An improved parameterisation of ozone dry deposition to the ocean and its impact in a global climate–chemistry model
Schemes used to parameterise ozone dry deposition velocity at the oceanic surface mainly differ in terms of how the dominant term of surface resistance is parameterised. We examine three such schemes and test them in a global climate–chemistry model that incorporates meteorological nudging and month...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/3749/2017/acp-17-3749-2017.pdf |
| Summary: | Schemes used to parameterise ozone dry deposition velocity at the oceanic
surface mainly differ in terms of how the dominant term of surface resistance
is parameterised. We examine three such schemes and test them in a global
climate–chemistry model that incorporates meteorological nudging and
monthly-varying reactive-gas emissions. The default scheme invokes the
commonly used assumption that the water surface resistance is constant. The
other two schemes, named the one-layer and two-layer reactivity schemes,
include the simultaneous influence on the water surface resistance of ozone
solubility in water, waterside molecular diffusion and turbulent transfer,
and a first-order chemical reaction of ozone with dissolved iodide. Unlike
the one-layer scheme, the two-layer scheme can indirectly control the degree
of interaction between chemical reaction and turbulent transfer through the
specification of a surface reactive layer thickness. A comparison is made of
the modelled deposition velocity dependencies on sea surface temperature
(SST) and wind speed with recently reported cruise-based observations. The
default scheme overestimates the observed deposition velocities by a factor
of 2–4 when the chemical reaction is slow (e.g. under colder SSTs in the
Southern Ocean). The default scheme has almost no temperature, wind speed, or
latitudinal variations in contrast with the observations. The one-layer
scheme provides noticeably better variations, but it overestimates deposition
velocity by a factor of 2–3 due to an enhancement of the interaction between
chemical reaction and turbulent transfer. The two-layer scheme with a surface
reactive layer thickness specification of 2.5 µm, which is
approximately equal to the reaction-diffusive length scale of the ozone–iodide reaction, is able to simulate
the field measurements most closely with respect to absolute values as well
as SST and wind-speed dependence. The annual global oceanic deposition of
ozone determined using this scheme is approximately half of the original
oceanic deposition obtained using the default scheme, and it corresponds to a
10 % decrease in the original estimate of the total global ozone
deposition. The previously reported modelled estimate of oceanic deposition
is roughly one-third of total deposition and with this new parameterisation
it is reduced to 12 % of the modelled total global ozone deposition.
Deposition parameterisation influences the predicted atmospheric ozone mixing
ratios, especially in the Southern Hemisphere. For the latitudes
45–70° S, the two-layer scheme improves the prediction of ozone
observed at an altitude of 1 km by 7 % and that within the
altitude range 1–6 km by 5 % compared to the default scheme. |
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| ISSN: | 1680-7316 1680-7324 |