[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 /dlna, 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 À3 relative to a background concentration of 100 cm À3 , ranges between À4 and À9 W m À2 . Further characterization of ACI and its variability is required to reduce uncertainties in global radiative forcing estimates.
The Atlantic Stratocumulus Experiment (ASTEX) was conducted over the northeast Atlantic Ocean during June 1992 with substantial international collaboration. The main goal of ASTEX was to study the climatologically important transition between solid stratocumulus and subtropical trade cumulus cloud regimes using island, aircraft, ship, and satellite measurements. Typically, the boundary layer was found to support cumulus clouds detraining into a patchy and fairly thin upper-stratocumulus layer. The substantial microphysical variability between clean marine and polluted continental air masses observed during ASTEX affected both drizzle and cloud properties. Highlights of the ASTEX research strategy included use of the ECMWF operational forecast model for assimilation of ASTEX soundings to obtain improved regional meteorological analyses; "Lagrangian" measurements of boundary-layer evolution following an air mass using aircraft and balloons, extensive coordinated use of surface, airborne, and satellite platforms; and an extensive suite of island-based remote sensing systems including millimeter-wavelength radars. A summary of ASTEX is presented and some initial results are presented.
Abstract. We show a method for determining stratus cloud liquid water profiles using a microwave radiometer and cloud radar. This method is independent of the radar calibration and the cloud-droplet size distribution provided that the sixth moment of the size distribution can be related to the square of the third moment. We have calculated these moments with a wide variety of in situ measurements and show that this is a reasonable assumption. Examples of droplet distributions that meet this requirement are the lognormal and gamma distributions. Frisch et al.[1995] retrieved a constant-with-height drop number concentration as well as an effective radius profile; then, using the assumption of a lognormal function, liquid water profiles were derived. We will show here that (1) the retrieval of liquid water profiles does not require a lognormal droplet distribution and (2) the accuracy of the liquid water profile retrieval is independent of radar calibration errors, provided we can relate the sixth moment to the third moment of the distribution through a power law. Thus this paper presents a generalization of the earlier method of Frisch et al. [1995].
MethodThe liquid water for a droplet distribution is given by
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