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"We are interested in this 'droplet activation zone,' where most cloud droplets are initially formed at the cloud base, because the number of droplets formed there will affect the later stages and properties of the cloud - including how much sunlight a cloud reflects and the likelihood of precipitation," said Brookhaven atmospheric scientist Fan Yang, the first author on the paper.
"If there are more aerosols in the atmosphere, clouds tend to have more droplets, but the droplets will each be smaller, which means they can reflect more sunlight," Yang said. "This might help to cool our warming Earth," he noted.
Yang highlighted that accurately predicting aerosol-cloud interactions' impact on climate requires measurements of cloud droplet concentrations without airborne sample collection.
"This remains one of the biggest challenges in our field," Yang said.
The team introduced a novel remote-sensing technique to estimate droplet concentration, advancing our understanding of atmospheric aerosols' influence on clouds and climate.
Conventional atmospheric lidars, which measure cloud base distance, lack the resolution to detail internal cloud structures, typically achieving resolutions around 10 meters.
"Ten meters is like the height of a building," said Yang, explaining that while this scale can identify large objects, finer details like floors or windows require greater precision.
The collaboration between Brookhaven, Stevens Institute of Technology, and Raymetrics S.A. led to the development of a new lidar technology with a remarkable 10-centimeter resolution, offering unprecedented views of the droplet formation zone.
"With such a high resolution, the T2 lidar observations reveal the transition zone where aerosol particles absorb water vapor to be transformed into cloud droplets," Yang said.
This high-resolution device, which employs a time-gating technique to focus observations on specific cloud regions, allows for sampling at various depths within the cloud by adjusting the laser pulse and detection timing.
Yang noted the high repetition rate of the device, firing 20,000 pulses per second, provides detailed data on cloud properties based on the distribution of back-scattered signals.
Further enhancing the technology's practical application, the team plans to calibrate the lidar using in-situ measurements from a lab-based cloud chamber. This step ensures the remote sensing data accurately reflect true cloud properties, increasing confidence in real-world atmospheric studies.
"Our study highlights the benefits of applying advanced technologies to observe atmospheric clouds at submeter scales, which can open up new avenues for advancing our understanding of cloud microphysical properties and processes that are crucial to weather and climate," Yang concluded.
The development of the T2 lidar was supported by Brookhaven National Laboratory Program Development and Laboratory Directed Research and Development funding, with additional support from the National Science Foundation and the DOE Office of Science (BER).
Research Report:A single-photon lidar observes atmospheric clouds at decimeter scales: resolving droplet activation within cloud base
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