[1] An assessment of aerosol-cloud interactions (ACI) from ground-based remote sensing under coastal stratiform clouds is presented. The assessment utilizes a long-term, high temporal resolution data set from the Atmospheric Radiation Measurement (ARM) Program deployment at Pt. Reyes, California, United States, in 2005 to provide statistically robust measures of ACI and to characterize the variability of the measures based on variability in environmental conditions and observational approaches. The average ACI_N (= dlnN_d/dlnα, the change in cloud drop number concentration with aerosol concentration) is 0.48, within a physically plausible range of 0-1.0. Values vary between 0.18 and 0.69 with dependence on (1) the assumption of constant cloud liquid water path (LWP), (2) the relative value of cloud LWP, (3) methods for retrieving N_d, (4) aerosol size distribution, (5) updraft velocity, and (6) the scale and resolution of observations. The sensitivity of the local, diurnally averaged radiative forcing to this variability in ACI_N values, assuming an aerosol perturbation of 500 cm~(-3) relative to a background concentration of 100 cm~(-3), ranges between -4 and -9 W m~(-2). Further characterization of ACI and its variability is required to reduce uncertainties in global radiative forcing estimates.
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