Assessing the mean atmospheric circulation with non-ideal winds

The TEM equations are a longstanding concept for interpreting middle atmospheric dynamics and explain especially the residual circulation as a circulation induced by small-amplitude waves with the help of the residual winds $v^*$ and $w^*$. The actual topic of the talk deviates from the small-amplitude wave viewpoint of the TEM equations and takes instead a viewpoint that considers nonlinear disturbances which are not necessarily waves in the sense that they exhibit a dispersion relation.

A flow is not accelerated in neither dynamical variable if it resides in a perfectly nonlinear balanced state. We denote the accompanying wind as the ideal wind. Any deviation from this ideal wind may contribute to a disturbance-induced acceleration of the flow. The deviations from the ideal wind are called non-ideal winds. The zonally averaged non-ideal winds are demonstrated to be a nonlinear generalization of the TEM residual winds.

This approach has two main advantages. Firstly, it is not confined to zonal means and allows thus for the analysis of data from regional domains (as for instance from regional models), and secondly, it describes flow phenomena that are further away from small-amplitude perturbations from geostrophic balance. A striking example of the latter advantage is the mean flow in the planetary boundary layer. There, the TEM residual winds describe a questionable equatorward return flow that can not explain frictional induced inflow and outflow for lows and highs, respectively.

The Held-Suarez test is performed with the ICON-IAP model in order to compare the diagnostics for the two mentioned methods. Similarities and differences are discussed. For this testcase it is possible to disentangle eddy, diabatic, and friction induced contributions to the non-ideal winds.

The future potential for applications of the new non-ideal wind diagnostics are three-dimensional aspects of the temporally averaged circulation and diagnostics in non-periodic domains as they are needed for regional-scale model output.