Effects of mixing on resolved and unresolved scales on stratospheric age of air
Mean age of air (AoA) is a widely used metric to describe the transport along the Brewer–Dobson circulation. We seek to untangle the effects of different processes on the simulation of AoA, using the chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) and the Chemical Lagrangian Mode...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/7703/2017/acp-17-7703-2017.pdf |
| Summary: | Mean age of air (AoA) is a widely used metric to describe the
transport along the Brewer–Dobson circulation. We seek to untangle the
effects of different processes on the simulation of AoA, using the
chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) and the Chemical Lagrangian Model of the Stratosphere
(CLaMS). Here, the effects of residual transport and two-way mixing on AoA are
calculated. To do so, we calculate the residual circulation transit time
(RCTT). The difference of AoA and RCTT is defined as aging by mixing.
However, as diffusion is also included in this difference, we further use a
method to directly calculate aging by mixing on resolved scales. Comparing
these two methods of calculating aging by mixing allows for separating the
effect of unresolved aging by mixing (which we term <q>aging by diffusion</q> in
the following) in EMAC and CLaMS. We find that diffusion impacts AoA by
making air older, but its contribution plays a minor role (order of 10 %)
in all simulations. However, due to the different advection schemes of the
two models, aging by diffusion has a larger effect on AoA and mixing
efficiency in EMAC, compared to CLaMS. Regarding the trends in AoA, in CLaMS
the AoA trend is negative throughout the stratosphere except in the Northern
Hemisphere middle stratosphere, consistent with observations. This slight
positive trend is neither reproduced in a free-running nor in a nudged
simulation with EMAC – in both simulations the AoA trend is negative
throughout the stratosphere. Trends in AoA are mainly driven by the
contributions of RCTT and aging by mixing, whereas the contribution of aging
by diffusion plays a minor role. |
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| ISSN: | 1680-7316 1680-7324 |