The Advection-Condensation Paradigm for Stratospheric Water Vapour

Atmospheric water vapour is of paramount importance for climate. The advection-condensation (A-C) paradigm postulates that water vapour concentrations are governed to leading order by the transport through the 4-dimensional temperature field, and condensate is assumed to be instantaneously removed. Brewer's (1949) qualitative deduction of the stratospheric circulation based on water vapour measurements is a prominent application of this paradigm. We evaluate the quantitative validity of the A-C paradigm by predicting water vapour distribution in the upper troposphere/lower stratosphere from the minimum saturation mixing ratio of trajectories ending in the stratosphere, calculated using ECMWF reanalysis data. We show that results are sensitive to small differences in temperatures and wind fields, and that interpretation of results (in terms of effects from deliberately neglected processes) requires a careful error calculation. Further, the timescale of troposphere-to-stratosphere transport and vertical dispersion affect results. The probability to encounter very low temperatures increases with slower ascent, and/or less vertical dispersion. Depending on error scenario, we find a dry bias of 20%-50% for the stratospheric overworld. Taking smaller scale temperature fluctuations (e.g. gravity waves) into account increases the bias by 10%. Our analysis suggests that the A-C paradigm is a very valuable concept for theoretical advances on atmospheric moisture distributions, but that the finite sedimentation of condensate (leading to re-evaporation) plays a substantial role at least for the stratospheric water budget. Finally, we analyse in detail the relationship between transport and water vapour in the context of troposphere-to-stratosphere transport (TST).