Kinematic and diabatic vertical velocity climatologies from a chemistry climate model

Abstract

Abstract. The representation of vertical velocity in chemistry climate models is a key element for the representation of the large-scale Brewer-Dobson circulation in the stratosphere. Here, we diagnose and compare the kinematic and diabatic vertical velocities in the ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model. The calculation of kinematic vertical velocity is based on the continuity equation, whereas diabatic vertical velocity is computed using diabatic heating rates. Annual and monthly zonal mean climatologies of vertical velocity from a 10-year simulation are provided for both kinematic and diabatic vertical velocity representations. In general, both vertical velocity patterns show the main features of the stratospheric circulation, namely, upwelling at low latitudes and downwelling at high latitudes. The main difference in the vertical velocity pattern is a more uniform structure for diabatic and a noisier structure for kinematic vertical velocity. Diabatic vertical velocities show higher absolute values both in the upwelling branch in the inner tropics and in the downwelling regions in the polar vortices. Further, there is a latitudinal shift of the tropical upwelling branch in boreal summer between the two vertical velocity representations with the tropical upwelling region in the diabatic representation shifted southward compared to the kinematic case. Furthermore, we present mean age of air climatologies from two transport schemes in EMAC using these different vertical velocities and analyze the impact of residual circulation and mixing processes on the age of air. The age of air distributions show a hemispheric difference pattern in the stratosphere with younger air in the Southern Hemisphere and older air in the Northern Hemisphere using the transport scheme with diabatic vertical velocities. Further, the age of air climatology from the transport scheme using diabatic vertical velocities shows a younger mean age of air in the inner tropical upwelling branch and an older mean age in the extratropical tropopause region.

Effects of mixing on resolved and unresolved scales on stratospheric age of air

“…The different calculation frameworks for EMAC and CLaMS data (kinematic vs. diabatic vertical velocities) causes differences in the results, with a noisier structure for the kinematic vertical velocity (see Hoppe et al, 2016). However, as the internal vertical coordinates in EMAC and CLaMS are pressure and potential temperature, respectively, calculating residual circulation and mixing diagnostics in the two different coordinate systems is more consistent with the respective model simulation.…”

“…Moreover, the choice of the vertical coordinate (pressure or potential temperature) may influence the AoA pattern (e.g., Mahowald et al, 2002;Hoppe et al, 2016). The recent work of Hoppe et al (2016) investigated the differences in AoA of two CCM simulations with the same underlying model, but one using a flux-form semiLagrangian scheme and corresponding kinematic vertical velocities and another simulation using a Lagrangian scheme and diabatic vertical velocities. They found out, that the difference pattern of AoA can be attributed both to the different vertical velocities and to the different transport schemes, leading to differences in aging by mixing.…”

ATTILA 4.0: Lagrangian Advective and Convective Transport of Passive Tracers within the ECHAM5/MESSy (2.53.0) Chemistry Climate Model

“…9), most pronounced near 25 the poles below 50 hPa. This difference is attributed to the Eulerian vertical velocity used in the flux-form semi-Lagrangian transport scheme for the GP simulations, that shows up-and downwelling at different high latitudes, which are not related to the net tracer transport (Hoppe et al, 2016).…”

“…In the vertical direction we may use either η-coordinate vertical velocities (η = p p0 ,η-kinematic velocity) calculated from the horizontal flux divergence using the continuity equation, or isentropic coordinates ξ , where the vertical velocitiesξ are calculated from the EMAC diabatic heating rates (diabatic velocity). The kinematic velocity is provided by default from EMAC, whereas the diabatic velocity was newly implemented similar to Eluszkiewicz et al (2000) and Hoppe et al (2016).…”

Reply to E. Ray

“…The core atmospheric model of EMAC is the 5th generation of the European Centre Hamburg general circulation model ECHAM5 (Roeckner et al, 2006). Atmospheric tracer management in MESSy is treated with the submodel TRACER (Jöckel et al, 2008), providing an interface structure (memory and data management) to couple chemical processes with the base model. In the standard setup of EMAC, tracers are transported by the flux-form semi-Lagrangian (FFSL) transport scheme of Lin and Rood (1996).…”