1Max Planck Institute for Meteorology, Hamburg, Germany
2International Max Planck Research School on Earth System Modelling, Hamburg, Germany
*now at: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Abstract. Atmospheric water vapour tracers (WVTs) are an elegant tool to determine source-sink relations of moisture "online" in atmospheric general circulation models (AGCMs). However, it is sometimes desireable to establish such relations "offline" based on already existing atmospheric data (e.g. reanalysis data). One simple and frequently applied offline method is 2-D moisture tracing. It makes use of the "well-mixed" assumption, which allows to treat the vertical dimension integratively.
Here we scrutinise the "well-mixed" assumption and 2-D moisture tracing by means of analytical considerations in combination with AGCM-WVT simulations. We find that vertically well-mixed conditions are seldomly met. Due to the presence of vertical inhomogeneities, 2-D moisture tracing (I) neglects a significant degree of fast-recycling, and (II) results in erroneous advection where the direction of the horizontal winds varies vertically. The latter is not so much the case in the extratropics, but in the tropics this can lead to large errors. For example, computed by 2-D moisture tracing, the fraction of precipitation in the Western Sahel that originates from beyond the Sahara is ~40%, whereas the fraction that originates from the tropical and Southern Atlantic is only ~4%. Full (i.e. 3-D) moisture tracing however shows that both regions contribute roughly equally, which reveals the results of an earlier study as spurious.
Moreover, we point out that there are subtle degrees of freedom associated with the implementation of WVTs into AGCMs because the strength of mixing between precipitation and the ambient water vapour is not completely provided by such models. We compute an upper bound for the resulting uncertainty and show that this uncertainty is smaller than the errors associated with 2-D moisture tracing.