Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus

TitleTwomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus
Publication TypeJournal Article
Year of Publication2014
AuthorsWerner F., Ditas F., Siebert H., Simmel M., Wehner B., Pilewskie P., Schmeissner T., Shaw R.A, Hartmann S., Wex H., Roberts GC, Wendisch M.
JournalJournal of Geophysical Research-Atmospheres
Date Published2014/02
Type of ArticleArticle
ISBN Number2169-897X
Accession NumberWOS:000332994600028
Keywordsaerosol; aerosols; albedo; cloud; Clouds; layer; maritime; optical-thickness; pollution; radiation; radiative-transfer; radius; susceptibility; Twomey effect

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 (), retrieved from helicopter-borne spectral cloud-reflected radiance measurements, and spectral cloud reflectivity () 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 and toward larger values is observed, while the mean values and observed ranges of retrieved r(eff) decrease. The relative susceptibilities (RS) of r(eff), , and 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 , and 0.27 for . 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.

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