Cloud Studies with the Droplet Aerosol Analyzer

Abstract: Climate change and atmospheric aerosols are a threat for human health and life. Reducing aerosol emissions would save human lives close to the aerosol source, but could lead to more death due to the implications of a warmer climate. Aerosol particles acting as cloud or ice nuclei can induce a change in cloud properties and thus indirectly induce a change in planetary albedo, which is considered the major reason of changes in planetary albedo associated with global warming. Because of the complexity of the interaction between aerosols and clouds, uncertainties in cloud parametrization remain the major cause of discrepancies between cloud observations and simulations. Thus, improving the understanding of aerosol-cloud-interactions is one of the keys for reducing uncertainty in the estimate of the total anthropogenic radiative forcing and climate sensitivity. Climate sensitivity is important for quantifying risks and probabilities, and the development of adaption strategies. The Droplet Aerosol Analyzer (DAA) was developed to study aerosol-cloud interaction and is unique in providing the number and the direct relationship between cloud droplet and residual particle size. For this purpose a more automatic version with better time resolution (10 min) and an improved and more automated inversion algorithm has been developed to better suit the needs of long-term measurements. Between June and October 2010 aerosol-cloud interaction measurements have been performed at the summit of Mt. Brocken (51.80 N, 10.62 E, 1142 m a.s.l.) in central Germany. For this period the aerosol and cloud properties and the droplet activation regime regarding the ratio between updraft velocity and particle number concentration (w=Ntot), have been determined. The relation between cloud droplet number concentration Nd;tot and total number concentration Ntot, updraft velocity wpred, and size distribution shape R_0.1um has been determined for three overlapping w=Ntot-intervals. As expected, for increasing w=Ntot-ratio (from the transitional regime towards aerosol limited regime) the relative sensitivity ofNd;tot against w decreases while the relative sensitivity of Nd;tot against Ntot increases. The influence of the size distribution shape R_0.1um was examined and the absolute relative sensitivity of Nd;tot against R_0.1um was observed to decrease from the transitional towards the aerosol limited regime. The onset of ’roll-off’, where an increase in Ntot does not lead to a proportional increase in Nd;tot, shifted towards higher total number concentration for increasing w=Ntot-ratio.