The GEWEX Aerosol Precipitation initiative
Contact: Philip Stier (University of Oxford), Sue van den Heever (Colorado State University), Guy Dagan (Hebrew University of Jerusalem)

Precipitation is of fundamental importance for all life on Earth. A significant body of research exists on the impact of greenhouse gas-induced global warming on precipitation. Beyond greenhouse gas effects, aerosols have been hypothesized to have a wide range of effects on precipitation, through i) radiative effects via modification of radiative fluxes and the energy balance and ii) microphysical effects via modification of the properties and processes associated with cloud droplets and ice crystals.

While it is widely recognized that aerosols could exert a significant impact on precipitation – in fact, the largest recorded decrease in global mean precipitation followed the injection of volcanic aerosol from the Mount Pinatubo volcanic eruption – evidence for systematic aerosol effects on regional and global precipitation remains ambiguous. This limited progress can be attributed, in part, to the prevailing gap between local process-driven approaches, such as those utilized in the Aerosol, Clouds, Precipitation and Climate (ACPC) initiative, and the more global perspectives of climate modelling and satellite studies.

The GEWEX Aerosol Precipitation (GAP) initiative aims to close this gap, with the goals to:

  1. enhance our understanding of aerosol-precipitation interactions on a regional to global scale with a focus on energy and water budget constraints
  2. facilitate connections between all GEWEX and related process-based activities focusing on cloud-aerosol-precipitation interactions, such as ACPC.

Significant opportunities now exist that allow us to capitalize on recent advances to make progress in this area: a large body of literature now underpins our theoretical understanding of aerosol effects on cloud microphysics and precipitation formation processes as well as the convective momentum budget [Marinescu et al., 2021] and on how energy and water budget constraints can be applied to understand aerosol effects on precipitation [Dagan et al., 2019; Dagan and Stier, 2020; Dagan et al., 2021]. Novel earth observations from geostationary satellites and fleets of active and passive remote sensing instruments will allow to constrain global cloud lifecycles as well as cloud microphysics and dynamics, and global km-scale atmospheric models are now available to overcome the structural limitations associated with the representation of clouds in traditional CMIP style climate models.

GAP Activities
GAP’s initial strategy was guided and refined through two initial expert workshops:

Guided by these expert workshops, we initially propose two initiatives concentrating on exploiting the opportunities provided by the advent of global km-scale atmospheric models to consistently simulate aerosol effects on clouds and precipitation across cloud regimes, including convection. This work will inevitably highlight the need for novel observational constraints on aerosol effects on precipitation at the km-scale, to be developed in subsequent GAP initiatives.


Dagan, G., P. Stier, and D. Watson-Parris (2019), Contrasting Response of Precipitation to Aerosol Perturbation in the Tropics and Extratropics Explained by Energy Budget Considerations, Geophysical Research Letters, 46(13), 7828-7837 doi: 10.1029/2019gl083479.

Dagan, G., and P. Stier (2020), Constraint on precipitation response to climate change by combination of atmospheric energy and water budgets, Npj Clim Atmos Sci, 3(1) doi: 10.1038/s41612-020-00137-8

Dagan, G., P. Stier, and D. Watson-Parris (2021), An Energetic View on the Geographical Dependence of the Fast Aerosol Radiative Effects on Precipitation, J Geophys Res-Atmos, 126(9) doi: 10.1029/2020JD033045.

Grosvenor, D. P., O. Sourdeval, P. Zuidema, A. Ackerman, M. D. Alexandrov, R. Bennartz, R. Boers, B. Cairns, J. C. Chiu, M. Christensen, H. Deneke, M. Diamond, G. Feingold, A. Fridlind, A. Hunerbein, C. Knist, P. Kollias, A. Marshak, D. McCoy, … J. Quaas (2018), Remote Sensing of Droplet Number Concentration in Warm Clouds: A Review of the Current State of Knowledge and Perspectives, Rev Geophys, 56(2), 409-453 doi: 10.1029/2017rg000593.

Herbert, R., P. Stier, and G. Dagan (2021), Isolating Large-Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution, J Geophys Res-Atmos, 126(13) doi: 10.1029/2021JD034615.

Hohenegger, C., P. Korn, L. Linardakis, R. Redler, R. Schnur, P. Adamidis, J. Bao, S. Bastin, M. Behravesh, M. Bergemann, J. Biercamp, H. Bockelmann, R. Brokopf, N. Brüggemann, L. Casaroli, F. Chegini, G. Datseris, M. Esch, G. George, … B. Stevens (2022), ICON-Sapphire: simulating the components of the Earth System and their interactions at kilometer and subkilometer scales, Geosci. Mod. Dev. Discuss. doi: 10.5194/gmd-16-779-2023.

Marinescu, P. J., S. C. van den Heever, M. Heikenfeld, A. I. Barrett, C. Barthlott, C. Hoose, J. W. Fan, A. M. Fridlind, T. Matsui, A. K. Miltenberger, P. Stier, B. Vie, B. A. White, and Y. W. Zhang (2021), Impacts of Varying Concentrations of Cloud Condensation Nuclei on Deep Convective Cloud Updrafts-A Multimodel Assessment, J Atmos Sci, 78(4), 1147-1172 doi: 10.1175/Jas-D-20-0200.1.

Stevens, B., S. Fiedler, S. Kinne, K. Peters, S. Rast, J. Müsse, S. J. Smith, and T. Mauritsen (2017), MACv2-SP: a parameterization of anthropogenic aerosol optical properties and an associated Twomey effect for use in CMIP6, Geosci. Model Dev., 10(1), 433-452 doi: 10.5194/gmd-10-433-2017.

Wing, A. A., K. A. Reed, M. Satoh, B. Stevens, S. Bony, and T. Ohno (2018), Radiative-convective equilibrium model intercomparison project, Geoscientific Model Development, 793-813

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