Atmos. Chem. Phys. Discuss., 13, 19921-19970, 2013
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Aerosol impacts on California winter clouds and precipitation during CalWater 2011: local pollution vs. long-range transported dust
J. Fan1, L. R. Leung1, P. J. DeMott2, J. M. Comstock1, B. Singh1, D. Rosenfeld3, J. M. Tomlinson1, A. White4, K. A. Prather5, P. Minnis6, J. K. Ayers7, and Q. Min8
1Climate Physics, Pacific Northwest National Laboratory, Richland, WA 99352, USA
2Department of Atmospheric Science, Colorado State University, Fort Collins, Co 80523, USA
3Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel
4NOAA/ESRL, R/PSD2, 325 Broadway, Boulder, CO, 80305, USA
5Department of Chemistry and Biochemistry, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0314, USA
6NASA Langley Research Center (LaRC), Hampton, VA, USA
7Science Systems and Applications Incorporated, Hampton, VA, USA
8Atmospheric Sciences Research Center, State University of New York at Albany, Albany, NY 12203, USA

Abstract. Mineral dust aerosols often observed over California in winter/spring, associated with long-range transport from Asia and Sahara, have been linked to enhanced precipitation based on observations. Local anthropogenic pollution, on the other hand, was shown in previous observational and modeling studies to reduce precipitation. Here we incorporate recent developments in ice nucleation parameterizations to link aerosols with ice crystal formation in a spectral-bin cloud microphysical model coupled with the Weather Research and Forecasting (WRF) model, to examine the relative and combined impacts of dust and local pollution particles on cloud properties and precipitation type and intensity. Simulations are carried out for two cloud cases with contrasting meteorology and cloud dynamics that occurred on 16 February (FEB16) and 2 March (MAR02) from the CalWater 2011 field campaign. In both cases, observations show the presence of dust or dust/biological particles in a relative pristine environment. The simulated cloud microphysical properties and precipitation show reasonable agreement with aircraft and surface measurements. Model sensitivity experiments indicate that in the pristine environment, the dust/biological aerosol layers increase the accumulated precipitation by 10–20% from the Central Valley to the Sierra Nevada Mountains for both FEB16 and MAR02 due to a 40% increase in snow formation, validating the observational hypothesis. Model results show that local pollution increases precipitation over the windward slope of the mountains by few percent due to increased snow formation when dust is present but reduces precipitation by 5–8% if dust is removed on FEB16. The effects of local pollution on cloud microphysics and precipitation strongly depend on meteorology including the strength of the Sierra Barrier Jet, and cloud dynamics. This study further underscores the importance of the interactions between local pollution, dust, and environmental conditions for assessing aerosol effects on cold season precipitation in California.

Citation: Fan, J., Leung, L. R., DeMott, P. J., Comstock, J. M., Singh, B., Rosenfeld, D., Tomlinson, J. M., White, A., Prather, K. A., Minnis, P., Ayers, J. K., and Min, Q.: Aerosol impacts on California winter clouds and precipitation during CalWater 2011: local pollution vs. long-range transported dust, Atmos. Chem. Phys. Discuss., 13, 19921-19970, doi:10.5194/acpd-13-19921-2013, 2013.
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