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Radiative impact of light absorbing impurity
in snow : some examples and uncertainty
assessment
Marie Dumont
Météo-France - CNRS, CNRM-GAME UMR3589,
Centre d’Études de la Neige (CEN), Grenoble, France
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Radiative impact of impurity in snow
Visible albedo decrease
Enhanced for coarse grains
NIR albedo decrease due to grain size effect
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Positive feedbacks
SSA
albedo ↓
Absorbedenergy
e.g. [Flanner and Zender, 2006]
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Positive feedbacks
SSA
Enhanced albedo decrease due to impurity
Absorbedenergy
e.g. [Doherty et al., 2013]
SSA
albedo ↓
Absorbedenergy
e.g. [Flanner and Zender, 2006]
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Colorado river basin [Painter et al., 2012 ; Skyles et al., 2012 ; Bryant et al., 2013]
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End of Little Ice Age [Painter et al., 2013]
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Greenland Ice Sheet [Dumont et al., 2014]
Darkening of the GrIS in late Spring due to impurities ?
Δm ≅ -27 Gt yr-1
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Modelling uncertainties
DISORT simulations, diffuse albedo
Flanner et al., 2012 formulation for BC internal mixture and physical properties
Model developed and used in Dumont et al., 2014 ; Ginot et al., 2014 ; Carmagnola
et al. 2013
SSA= 10 m²kg-1, ρ= 300 kg m-3, d=5 cm
SSA= 5 m²kg-1, ρ= 300 kg m-3, d=10 m
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Localisation of impurities
[Flanner et al., 2012]
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Impurities physical properties
[Flanner et al., 2012]
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Impurities refractive indices
[Dang et al., 2014]
Soot refractive index is relatively well constrained but its not the case for dust …
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Impurities refractive indices
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Impurities refractive indices
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Ice refractive index
[Warren and Brandt, 2008]
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Snow physical properties
Snow simulations
Accuracy of SSA measurements in the field
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Vertical profile of impurity content
Current measurements methods for impurity content in snow often limitsthe vertical resolution of the impuritycontent profile (volume needed for filtration for exemple)
The impurity content might vary a lot in the first centimeters of the snowpack (dry deposition, impurityconcentration due to melt …)
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Conclusion
All these simulations have been performed with the same dust content …
Albedo decrease varies between -0.01 and -0.09 ~ in the visible wavelengths