Articles | Volume 16, issue 4
Atmos. Chem. Phys., 16, 2221–2241, 2016
Atmos. Chem. Phys., 16, 2221–2241, 2016

Research article 26 Feb 2016

Research article | 26 Feb 2016

What controls the vertical distribution of aerosol? Relationships between process sensitivity in HadGEM3–UKCA and inter-model variation from AeroCom Phase II

Zak Kipling1, Philip Stier1, Colin E. Johnson2, Graham W. Mann3,4, Nicolas Bellouin5, Susanne E. Bauer6,7, Tommi Bergman8, Mian Chin9, Thomas Diehl10, Steven J. Ghan11, Trond Iversen12,13, Alf Kirkevåg12, Harri Kokkola8, Xiaohong Liu14, Gan Luo15, Twan van Noije16, Kirsty J. Pringle4, Knut von Salzen17, Michael Schulz12, Øyvind Seland12, Ragnhild B. Skeie18, Toshihiko Takemura19, Kostas Tsigaridis6,7, and Kai Zhang20,11 Zak Kipling et al.
  • 1Department of Physics, University of Oxford, Oxford, UK
  • 2Met Office Hadley Centre, Exeter, UK
  • 3National Centre for Atmospheric Science, University of Leeds, Leeds, UK
  • 4Institute of Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 5Department of Meteorology, University of Reading, Reading, UK
  • 6Center for Climate Systems Research, Columbia University, New York, NY, USA
  • 7NASA Goddard Institute for Space Studies, New York, NY, USA
  • 8Finnish Meteorological Institute, Atmospheric Research Centre of Eastern Finland, Kuopio, Finland
  • 9NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 10European Commission, Joint Research Centre, Institute for Environment and Sustainability, Climate Risk Management Unit, Ispra, Italy
  • 11Pacific Northwest National Laboratory, Richland, WA, USA
  • 12Norwegian Meteorological Institute, Oslo, Norway
  • 13Department of Geosciences, University of Oslo, Oslo, Norway
  • 14Department of Atmospheric Science, University of Wyoming, Laramie, WY, USA
  • 15Atmospheric Sciences Research Center, the State University of New York, Albany, NY, USA
  • 16Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 17Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, BC, Canada
  • 18Center for International Climate and Environmental Research – Oslo (CICERO), Oslo, Norway
  • 19Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
  • 20Max Planck Institute for Meteorology, Hamburg, Germany

Abstract. The vertical profile of aerosol is important for its radiative effects, but weakly constrained by observations on the global scale, and highly variable among different models. To investigate the controlling factors in one particular model, we investigate the effects of individual processes in HadGEM3–UKCA and compare the resulting diversity of aerosol vertical profiles with the inter-model diversity from the AeroCom Phase II control experiment.

In this way we show that (in this model at least) the vertical profile is controlled by a relatively small number of processes, although these vary among aerosol components and particle sizes. We also show that sufficiently coarse variations in these processes can produce a similar diversity to that among different models in terms of the global-mean profile and, to a lesser extent, the zonal-mean vertical position. However, there are features of certain models' profiles that cannot be reproduced, suggesting the influence of further structural differences between models.

In HadGEM3–UKCA, convective transport is found to be very important in controlling the vertical profile of all aerosol components by mass. In-cloud scavenging is very important for all except mineral dust. Growth by condensation is important for sulfate and carbonaceous aerosol (along with aqueous oxidation for the former and ageing by soluble material for the latter). The vertical extent of biomass-burning emissions into the free troposphere is also important for the profile of carbonaceous aerosol. Boundary-layer mixing plays a dominant role for sea salt and mineral dust, which are emitted only from the surface. Dry deposition and below-cloud scavenging are important for the profile of mineral dust only.

In this model, the microphysical processes of nucleation, condensation and coagulation dominate the vertical profile of the smallest particles by number (e.g. total CN  >  3 nm), while the profiles of larger particles (e.g. CN  >  100 nm) are controlled by the same processes as the component mass profiles, plus the size distribution of primary emissions.

We also show that the processes that affect the AOD-normalised radiative forcing in the model are predominantly those that affect the vertical mass distribution, in particular convective transport, in-cloud scavenging, aqueous oxidation, ageing and the vertical extent of biomass-burning emissions.

Short summary
The vertical distribution of atmospheric aerosol is an important factor in its effects on climate. In this study we use a sophisticated model of the many interacting processes affecting aerosol in the atmosphere to show that the vertical distribution is typically dominated by only a few of these processes. Constraining these physical processes may help to reduce the large differences between models. However, the important processes are not always the same for different types of aerosol.
Final-revised paper