Atmospheric Sciences & Global Change Division
Distant Desert Dust Increases Snowpack in California
Local pollution's influence less impressive on Sierra Nevada Mountains' precipitation
Snow melt from the Sierra Nevada Mountains is the major fresh water supply for California. The Tinemaha Reservoir is at the base of the Eastern Sierra Nevada and is part of the Los Angeles Aqueduct system. Photo courtesy of Creative Commons license. Enlarge Image.
Results: Overseas dust influences the Sierra Nevada Mountains snowpack far more than home-produced pollution, according to scientists at Pacific Northwest National Laboratory. By modeling two cloud conditions during a field experiment called CalWater 2011, their results showed that dust blown in possibly from Asia and Africa increases snow in winter clouds over California and increases precipitation by 10 to 20 percent. However, local pollution from California cities and the Central Valley exerts a very small impact on snowfall. Further, the local pollution's influence on rain from warm clouds heavily depends on cloud conditions and the strength of a low level jet caused by mountain blocking that blows parallel to the Sierra Nevada Mountains.
"We used observational data captured in the clouds and on the surface to better understand how cloud ice forms in the presence of dust particles and simulated this process in models," said Dr. Jiwen Fan, PNNL atmospheric scientist and lead author of the study. "We compared the impact of long-range transported dust and local pollution on cold-season precipitation in contrasting cloud and meteorological conditions."
Why It Matters: The largest source of fresh water and irrigation in California is snow melt from the Sierra Nevada Mountains. Regional water resource managers and planners must understand the environmental and climate influences on California's precipitation and water supply sources. This need propelled the CalWater 2011 field campaign. Understanding the complex factors that impact precipitation and mountain snowpack will help improve regional climate predictions and far-reaching effects on populations and food supply.
Data collected in February and March, 2011, show dust has a greater effect on precipitation than does pollution. Dust effects on precipitation under clean (solid red line) and polluted (dashed red line) conditions; pollution effects on precipitation under dust (solid blue line) and no-dust (dashed blue line) conditions. Enlarge Image.
Methods: The research team led by PNNL's Dr. Jiwen Fan conducted high-resolution simulations using the Weather Research and Forecasting (WRF) model with a very detailed cloud microphysical model for two winter mixed-phase (liquid and ice) cloud cases. They used ground-based, aircraft and satellite data collected during the CalWater field campaign on February 16 and March 2, 2011. The simulations were highly detailed and computationally expensive. To improve the model representations of ice nucleation processes, they employed a state-of-the-art ice formation representation to connect dust with ice nucleation processes and used field campaign data from PNNL-operated Gulfstream-1 aircraft to validate the simulated clouds.
What's Next? A second field campaign called CalWater 2 / ARM Aerosol-Cloud-Precipitation Experiment (ACAPEX) will be conducted over the same region, but with additional measurements over the Pacific Ocean. Researchers will collect more data to systematically study the aerosol impact on snow and regional precipitation in California and improve models that can be used to better simulate aerosol effects on precipitation regionally and globally.
Sponsors: This study was supported by the California Energy Commission (CEC) and the U.S. Department of Energy (DOE) Office of Science as part of the Regional and Global Climate Modeling program. The G-1 is base-funded by the DOE's Atmospheric Radiation Measurement (ARM) Climate Research Facility and the deployment of the G-1 during the CalWater campaign was supported by CEC. PNNL Institutional Computing resources were used for the study simulations.
Research Team: Jiwen Fan, L. Ruby Leung, Jennifer Comstock, Balwinder Singh and Jason Tomlinson, PNNL; Paul DeMott, Colorado State University; Daniel Rosenfeld, The Hebrew University of Jerusalem; A. White, National Oceanic and Atmospheric Administration; Kim Prather, Scripps Institution of Oceanography, UC San Diego; P Minnis, NASA Langley Research Center; JK Ayers, Science Systems and Applications, Inc.; and Q Min, State University of New York at Albany.
Research Area: Climate & Earth Systems Science
Reference: Fan J, LR Leung, PJ DeMott, JM Comstock, B Singh, D Rosenfeld, JM Tomlinson, A White, KA Prather, P Minnis, JK Ayers and Q Min. 2014. "Aerosol Impacts on California Winter Clouds and Precipitation during CalWater 2011: Local Pollution versus Long-range Transported Dust." Atmospheric Chemistry and Physics 14:81-101. DOI:10.5194/acp-14-81-2014