Effective Radius of Cloud Droplets by Ground-Based Remote Sensing: Relationship to Aerosol

Enhancement of cloud droplet number concentration by anthropogenic aerosols has previously been demonstrated by in-situ measurements, but there remains large uncertainty in the resultant enhancement of cloud optical depth and reflectivity. Detection of this effect is made difficult by the large inherent variability in cloud liquid water path (LWP); the dominant influence of LWP on optical depth and albedo masks any aerosol influences. Here we use ground-based remote sensing of cloud optical depth (τc) by narrowband radiometry and LWP by microwave radiometry to determine the dependence of optical depth on LWP, thereby permitting examination of aerosol influence; the method is limited to complete overcast conditions with single layer clouds, as determined mainly by millimeter wave cloud radar. Measurements in north central Oklahoma on 13 different days in the year 2000 show wide variation in LWP and optical depth on any given day, but with near linear proportionality between the two quantities; variance in LWP accounts as much as 97% of the variance in optical depth on individual days and for about 63 % of the variance in optical depth for the whole data set. The slope of optical depth vs. LWP is inversely proportional to the effective radius of cloud droplets (re); event-average cloud droplet effective radius ranged from 5.6 ± 0.1 to 12.3 ± 0.6 μm (average ± uncertainty in the mean). This effective radius is negatively correlated with aerosol light scattering coefficient at the surface as expected for the aerosol indirect effect; the weak correlation (R = 0.24) might be due in part to vertically decoupled structure of aerosol particle concentration and possible meteorological influence such as vertical wind shear. Cloud albedo and radiative forcing for a given LWP are highly sensitive to effective radius; for solar zenith angle 60° and typical LWP of 100 g m, as effective radius decreases from 10.2 to 5.8 μm determined on different days, the resultant decrease in calculated net shortwave irradiance at the top of the atmosphere (Twomey forcing) is about 50 W m.