The water vapour mass balance (flux) of Switzerland

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Abstract

Just like a river or a glacier, the Swiss atmospheric water system has a “mass balance” - what comes in must equal what goes out, with storage in-between. The objective of this project was, therefore, to quantify the mass balance (or flux) of atmospheric water (mostly consisting of water vapour) over
Switzerland using a combination of ECMWF analysis data and MeteoSwiss rainfall observations (both rain gauge observations and precipitation radar). For this study, the Swiss atmosphere was considered to be a three-dimensional box, occupying the area from 46.0 to 48.0° N, 6.0 to 10.5° E, with fifteen atmospheric levels up to 150 hPa in height.
The results confirm that the water vapour flux through Switzerland is highly temporally variable, ranging from 1 to 5 X 10 7 kg/sec during quiet spells of weather, but increasing in size by a factor of ten or more during high speed currents of water vapour, known as “warm air conveyer belts“ or “atmospheric rivers“.
Overall, the water vapour flux into Switzerland (Wv in ) is generally greater than
the flux out (Wv out). However, the difference is only a small fraction (1% to 5%) of the total water vapour flux. This suggests that Switzerland's atmosphere “imports“ more water than it “exports“, but the amount gained at the surface through precipitation remains only a small fraction of the total
available water vapour passing by. Another important finding is that high inward water vapour fluxes (Wv in) are not necessarily linked to high precipitation episodes; a precipitation mechanism is also
needed.
A special case study of the water vapour flux during the August 2005 floods, which devastated large parts of Central Switzerland, is examined. There is a noticeable shortfall (~25%) in water vapour exiting Switzerland compared to that incoming, suggesting about a quarter of the incoming water vapour got
precipitated during the flood event (assuming zero evaporation at that time). During the same event, the water vapour mass balance (WV in – WV out) shows a better qualitative agreement with the mean ANETZ rain gauge observations, than with the MeteoSwiss radar precipitation product. This should not be
surprising given the limitations of using radar in a mountainous country, but it may also suggest that the radar product is underestimating precipitation in the mountains.
Original languageEnglish
PublisherInstitut für angewandte Physik, Universität Bern
Number of pages37
Publication statusPublished - Mar 2010

Publication series

Name2010-01-MW

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