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Atmospheric Sciences & Global Change
Research Highlights

April 2015

Unlocking Cloud Gridlock

Handling climate-important cumulus clouds, regardless of model scale

convective transport
Researchers at PNNL developed a new way for global climate and weather forecasting models to represent cumulus clouds, accounting for updrafts and downdrafts in a manner that is far more accurate, regardless of the scale of the model. zoom Enlarge Image.

Results: Even as computing power increases, current climate model formulas struggle to handle storm clouds at today's higher resolutions and smaller model grid sizes. Cumulus storm cloud systems are still only partially resolved. Armed with a new formula developed by a research team led by Pacific Northwest National Laboratory, scientists can now represent cumulus in grid sizes as fine as 2 kilometers to as coarse as 256 kilometers. The team's approach breaks the storm cloud gridlock by more accurately depicting how cumulus clouds transport moisture through the atmosphere.

"This study helps us understand the scale-dependency of moisture and heat transported by cumulus clouds," said Dr. Jiwen Fan, an atmospheric scientist at PNNL, who led the team. "Our new formulation improves how the vertical transport of moisture by cumulus clouds is depicted in climate models at all scales."

Why It Matters: Representing these ubiquitous storm clouds in large-scale global climate models is crucial to obtaining accurate simulation of the climate, how it varies, and how it could change in the future. The old formulas can miss vital information necessary for accurate weather and climate prediction. Published in the Journal of Geophysical Research: Atmospheres, the new approach accounts for the variability of strong lifting currents in cumulus clouds.

Methods: Researchers at PNNL and collaborators from Scripps Institution of Oceanography and NASA Langley Research Center plugged real-world data into the Weather Research and Forecasting (WRF) model to simulate three storms: two over the U.S. Southern Great Plains in May of 2011 during the Midlatitude Continental Convective Clouds Experiment and one in the western Pacific near Australia in January 2006 during the Tropical Warm Pool International Cloud Experiment. The scientists started with a model grid size of 1 kilometer over an area 560 kilometers square. The researchers divided that 560 by 560 kilometer region into smaller squares with the lengths of 2, 4, 8, 16, 32, 64, 128, 256, and 512 kilometers to emulate the WRF and climate model grid sizes, and then examined the vertical transport of moisture as a benchmark at each of these scales.

The researchers discovered that the vertical transport by the cumulus clouds that cannot be resolved is strongly dependent on grid size. Evaluating the conventional formula that is used to represent the unresolved transport in the climate model, they found that it underestimates the transport at all scales. The new formula's accounting for the variability of ascending motions in convective updrafts much more closely approximates the benchmark results at all scales.

What's Next? Scientists will add the new formula to the National Center for Atmospheric Research's Community Atmosphere Model to improve climate and weather modeling.

Acknowledgments

Sponsors: The research was sponsored by the U.S. Department of Energy's Office of Science Office of Biological and Environmental Research Earth System Modeling program, along with the Office of Advanced Scientific Computing Research's Scientific Discovery through Advanced Computing program. Additional funding was provided by NASA's Modeling, Analysis and Prediction Program.

Facilities: PNNL's Institutional Computing program and National Energy Research Scientific Computing Center provided computing resources for the simulations.

Research Team: Yi-Chin Liu, Jiwen Fan, and Steven Ghan, PNNL; Guang Zhang, Scripps Institution of Oceanography, University of California at San Diego; and Kuan-Man Xu, NASA Langley Research Center

Research Area: Climate & Earth Systems Science

Reference: Liu Y-C, J Fan, G Zhang, K-M Xu, and SJ Ghan. 2015. "Improving Representation of Convective Transport for Scale-Aware Parameterization, Part II: Analysis of Cloud-Resolving Model Simulations." Journal of Geophysical Research Atmospheres, accepted. DOI:10.1002/2014JD022145


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Cloud Convection: Building a Storm

Imagine heating a pot of water and feeling the steam and warm air rise above the pot into the cooler air as the water begins to boil. The pot water churns in a circular motion; the water on the surface cools as it hits the air and sinks to the bottom only to be heated again and rise. That's what happens in convective clouds. Similar to hot air balloons, warm and moist air rises into cooler air, which then condenses and builds the cloud structure further.

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