Mesoscale modelling of vertical atmospheric transport in the Alps associated with the advection of a tropopause fold – a winter ozone episode

The role of vertical atmospheric transport (VAT) for a winter ozone episode (January 1988) in the Alps, observed both on the mountain crest (3580 m asl) and in the lee of the Alps (209 m asl), is investigated. Numerical simulations were carried out with a doubly nested mesoscale model (14 km horizontal resolution), characterised by a refined orography scheme. A comparison between the modelled vertical profiles with features observed in the aerological soundings over Payerne indicated that the model is able to reproduce the fold in the Alps down to 650 hPa. Frictional processes are found to be responsible for the fragmentation of the lower parts of the fold when it is advected over the mountains, thus enhancing ozone concentrations at the mountain crest with likely cross-tropopause mixing. Vertically propagating gravity waves and strong mesoscale vertical winds (nordfoehn) are responsible for the further downward transport of ozone-rich air to the lower troposphere. This mechanism leads to ozone concentrations up to 70 ppb(v) in the leeside. The study is important because tropospheric ozone trends in the lee of the Alps have been inferred, in the past, from ozone peaks associated with such nordfoehn conditions. It is now shown that nordfoehn may entrain air which has been recently exchanged across the tropopause.