1. Carbonaceous aerosol absorption in Hadgem3
Department of Meteorology
Carbonaceous aerosol absorption in Hadgem3
Will Davies and Nicolas Bellouin
CLARIFY-2017: 1 year on, Reed Hall, University of Exeter, 4 October 2018
MODIS image 1 October 2018

2. Absorption constraints
The strength of RFari and its semi-direct adjustments depends on the absorption aerosol optical depth. But it is useful to characterise the single-scattering albedo ϖ0 as it is less dependent of biases in extinction aerosol optical depth.
The CLARIFY observation period of Aug-Sep 2017 brought us two constraints on aerosol single-scattering albedo:
Aircraft retrievals, ϖ0 ~ 0.85 for dry aerosol
AERONET inversions on the African continent and at Ascension, for ambient aerosol

3. HadGEM3 MAP August September
Single-scattering albedo at 0.55 μm in HadGEM3 GA7.1 N96
August
September
Clear gradient of increasing SSA as distance from source increases.

4. HadGEM3 vS AERONET August August AERONET
Single-scattering albedo at 0.55 μm in HadGEM3 GA7.1 N96
August
August
AERONET
Suggests model is too absorbing near source and not absorbing enough in Ascension Island – although the latter may be due to comparing biomass only to a mixture of biomass/marine aerosol from the model. Possible dust contamination in AERONET sites to the south of the continent – modelled SSA excludes the dust contribution.

5. HadGEM3 VS AERONET September AERONET
Single-scattering albedo at 0.55 μm in HadGEM3 GA7.1 N96
September
AERONET
Essentially same as August.

6. BC MAC
Zonal structure of BC MAC suggests that lensing enhancement of BC is driven by coating of water itself.

7. HadGEM3 VERTICAL Dry SSA Aircraft
Biomass burning plume in blue – absorption decreases with transport. Values near Ascension between 0.82 and 0.86, so not incompatible with aircraft constraint. Local minimum near the surface just off the coast due to shipping. Plume between 7 and 10 km longer range transport.
Aircraft

8. Sensitivities in Ascension
Ambient SSA
What happens to SSA when:
More BC is emitted? (x-axis)
OC is made moderately absorbing? (BR)
OC hygroscopic growth is altered (H2)
AERONET
Hygroscopic changes: control sees POM have 65% of sulphate water uptake. Here changed to 20%.

9. Preliminary conclusions
Modelled dry SSA matches the aircraft estimate of ~0.85 well with no changes required to the model.
However, to match ambient SSA retrieved by AERONET in Ascension, one would need to multiply BC emissions by 3 and make OC absorbing.