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R. Kouznetsov et al.
AoA can be inferred form the observations of some tracer that changes in the
troposphere and then remains unchanged along the transport of an air parcel in the
stratosphere. One of such tracers is sulphur hexafluoride SF 6 that is practically passive
in the troposphere and in stratosphere. SF 6 accumulates in the atmosphere and have
increased by more than an order of magnitude since 1960s [7], and thus the delay
between SF 6 mixing ratio in troposphere and in arbitrary point in stratosphere gives
AoA at that point. The AoA derived from observations of an atmospheric tracer in
assumption of its passivity is called “apparent AoA”, which is subject for corrections
to get “true” AoA [3].
There exist substantial discrepancies between the AoA derived from observations
and AoA derived from various modelling approaches: the experimental studies [3]
report ages derived from SF 6 observations that exceed 10 years in polar regions,
whereas the model studies [2, 6] report them below 6 years. In the current study
we used the SILAM Chemistry-Transport Model (CTM) driven by ERA-interim
reanalysis to simulate both AoA and the procedure of its evaluation from observations
of SF 6 distribution in the stratosphere. The main goal was to identify the causes of
disagreement between simulations and observations of AoA and quantify relative
importance of mesospheric depletion of SF 6 , gravitational separation of SF 6 in upper
stratosphere for accurate inferring of AoA from observations.
11.2 Materials and Methods
SILAM (System for Integrated modeLling of Atmospheric coMposition) is an offline chemistry-transport model [8]. Global Eulerian simulations were performed for
the period of 1980–2015 with 1.44
◦ resolution. The model was driven with ERAinterim reanalysis [1], that has T255 spectral resolution (∼0.72
◦ ) and 60 hybrid
sigma-pressure levels [1], having nominal pressure at the uppermost level of 10 Pa.
Same levels were used in SILAM, except for the uppermost layer bounded with
pressure levels of 20 and 10 Pa.
In this study two SF 6 tracers are used: fully passive “sf6pass” and “sf6” that is
subject for gravitational separation and destruction in the mesosphere, that was implemented via effective lifetime of 30 days in the uppermost model layer. Both tracers
had the same emissions [7]. The inventory covers 1970–2008, and has been extrapolated with linearly growing trend until 2015. The reference AoA was derived from
“ideal age” [10], a tracer whose mixing ratio linearly increases with time everywhere
and continuously forced to zero at the surface.
The simulation results has been evaluated with the results of SF 6 retrieval from
the limb-viewing MIPAS instrument [9] operated on-board of the Envisat satellite
in 2002-2012. The conversion of retrieved SF 6 to AoA was performed by referring
to a global mean of in situ SF 6 measurements at the surface as provided by NOAA/
ESRL [4].
R. Kouznetsov et al.
AoA can be inferred form the observations of some tracer that changes in the
troposphere and then remains unchanged along the transport of an air parcel in the
stratosphere. One of such tracers is sulphur hexafluoride SF 6 that is practically passive
in the troposphere and in stratosphere. SF 6 accumulates in the atmosphere and have
increased by more than an order of magnitude since 1960s [7], and thus the delay
between SF 6 mixing ratio in troposphere and in arbitrary point in stratosphere gives
AoA at that point. The AoA derived from observations of an atmospheric tracer in
assumption of its passivity is called “apparent AoA”, which is subject for corrections
to get “true” AoA [3].
There exist substantial discrepancies between the AoA derived from observations
and AoA derived from various modelling approaches: the experimental studies [3]
report ages derived from SF 6 observations that exceed 10 years in polar regions,
whereas the model studies [2, 6] report them below 6 years. In the current study
we used the SILAM Chemistry-Transport Model (CTM) driven by ERA-interim
reanalysis to simulate both AoA and the procedure of its evaluation from observations
of SF 6 distribution in the stratosphere. The main goal was to identify the causes of
disagreement between simulations and observations of AoA and quantify relative
importance of mesospheric depletion of SF 6 , gravitational separation of SF 6 in upper
stratosphere for accurate inferring of AoA from observations.
11.2 Materials and Methods
SILAM (System for Integrated modeLling of Atmospheric coMposition) is an offline chemistry-transport model [8]. Global Eulerian simulations were performed for
the period of 1980–2015 with 1.44
◦ resolution. The model was driven with ERAinterim reanalysis [1], that has T255 spectral resolution (∼0.72
◦ ) and 60 hybrid
sigma-pressure levels [1], having nominal pressure at the uppermost level of 10 Pa.
Same levels were used in SILAM, except for the uppermost layer bounded with
pressure levels of 20 and 10 Pa.
In this study two SF 6 tracers are used: fully passive “sf6pass” and “sf6” that is
subject for gravitational separation and destruction in the mesosphere, that was implemented via effective lifetime of 30 days in the uppermost model layer. Both tracers
had the same emissions [7]. The inventory covers 1970–2008, and has been extrapolated with linearly growing trend until 2015. The reference AoA was derived from
“ideal age” [10], a tracer whose mixing ratio linearly increases with time everywhere
and continuously forced to zero at the surface.
The simulation results has been evaluated with the results of SF 6 retrieval from
the limb-viewing MIPAS instrument [9] operated on-board of the Envisat satellite
in 2002-2012. The conversion of retrieved SF 6 to AoA was performed by referring
to a global mean of in situ SF 6 measurements at the surface as provided by NOAA/
ESRL [4].