EGU2020-5254
https://doi.org/10.5194/egusphere-egu2020-5254
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Large variations in volcanic aerosol forcing efficiency due to eruption source parameters and rapid adjustments

Lauren Marshall1, Christopher Smith2, Piers Forster2, Thomas Aubry3, and Anja Schmidt3
Lauren Marshall et al.
  • 1Department of Chemistry, University of Cambridge, Cambridge, UK (lrm49@cam.ac.uk)
  • 2Priestely International Centre for Climate, University of Leeds, Leeds, UK
  • 3Department of Geography, University of Cambridge, Cambridge, UK

The relationship between volcanic stratospheric aerosol optical depth (SAOD) and volcanic forcing is key to quantify the climate impacts of volcanic eruptions. In their fifth assessment report, the Intergovernmental Panel on Climate Change uses a single scaling factor between volcanic SAOD and effective radiative forcing (ERF) based on climate model simulations of the 1991 Mt. Pinatubo eruption, which may not be appropriate for eruptions of different magnitudes. Using a large-ensemble of aerosol-chemistry-climate simulations of eruptions with different SO2 emissions, latitudes, emission altitudes and seasons, we find that the effective radiative forcing is on average 21% less than the instantaneous radiative forcing, predominantly due to a positive shortwave cloud adjustment.  In our model, the volcanic SAOD to ERF relationship is non-unique and depends strongly on eruption latitude and season. We recommend a power law fit in the form of ERF = -15.1 × SAOD0.88 to convert SAOD (in the range of 0.01-0.7) to ERF.

How to cite: Marshall, L., Smith, C., Forster, P., Aubry, T., and Schmidt, A.: Large variations in volcanic aerosol forcing efficiency due to eruption source parameters and rapid adjustments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5254, https://doi.org/10.5194/egusphere-egu2020-5254, 2020

How to cite: Marshall, L., Smith, C., Forster, P., Aubry, T., and Schmidt, A.: Large variations in volcanic aerosol forcing efficiency due to eruption source parameters and rapid adjustments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5254, https://doi.org/10.5194/egusphere-egu2020-5254, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-5254, Matthew Toohey, 05 May 2020

    Hi Lauren! It's clear that the IPCC AR5 scaling doesn't well represent the overall relationship coming from your simulations, but interestingly, it is consistent with a rather defined edge to the scatter. Do you know what simulations are lying close to the IPCC AR5 scaling line, e.g., is it the tropical eruptions?

    • AC1: Reply to CC1, Lauren Marshall, 05 May 2020

      Hi Matt, thanks for taking a look! The IPCC AR5 line is consistent with the instantaneous radiative forcing (IRF) only, since rapid adjustments that weaken the forcing are not considered. This defined edge is indeed predominantly from tropical eruptions, although at SAOD < ~0.25 there are also extratropical winter and summer eruptions that fall on this line. In this case, these points are from SAOD and radiative forcing values in year 2 of the simulations (the regression is over 3 years). Spread in year 1 from extratropical eruptions forms the main scatter seen on the figure and comes from the interplay between the timing of the peak aerosol burden and insolation – for summer eruptions, the bulk of the aerosol is present in winter when insolation is low, and therefore forcing is low.  In another study, Schmidt et al. (2018) also found that the IPCC AR5 IRF value appears consistent with smaller-magnitude eruptions that occurred between 2000-2015.

      Schmidt et al. (2018), https://doi.org/10.1029/2018JD028776

  • CC2: Comment on EGU2020-5254, Kirstin Krüger, 06 May 2020

    Hi Lauren,

    nice work. Some clarification question: Were all 82 aerosol-microphysical simulations based on one model? How different would the SAOD ERF factors look like including other models?

    Kicki

     

     

    • AC2: Reply to CC2, Lauren Marshall, 06 May 2020

      Hi Kicki, thank you! Yes, all simulations were using UM-UKCA. Previous work using other models has also found reduced slopes when accounting for rapid adjustments (Gregory et al., 2016; Larson & Portman, 2016; Schmidt et al. 2018) but have not examined the wide range of eruptions as we do here. This would be an interesting future comparison. Thanks, Lauren