Twomey Effect Observed from Collocated Microphysical and Remote Sensing Measurements over Shallow Cumulus

Clear experimental evidence of the Twomey effect for shallow trade wind cumuli near Barbados is presented. Effective droplet radius (r_eff) and cloud optical thickness (τ), retrieved from helicopter-borne spectral cloud-reflected radiance measurements, and spectral cloud reflectivity (γ_λ) are correlated with collocated in situ observations of the number concentration of aerosol particles from the subcloud layer (N). N denotes the concentration of particles larger than 80 nm in diameter and represents particles in the activation mode. In situ cloud microphysical and aerosol parameters were sampled by the Airborne Cloud Turbulence Observation System (ACTOS). Spectral cloud-reflected radiance data were collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART-HELIOS). With increasing N a shift in the probability density functions of τ and γ_λ toward larger values is observed, while the mean values and observed ranges of retrieved r_eff decrease. The relative susceptibilities (RS) of r_eff, τ, and γ_λ to N are derived for bins of constant liquid water path. The resulting values of RS are in the range of 0.35 for r_eff and τ, and 0.27 for γ_λ. These results are close to the maximum susceptibility possible from theory. Overall, the shallow cumuli sampled near Barbados show characteristics of homogeneous, plane-parallel clouds. Comparisons of RS derived from in situ measured r_eff and from a microphysical parcel model are in close agreement.