The importance of accurately representing turbulence in atmospheric models
A team of researchers supported by the Atmospheric System Research (ASR) program has made significant progress in understanding rain formation, with findings published in the Proceedings of the National Academy of Sciences (PNAS).
The study reveals how turbulence influences raindrop growth by driving droplet collision and coalescence in cumulus clouds—an insight that could improve weather prediction and climate models.
Rain formation is pivotal in cloud life cycles and their effects on Earth’s radiative balance. However, the microphysical processes behind rain initiation, especially turbulence-induced coalescence, remain challenging to represent in current atmospheric models. This research takes a significant step forward in bridging that gap.
The study was led by Kamal Kant Chandrakar of the National Science Foundation National Center for Atmospheric Research (NSF NCAR). The work harnessed advanced observational data and used large-eddy simulations (LES), which model detailed wind patterns in and around the clouds, with a particle-based microphysics scheme that tracks representative cloud and raindrops in the flow.
Chandrakar emphasizes the significance of their findings: “Understanding the role of turbulence in rain formation allows us to better predict the onset of rainfall, which has critical implications for both weather forecasting and long-term climate projections.”
With support from ASR, the NSF NCAR team integrated a state-of-the-art Lagrangian particle-based scheme in one of NSF NCAR’s community atmospheric models. This unique tool enabled the researchers to investigate the problem of rain formation more precisely, which is difficult with traditional approaches.
Chandrakar’s co-authors include Hugh Morrison and Wojciech W. Grabowski, also of NSF NCAR, along with R. Paul Lawson of Stratton Park Engineering Company (SPEC). The work was partially funded through the ASR project From Clouds to Precipitation: Multiscale Dynamics-Microphysics Interactions in Cumulus Clouds.
The research team also integrated high-resolution data from NASA’s Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) with LES and theoretical scaling analyses. Their results provide strong evidence that turbulent air currents drive raindrop growth through a “rain formation bottleneck” in cumulus congestus clouds.
“By capturing how turbulence accelerates droplet coalescence, we can more accurately simulate the generation and growth of embryo raindrops, especially near the cloud base,” explains Morrison.
The study found that these turbulent effects significantly influence droplet-size distribution, ultimately driving rain initiation. Moreover, the research dispels previous assumptions about the role of large aerosols—known as “giant cloud condensation nuclei” (giant CCN)—which, according to the team’s findings, have a negligible impact on rain formation in cumulus congestus clouds and are not needed to explain observed rain formation when turbulence is factored in.
This study not only informs the understanding of cloud microphysics but also underscores the importance of accurately representing turbulence in atmospheric models. By doing so, researchers can enhance the fidelity of weather forecasts and climate simulations, improving predictions of cloud dynamics and precipitation patterns in a warming world.
# # #Author: Mike Wasem, Staff Writer, Pacific Northwest National Laboratory
This work was supported by the U.S. Department of Energy’s Office of Science, through the Biological and Environmental Research program as part of the Atmospheric System Research program.