TY - JOUR
T1 - Gas transport in firn: Multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland
AU - Buizert, C
AU - Martinerie, P
AU - Petrenko, VV
AU - Severinghaus, JP
AU - Trudinger, CM
AU - Witrant, E
AU - Rosen, JL
AU - Orsi, AJ
AU - Rubino, M
AU - Etheridge, DM
AU - Steele, LP
AU - Hogan, C
AU - Laube, JC
AU - Sturges, WT
AU - Levchenko, VA
AU - Smith, AM
AU - Levin, I
AU - Conway, TJ
AU - Dlugokencky, EJ
AU - Lang, PM
AU - Kawamura, K
AU - Jenk, TM
AU - White, JWC
AU - Sowers, T
AU - Schwander, J
AU - Blunier, T
PY - 2011
Y1 - 2011
N2 - Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO2). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the model tuning, and define a root mean square criterion that is minimised in the tuning. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1Ï Gaussian distribution. The modern day Î"age, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.
AB - Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO2). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the model tuning, and define a root mean square criterion that is minimised in the tuning. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1Ï Gaussian distribution. The modern day Î"age, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.
U2 - 10.5194/acpd-11-15975-2011
DO - 10.5194/acpd-11-15975-2011
M3 - Article
VL - 11
SP - 15975
EP - 16021
JO - Atmospheric Chemistry and Physics Discussions
JF - Atmospheric Chemistry and Physics Discussions
SN - 1680-7375
IS - 5
ER -