Publications

Export 2 results:
Sort by: Author Title Type [ Year  (Desc)]
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.

2006
Wilcox, EM, Roberts G, Ramanathan V.  2006.  Influence of aerosols on the shortwave cloud radiative forcing from North Pacific oceanic clouds: Results from the Cloud Indirect Forcing Experiment (CIFEX). Geophysical Research Letters. 33   10.1029/2006gl027150   AbstractWebsite

Aerosols over the Northeastern Pacific Ocean enhance the cloud drop number concentration and reduce the drop size for marine stratocumulus and cumulus clouds. These microphysical effects result in brighter clouds, as evidenced by a combination of aircraft and satellite observations. In-situ measurements from the Cloud Indirect Forcing Experiment (CIFEX) indicate that the mean cloud drop number concentration in low clouds over the polluted marine boundary layer is greater by 53 cm(-3) compared to clean clouds, and the mean cloud drop effective radius is smaller by 4 mm. We link these in-situ measurements of cloud modification by aerosols, for the first time, with collocated satellite broadband radiative flux observations from the Clouds and the Earth's Radiant Energy System to show that these microphysical effects of aerosols enhance the top-of-atmosphere cooling by similar to 9.9 +/- 4.3 W m(-2) for overcast conditions.