10 March 2016

Rain-aerosol Relationships Influenced by Wind Speed


Isolating aerosol particle effects on precipitation in data is tricky because aerosol-precipitation interactions work both ways. This is because most aerosol particles are removed from the atmosphere by precipitation. Moreover, winds associated with precipitation are a major source of natural aerosol particles, both over deserts (dust) and over the ocean (sea salt), and the optical signature of the aerosol particles (aerosol optical depth) increases as the aerosol swells with accumulated water when relative humidity is high during rainy conditions.

Recently a team including researchers from at the Scripps Institution of Oceanography at the University of California – San Diego and scientists at the DOE’s Pacific Northwest National Laboratory applied a partial correlation analysis to two long climate simulations. One of the simulations had prescribed sea salt emissions and one had sea salt emissions that fluctuate depending on surface wind speed. Precipitation is lighter during polluted conditions in both simulations, partly because increasing amounts of aerosols suppress precipitation, and partly because fewer aerosol particles are scavenged (removed) from the atmosphere when there is less precipitation. But the team found that the rain-aerosol linkage is much stronger when sea salt emissions are prescribed. This stronger link suggests that the increase of wind speed associated with storms comes with a number of effects: it increases sea salt emissions and diminishes the reduction in aerosol particles due to precipitation removing them from the cloud.

Researchers in this study found that wind speed makes a large contribution to the rain-aerosol relationship by changing sea salt emissions.

Steven J. Ghan
Scripps Institution of Oceanography (SIO)
Yang, Y, LM Russell, S Lou, Y Liu, B Singh, and SJ Ghan.  2016.  "Rain-Aerosol Relationships Influenced by Wind Speed."  Geophysical Research Letters.  https://doi.org/10.1002/2016GL067770.

This research was supported by NSF AGS1048995 and by DOE DE-SC0006679 as part of the U.S. Department of Energy, Office of Science, Biological and Environmental Research, Decadal and Regional Climate Prediction using Earth System Models (EaSM) program. The Pacific Northwest National Laboratory is operated for the DOE by Battelle Memorial Institute under contract DE-AC05-76RLO 1830. Observed 10-m wind speed data are obtained from the ERA-Interim re-analysis at the European Center for Medium Range Weather Forecasts (ECMWF). CMAP precipitation data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.esrl.noaa.gov/psd/. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The data and codes for these results are posted at: http://portal.nersc.gov/project/m1374/AOD_R.