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2014
Werner, F, Ditas F, Siebert H, Simmel M, Wehner B, Pilewskie P, Schmeissner T, Shaw RA, Hartmann S, Wex H, Roberts GC, Wendisch M.  2014.  Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus. Journal of Geophysical Research-Atmospheres. 119:1534-1545.   10.1002/2013jd020131   AbstractWebsite

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.

2004
Conant, WC, VanReken TM, Rissman TA, Varutbangkul V, Jonsson HH, Nenes A, Jimenez JL, Delia AE, Bahreini R, Roberts GC, Flagan RC, Seinfeld JH.  2004.  Aerosol-cloud drop concentration closure in warm cumulus. Journal of Geophysical Research-Atmospheres. 109   10.1029/2003jd004324   AbstractWebsite

[1] Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE). An "aerosol-cloud'' closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm(-3), drives large microphysical differences ( 250 2300 cm(-3)) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties ( effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosol's effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.