April 2015
Original link: http://www.pnnl.gov/science/highlights/highlight.asp?id=3966
Unlocking Cloud Gridlock
Handling climate-important cumulus clouds, regardless of model scale
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.
"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
