A team at the U.S. Department of Energy’s Brookhaven National Laboratory has made groundbreaking remote observations of the fine-scale structure at the base of clouds. Published in NPJ Climate and Atmospheric Science, the research shows that the air-cloud interface is a transition zone where aerosol particles lead to the formation of cloud droplets. This ‘droplet activation zone’ is crucial for understanding how clouds affect climate by reflecting sunlight and influencing precipitation.
Cloud droplet number concentrations are difficult to measure accurately, but the team’s new remote-sensing measurements provide a novel way to estimate droplet concentration. This insight into how changes in atmospheric aerosol levels could impact clouds and climate is essential for predicting the effects of aerosol-cloud interactions. Traditional lidars can only measure the distance to the cloud base, not the detailed structures within it. The team developed a new high-resolution lidar with a resolution down to 10 centimeters, allowing them to see the transition zone where aerosol particles become cloud droplets.
The time-gated, time-correlated, single-photon counting lidar (T2 lidar) developed by the team provides unprecedented fine-scale observations of the cloud base region. This high-resolution lidar enables scientists to develop a theoretical model to estimate cloud droplet concentration based on backscatter signals. By setting different time intervals for the lidar’s observation window and using a high repetition rate, the team can sample signals at different regions through the cloud, giving them valuable information about cloud properties.
To make the T2 lidar technique useful for real-world measurements, it needs to be properly calibrated to match light signals with actual cloud properties. Traditional lidar measurements are sometimes calibrated by in-situ aircraft measurements of cloud droplets, but this method is not always accurate. The team is working on building a lidar with even finer resolution to observe cloud properties in a lab-based cloud chamber. This will allow them to match up backscattering signals with in-situ measurements, improving their understanding of how lidar measurements relate to cloud properties.
The development of the T2 lidar was supported by funding from Brookhaven National Laboratory, the National Science Foundation, and the DOE Office of Science (BER). By advancing technologies to observe atmospheric clouds at submeter scales, the team hopes to open new avenues for understanding cloud microphysical properties and processes crucial to weather and climate. The ability to measure cloud droplet concentrations remotely and accurately will greatly enhance scientists’ understanding of how aerosol-cloud interactions impact the climate system.
