Using volcanic eruptions to unlock the secrets of aerosol-cloud interactions. Jim Haywood

Transcription

1 Using volcanic eruptions to unlock the secrets of aerosol-cloud interactions Jim Haywood Malavelle, F., et al., Strong constraints on aerosol-cloud interactions from volcanic eruptions, Nature, June 22 nd, Long author list including contributions from >20 institutions

2 Setting the scene What determines whether the climate warms/cools? IPCC, 2007 The radiative forcing is the change in the energy budget. Calculated since pre-industrial times. cooling warming Tug-of-war between emissions that warm climate and those that cool climate

3 Setting the scene The uncertainty is so large because clouds are complex systems (IPCC, 2001) 1 st and 2 nd indirect effects: this description is a little unfashionable (aerosol-cloud-interactions) but it helps a lot in the arguments that follow because models differ wildly in the relative contribution of the 1 st and 2 nd indirect effect

4 Setting the scene Models agree on impacts on cloud effective radius (1 st indirect effect) Models disagree on impacts on cloud liquid water (2 nd indirect effect) Ghan et al, PNAS, 2016

5 The poster boy for aerosol-cloudinteractions for the last couple of decades has been the observation of ship-tracks Setting the scene

6 Setting the scene The problems:- Ship-tracks are very small scale phenomena. Observational studies have shown a change in the cloud effective radius, but the liquid water path can go up or down. Chen et al., Atmos. Chem. Phys., 12, , 2012, analysed 589 individual ship-tracks and found the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. GCMs have a very coarse resolution Ship-tracks can be modelled in large eddy simulation and cloud resolving models, but not GCMs

7 Setting the scene Not surprisingly, given disparity in impacts on reff and LWP, impacts on precipitation are all over the place: GeoMIP sea salt experiments Hashed regions are areas where the SIGN of the precipitation change is not known Ahlm et al; Atmos. Chem. Phys. Discuss., doi: /acp , 2017

8 Ease of Solving Setting the scene UNIMPORTANT INTRACTABLE IMPORTANT INTRACTABLE ACI UNIMPORTANT TRACTABLE IMPORTANT TRACTABLE ACI Importance Without adequate constraints, we will not make progress on ACI

9 Malavelle, F., et al., Strong constraints on aerosol-cloud interactions from volcanic eruptions, Nature, June 22 nd, Gettelman, A., A. Schmidt, and J.-E. Kristjánsson, Icelandic volcanic emissions and climate. Nature Geoscience, 8, 243, doi: /ngeo2376 (2015). Initial assessment of potential climate impacts using a free-running model McCoy, D. T., and D. L. Hartmann, Observations of a substantial cloud-aerosol indirect effect during the Bárðarbunga-Veiðivötn fissure eruption in Iceland. Geophys. Res. Lett., 42, 10,409 10,414, doi: /2015gl (2015). Initial assessment of the impacts on cloud properties retrieved from satellite (MODIS collection 6).

10 Holuhraun fissure eruption: Up to 100ktSO2/day Up to x10 emission rate from all of 28 European countries put together Sustained for ~ 6 months A very different eruption to Pinatubo.. Plume is driven only by buoyancy, not momentum

11 Images courtesy of Anja Schmidt If we can t detect/model the impact on clouds then the impact is not important.

12 At each stage in the study we make efforts to validate the model performance. This requires synergistic working by a number of teams. Eruption SO2 Sulphuric acid aerosols Cloud droplet Number N d Cloud top effective radius, r eff Emissions and distributions validated using IASI AODs and plume height validated using MISR Impact validated using MODIS Impact validated using MODIS Impact on cloud optical depth, t cloud Impact validated using MODIS Impact on TOA SW Impact validated using CERES Impact on precipitation Impact validated using CloudSat

13 Assessment of emission rates

14 Malavelle et al (2017):- Nudged version of proto-hadgem3 to replicate the meteorology. 2-moment UKCA aerosol scheme. Empirical relationship between degassed sulphur and TiO 2 /FeO ratios and lava production derived from Icelandic basaltic flood lava eruptions IASI retrievals of SO2 MODIS Collection 5 for cloud (Aqua as TERRA has drift from sensor degradation). Untangles the impacts of meteorology Allows proper assessment of statistical significance Allows us to accept/reject results from GCMs

15 Grey = 1 stdev. Similar results for September (except there is some contribution from continental pollution to south of the region)

16 Grey = 1 stdev. Similar results for September (except there is some contribution from continental pollution to south of the region)

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19 Assessment of statistical significant is pretty straightforward as we can use the results from to build up a picture of variability in both the satellite observations and the models. These are from the models:- Dark grey = 1 stdev. Sept & Oct

20 HadGEM3-UKCA: HadGEM3-CLASSIC CAM5.4: CAM5-Oslo: MODIS:

21 There are some big implications for the believability of ACI in climate models These models are all OK These models are all wrong Ghan et al, PNAS, 2016

22 Cloud regime analysis update to ISCCP: Lazarus Oreopolis et al. (2016)

23 We are far from examining a meteorological special case the area consists of a mix of all cloud types

24 What can we conclude from the study? A massive plume (1/3 of global emission rate!) was emitted from Iceland in 2014 The emissions clearly perturbs cloud effective radius The radiative forcing was a modest -0.2Wm -2 during Sept/Oct 2014 The radiative forcing could be *2.9 (June) *2.3 (Sc region) * 1.5 (pre-industrial) = -2Wm -2 The emission have no detectable net influence on cloud liquid water The emissions have no detectable net influence on precipitation SOME models are accurate, some are not...

25 Additional Material Little impact on precipitation Kilauea provides the same conclusion ( but less dramatically) Are there any climate impacts e.g. SST impacts?

26 More detail on precipitation Impacts on precipitation over during September/ October are very unremarkable...

27 Unlike the change in effective radius that is very obvious in the observations... A very obvious outlier

28 The precipitation is actually the most average October in the satellite record...

29 The precipitation is actually the most average October in the satellite record and the zonal mean anomaly does not stand out.

30 Kilauea

31 Kilauea Grey envelope = 1 stdev

32 Kilauea

33 Kilauea Grey envelope = 1 stdev

34 Coupling to the N. Atlantic SSTs?

35 Coupling to the N. Atlantic SSTs? Effective radiative forcing calculation suggests a reduction in SW surface fluxes of up to -20Wm-2. Assuming a mixed layer ocean depth, this equates to a cooling of ~1K/month. Grist et al (2015) have suggested that the SST anomaly is due to resurfacing of anomalously cold water. Did the eruption help maintain it???

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