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.
Highlights
The numerical representation of vertical velocity in meteorological models can be established in various ways
This work presents diagnostics to obtain the vertical velocity of the tracer transport scheme in the chemistry climate models (CCMs) EMAC (Röckner et al, 2006; Jöckel et al, 2010), and in the coupled model system EMAC–CLaMS (Chemical Lagrangian Model of the Stratosphere) (Hoppe et al, 2014) in Sect
This vertical velocity ωFFSL differs from the vertical velocity ωspec deduced from the wind field, since different advection schemes are used for the air-mass density and for trace gases: the spectral advection is used for air-mass density, whereas the grid-pointbased flux-form semi-Lagrangian transport scheme (FFSL) transport is used for the tracers
Summary
The numerical representation of vertical velocity in meteorological models can be established in various ways. If a pressure-based vertical coordinate system is implemented, the associated vertical velocity ω is calculated as a residual from the horizontal flux divergence using the continuity equation This method is denoted kinematic vertical velocity representation and most commonly used in CCMs. The potential temperature θ can be used as the vertical coordinate in a model, forming isentropic vertical model layers. They occur due to numerical discretization of the underlying equations, limited accuracy of representation of numbers in computers, and parametrizations of sub-grid scale processes These inaccuracies lead to differences in vertical velocity fields when using different vertical velocity representations. This work presents diagnostics to obtain the vertical velocity of the tracer transport scheme in the CCM EMAC (Röckner et al, 2006; Jöckel et al, 2010), and in the coupled model system EMAC–CLaMS (Chemical Lagrangian Model of the Stratosphere) (Hoppe et al, 2014) in Sect.
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