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The impact of humidity above stratiform clouds on indirect aerosol climate forcing

Nature volume 432, pages 1014–1017 (2004)Cite this article

Abstract

Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei1. The resulting cooling tendency, termed the indirect aerosol forcing, is thought to be comparable in magnitude to the forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood2. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water3,4. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water5,6,7. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect climate forcing.

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Figure 1: Liquid-water path and surface precipitation and entrainment rate as a function of cloud droplet number concentration.
Figure 2: Horizontal average profiles of precipitation rate and liquid-water mixing ratio averaged over the last two hours of 8-h simulations.

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Acknowledgements

We thank A. Fridlind and E. Jensen for comments on the manuscript, N. Mansour, J. Ferziger, and R. Street for discussions regarding the subgrid-scale model, and B. Stevens for providing the DYCOMS-II measurements to the Eighth GCSS Boundary Layer Cloud Workshop. This work was supported by the Radiation Science Program of NASA.

Author information

Authors and Affiliations

  1. NASA Ames Research Center, Moffett Field, California, 94035, USA

    Andrew S. Ackerman

  2. University of Tasmania, Hobart, TAS, 7001, Australia

    Michael P. Kirkpatrick

  3. Lawrence Livermore National Laboratory, Livermore, California, 94552, USA

    David E. Stevens

  4. University of Colorado, Boulder, Colorado, 80309, USA

    Owen B. Toon

Authors
  1. Andrew S. Ackerman
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  2. Michael P. Kirkpatrick
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  3. David E. Stevens
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Correspondence to Andrew S. Ackerman.

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Supplementary information

Supplementary Table 1

Relative change of liquid water path with respect to cloud droplet concentration between averages over the last two hours of 8-h simulations with aerosol concentrations of 150 and 300 per cubic centimetre. (PDF 9 kb)

Supplementary Figure 1

Differences between the boundary layer and overlying air of total water mixing ratio and liquid water potential temperature for the meteorological conditions used. (PDF 9 kb)

Supplementary Figure 2

Dependence on grid spacing of domain average liquid water path evolution for DYCOMS-II simulations. (PDF 48 kb)

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Ackerman, A., Kirkpatrick, M., Stevens, D. et al. The impact of humidity above stratiform clouds on indirect aerosol climate forcing. Nature 432, 1014–1017 (2004). https://doi.org/10.1038/nature03174

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