Tropospheric control of tropopause folding and the efficiency of mixing across tropopause in idealized simulations of baroclinc-wave life cycles

We investigate the sensitivities of tropopause folding and the efficiency of stratosphere-troposphere exchange along a series of idealised atmospheric jet streams using a mesoscale non-hydrostatic simulation model, from which moist processes and boundary layer parameterisations have been removed. The activity of non-isentropic mixing is highly localised and clearly structured due to isolated events that have been triggered naturally by a variety of hydrodynamic instabilities. The most active irreversible mixing is observed along the surface front where the density and thermal contrast are highest, followed by the region along the tropopause. In particular, the structure of the potential vorticity in the entire troposphere is practivally insensitive to the change of stratostification in the stratosphere. This includes the structure of the tropopause folds. We also find the stratosphere-troposphere (S-T) exchanges of net heat and mass are always from the troposphere into the stratosphere, and the most efficient S-T exchange occurs when the stratospheric stratification is set to the climatological level. We also find the climatological background state leads to the minimum non-isentropic mixing in terms of the integral over the whole domain. It appears that the climatological characteristics of the lower stratosphere are optimised for optimal efficiency of S-T exchange. This might be taken to imply that the S-T exchange and the level of stratification in the lower stratosphere are also controlled by troposphere, although further study is needed for confirmation.