Quantifying transport in the troposphere and lower stratosphere

We study the transport of air within the troposphere and the lower stratosphere by following large numbers of air parcels using analysed wind fields. We define different regions (stratosphere, troposphere, boundary layer) using criteria based on combinations of potential vorticity (PV), water vapour and geometric height and consider the statistics of the time since visiting those regions as a function of position (defined using combinations of pressure, latitude, potential temperature and PV-based equivalent latitude) - in effect an age distribution.

We find that the troposphere is clearly defined as a region for which average times for transport from the boundary layer are relatively short (less than 30 days). These average times increase sharply across the tropopause and indeed the sharp increase provides a useful objective definition of the position of the tropopause (e.g. in terms of equivalent latitude as a function of potential temperature).

We compare the quantitative picture of transport obtained by our analysis with that obtained by previous analyses based on isentropic velocity fields. We use previous estimates of timescales for convective transport as a basis for discussion of the relative importance of convective and non-convective transport within the troposphere.