The thermal and dynamical tropopause definition in baroclinic wave life-cycles

In this study we introduce a new tropopause definition, based on mixing considerations, and discuss the effect of baroclinic wave-breaking on tropopause height and sharpness. Accurate identification of the tropopause is important for many applications ranging from estimates of dynamical stratosphere-troposphere exchange to the retrieval of atmospheric composition. However, current definitions of the tropopause rely an empirically chosen threshold values. Moreover the evolution of the thermal and the dynamical tropopause during a baroclinic wave life-cycle is not consistent. Here we propose an objective way to identify the tropopause that does not rely on an empirically chosen parameter. The physical basis of our method is a change in mixing characteristics, which is reflected in the gradients of materially conserved quantities. Potential temperature can be used to define an equivalent to the thermal tropopause, and PV to define a dynamical tropopause. Using this method, we provide an analysis of the tropopause evolution during baroclinic wave--breaking in high resolution ECMWF analysis data and idealized baroclinic life--cycle experiments. The height and sharpness of the tropopause shows high spatial and temporal variability and we find a net tropopause rise during wave--breaking. Furthermore, we show that the tropopause, defined according to our method, characterizes the vertical structure much better, and the evolution of the dynamical and thermal tropopause is more consistent; major differences are limited to regions where the tropopause is physically not meaningful. Finally we comment on some implications for stratosphere-troposphere exchange and the fine-scale structure of the tropopause.