| Summary: | A new technique is presented for quantifying the impacts of aerosols on clouds while controlling for variations in meteorology. The recent work of Kaufman et al (2005a) has shown observational evidence for large aerosol effects on clouds. We present work that builds on these results by separating aerosol from meteorological effects on cloud forcing. The new technique uses parcel back-trajectories to account for differences in cloud history. Observations are obtained from the MODIS instrument aboard Terra, and are supplemented with ECMWF reanalyses. Geographic and seasonal biases are removed so that climatological variations cannot contribute to false correlations between aerosols and cloud properties. The present work is focused specifically on the stratocumulus cloud region of the Northeast Atlantic for June through August 2002, the season of maximum cloud cover. Trajectories are grouped into high and low terciles of aerosol optical depth (AOD) and cloud fraction (CF), and evaluated for systematic aerosol-meteorology correlations. Results show statistically significant differences in the meteorology of polluted versus pristine aerosol cases, indicating that variations in the dynamics are contributing to the observed correlation between aerosols and cloud forcing. Specifically, lower tropospheric stability (LTS) is shown to correlate significantly with both aerosol optical depth and cloud fraction. Resampling while holding LTS constant removes almost the entire aerosol-cloud correlation. We conclude that meteorological variations must be accounted for in assessing aerosol microphysical impacts on cloud forcing.
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