2019, Karpińska, Katarzyna, Bodenschatz, Jonathan F.E., Malinowski, Szymon P., Nowak, Jakub L., Risius, Steffen, Schmeissner, Tina, Shaw, Raymond A., Siebert, Holger, Xi, Hengdong, Xu, Haitao, Bodenschatz, Eberhard

The phenomenon of "cloud voids", i.e., elongated volumes inside a cloud that are devoid of droplets, was observed with laser sheet photography in clouds at a mountain-top station. Two experimental cases, similar in turbulence conditions yet with diverse droplet size distributions and cloud void prevalence, are reported. A theoretical explanation is proposed based on the study of heavy inertial sedimenting particles inside a Burgers vortex. A general conclusion regarding void appearance is drawn from theoretical analysis. Numerical simulations of polydisperse droplet motion with realistic vortex parameters and Mie scattering visual effects accounted for can explain the presence of voids with sizes similar to that of the observed ones. Clustering and segregation effects in a vortex tube are discussed for reasonable cloud conditions. © Author(s) 2019.

2016, Wex, Heike, Dieckmann, Katrin, Roberts, Greg C., Conrath, Thomas, Izaguirre, Miguel A., Hartmann, Susan, Herenz, Paul, Schäfer, Michael, Ditas, Florian, Schmeissner, Tina, Henning, Silvia, Wehner, Birgit, Siebert, Holger, Stratmann, Frank

The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (Ntotal) ranged from as low as 100  up to 800 cm−3, while number concentrations for cloud condensation nuclei (NCCN) at a supersaturation of 0.26 % ranged from some 10 to 600 cm−3. Ntotal and NCCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both Ntotal and NCCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for NCCN below 200 cm−3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ  =  0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive NCCN from measured number size distributions, showing that this is a valid approach to obtain NCCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl−, this was found to be contributed by a small number of large particles ( >  500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to Ntotal and up to 15 % to NCCN. This is in accordance with finding no correlation between Ntotal and wind speed.