Séminaire
The advection-condensation (A-C) paradigm (e.g. Pierrehumbert and Roca [2006]) is a starting point for a theoretical framework for analysis of atmospheric water vapour distributions. It postulates that water vapour concentrations are governed to leading order by the transport through the full 4-dimensional temperature (hence saturation mixing ratio) eld, and the condensates formed by moisture in excess of saturation is assumed to be instantaneously removed by sedimentation. Brewer's [1949] deduction of the stratospheric circulation based on water vapour measurements may be considered the rst successful application of this paradigm. In this work we carry out a quantitative evaluation of the validity of the A-C paradigm for stratospheric water vapour. We predict stratospheric water vapour values using trajectories calculated with di erent ECMWF reanalysis data sets, and use a semi-empirical approach to analyse errors in the Lagrangian pre- diction of water vapour. We nd the results are sensitive to seemingly small di erences in temperatures and wind elds, and that the stratospheric wa- ter vapour predictions based on the A-C paradigm have a consistent dry bias of 20%-50%, which might be attributed to processes neglected by the model (e.g. re-evaporation of condensate). Interestingly, a simple correction with a constant frostpoint temperature o set can accommodate much of the di erence between model and observation.
Riwal Plougonven
LMD-ENS, 24 rue Lhomond, 75 005 Paris,
tel 01 44 32 27 31, mel plougon@lmd.ens.fr