author and director of the Center for Multiscale Applied Sensing at Brookhaven Lab.
"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." In-cloud air motion has long been hypothesized as important in shaping rain formation, but evidence of its importance has been limited. Multiple ARM sites and the Barbados Cloud Observatory provided the measurements needed to investigate this relationship across large populations of clouds containing similar amounts of liquid water.
At each site, the same instruments contributed the different pieces of the puzzle. Lidars identified where clouds ended and precipitation began. Doppler radars provided information about the size and number of raindrops, as well as air motions inside clouds.
Microwave radiometers measured how much liquid water the clouds contain. Together, these measurements allowed the scientists to examine how clouds with similar amounts of water can produce very different precipitation. By comparing precipitation properties beneath stratocumulus and shallow cumulus—two types of clouds with contrasting air motions and moisture field profiles—the authors found that the journey of water drops through a cloud matters in determining precipitation properties.
As air carries drops in a cloud, some may encounter regions rich in cloud droplets where they can collide and grow, while others pass through dry patches and evaporate. These different paths shape the number and size of drops that fall below the cloud base and impact the rain that reaches the surface. When comparing clouds containing the same amount of liquid water, shallow cumulus—which have more vigorous mixing and a patchwork of dry and moist regions—produced precipitation beneath the cloud much less frequently than stratocumulus—which are gentler and more uniform in moisture.
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