Quantifying the effect of mixing on the mean age of air in CCMVal-2 and CCMI-1 models

Abstract

Abstract. The stratospheric age of air (AoA) is a useful measure of the overall capabilities of a general circulation model (GCM) to simulate stratospheric transport. Previous studies have reported a large spread in the simulation of AoA by GCMs and coupled chemistry-climate models (CCMs). Compared to observational estimates, simulated AoA is mostly too low. Here we attempt to untangle the processes that lead to the AoA differences between the models and between models and observations. AoA is influenced by both mean transport by the residual circulation and two-way mixing; we quantify the effects of these processes using data from the CCM inter-comparison projects CCMVal-2 (Chemistry-Climate Model Validation Activity 2) and CCMI-1 (Chemistry-Climate Model Initiative, phase 1). Transport along the residual circulation is measured by the residual circulation transit time (RCTT). We interpret the difference between AoA and RCTT as additional aging by mixing. Aging by mixing thus includes mixing on both the resolved and subgrid scale. We find that the spread in AoA between the models is primarily caused by differences in the effects of mixing and only to some extent by differences in residual circulation strength. These effects Published by Copernicus Publications on behalf of the European Geosciences Union. are quantified by the mixing efficiency, a measure of the relative increase in AoA by mixing. The mixing efficiency varies strongly between the models from 0.24 to 1.02. We show that the mixing efficiency is not only controlled by horizontal mixing, but by vertical mixing and vertical diffusion as well. Possible causes for the differences in the models' mixing efficiencies are discussed. Differences in subgrid-scale mixing (including differences in advection schemes and model resolutions) likely contribute to the differences in mixing efficiency. However, differences in the relative contribution of resolved versus parameterized wave forcing do not appear to be related to differences in mixing efficiency or AoA.

Description and Evaluation of the Specified-Dynamics Experiment in the Chemistry-Climate Model Initiative (CCMI)

“…As described earlier, the differences in the w * fields may be exaggerated by the fact they also potentially reflects inconsistencies in how that calculation was performed among modeling groups. Therefore, we also derive w * from continuity as outlined Dietmuller et al (2018) and, consistent with their results, we find that the independent derivation of w * using v * does produce noticeable differences in the values of w * (Supplementary Figure 2). (Note that, in order to facilitate comparisons with Figure S2) we also show averages over 20 • S and 20 • N).…”

“…In the stratosphere, the dynamical circulation is more naturally quantified using the Transformed Eulerian Mean (TEM) residual meridional (v * ) and vertical (w * ) velocities (Andrews et al, 5 1987). Following Dietmuller et al (2018) we note that since w * was calculated slightly differently among models, specifically with respect to the conversion of the Lagrangian tendency of air pressure, we also derived w * independently from v * by continuity, as in that study. Comparisons of w * between the model output and the values inferred from v * are presented in Section IV and result in no major differences with respect to our main findings.…”

“…We make the ages more comparable by subtracting off a reference mean age value, evaluated here as the mean age at 100 hPa, averaged over 10 • S to 10 • N. This also corrects for the fact that the stratospheric mean age tracer was implemented differently among different models, with some models applying the lower boundary condition globally at the surface (versus only in the tropics). A similar approach was used by Dietmuller et al (2018), except their reference was 20 defined relative to the tropical tropopause in each model. For our analysis of tropospheric variables we interpolated all output from native model levels to a standard pressure vector with four pressure levels in the stratosphere (10,30,50 and 80 hPa) and 19 pressure levels in the troposphere spaced every 50 hPa between 100 hPa and 1000 hPa (Orbe et al (2018)).…”

Description and Evaluation of the specified-dynamics experiment in the Chemistry-Climate Model Initiative

“…In the stratosphere, the dynamical circulation is more naturally quantified using the transformed Eulerian mean (TEM) residual meridional (v * ) and vertical (w * ) velocities (Andrews et al, 1987). Following Dietmüller et al (2018) we note that since w * was calculated slightly differently among models, specifically with respect to the conversion of the Lagrangian tendency of air pressure, Table 1. List of the REF-C1SD model simulations that were conducted in support of CCMI phase 1.…”

“…The goals of this study, therefore, are twofold: (1) document how the specified-dynamics hindcast simulations were implemented and (2) quantify key differences in first-order measures of the tropospheric and stratospheric dynamical and transport circulations. Via (2) our goal is to present a more comprehensive evaluation of the large-scale flow than presented in Orbe et al (2018) and Yang et al (2019) and to extend our analysis to the stratosphere, which has been evaluated in CCMI models primarily using the free-running REF-C1 experiment (Dietmüller et al, 2018). Note that, while Chrysanthou et al (2019) presented the first comparison of the stratospheric residual circulation among the nudged CCMI hindcast runs, our analysis, which complements the findings presented in that study, has a broader scope by focusing on both the troposphere and the stratosphere and including discussions of large-scale transport.…”