Evidence of the First Indirect Effect in Clouds Downwind of a Mid-size North American City

Carl Berkowitz Pacific Northwest National Laboratory
Larry Berg Pacific Northwest National Laboratory
James Barnard University of Nevada Reno
Gunnar Senum Brookhaven National Laboratory
Stephen Springston Brookhaven National Laboratory

Category: Aerosol-Cloud-Radiation Interactions

Working Group: Cloud-Aerosol-Precipitation Interaction

The first indirect effect refers to changes in the cloud droplet spectrum resulting from enhanced particle loading that provides additional cloud condensation nuclei on which droplets can form. While past observations have found this effect in stratiform clouds downwind of large urban areas or associated with ship plumes, we will present evidence of this feature occurring in shallow convective clouds downwind of a mid-size North American city. By binning observations of pollutant loading and updraft velocity made within hundreds of clouds sampled during the U.S. Department of Energy’s Cumulus Humilis Aerosol Processing Study (CHAPS), we found an increase in the cloud droplet number concentration, CDNC, and a decrease in cloud droplet effective radius, r(eff), in polluted clouds relative to the CDNC and r(eff) measured in non-polluted clouds. Consistent with these observations was an increase in the total number of interstitial (non-activated) particles in polluted clouds relative to non-polluted clouds. Our presentation will include details of these observations and the analysis techniques used to arrive at these results. We will also present Mie calculations suggesting an increase in the scattering from the droplet spectra of polluted clouds. These results support the argument that to accurately simulate the microphysical properties of shallow cumuli requires treating both cloud dynamics and particle loading.


This poster will be displayed at ASR Science Team Meeting.

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