1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
2WSL Institute for Snow and Avalanche Research SLF Davos, Switzerland
3Wegener Center for Climate and Global Change and IGAM/Department of Physics, University of Graz, Austria
4Austrian Polar Research Institute, Vienna, Austria
5School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
6School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA
7Institute of Chemical Engineering Sciences, Foundation for Research and Technology, Hellas, 26504 Patras, Greece
8Institute for Environmental Research and Sustainable Development, National Observatory of Athens, 15236 Palea Penteli, Greece
9Laboratory for Air Pollution/Environmental Technology, Empa – Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland
anow at: Department of Geography, Faculty of Engineering and Environment, Northumbria University, Newcastle Upon Tyne, UK
bnow at: Grolimund and Partner – Environmental Engineering, Thunstrasse 101a, 3006 Bern, Switzerland
cnow at: Institute of Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Windisch, Switzerland
Received: 01 May 2015 – Discussion started: 09 Jun 2015
Abstract. A simple statistical model to predict the number of aerosols which activate to form cloud droplets in warm clouds has been established, based on regression analysis of data from four summertime Cloud and Aerosol Characterisation Experiments (CLACE) at the high-altitude site Jungfraujoch (JFJ). It is shown that 79 % of the observed variance in droplet numbers can be represented by a model accounting only for the number of potential cloud condensation nuclei (defined as number of particles larger than 80 nm in diameter), while the mean errors in the model representation may be reduced by the addition of further explanatory variables, such as the mixing ratios of O3, CO, and the height of the measurements above cloud base. The statistical model has a similar ability to represent the observed droplet numbers in each of the individual years, as well as for the two predominant local wind directions at the JFJ (northwest and southeast). Given the central European location of the JFJ, with air masses in summer being representative of the free troposphere with regular boundary layer in-mixing via convection, we expect that this statistical model is generally applicable to warm clouds under conditions where droplet formation is aerosol limited (i.e. at relatively high updraught velocities and/or relatively low aerosol number concentrations). A comparison between the statistical model and an established microphysical parametrization shows good agreement between the two and supports the conclusion that cloud droplet formation at the JFJ is predominantly controlled by the number concentration of aerosol particles.
Revised: 11 Mar 2016 – Accepted: 14 Mar 2016 – Published: 29 Mar 2016
Hoyle, C. R., Webster, C. S., Rieder, H. E., Nenes, A., Hammer, E., Herrmann, E., Gysel, M., Bukowiecki, N., Weingartner, E., Steinbacher, M., and Baltensperger, U.: Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland, Atmos. Chem. Phys., 16, 4043-4061, doi:10.5194/acp-16-4043-2016, 2016.