The present note discusses physical mechanisms which may contribute to cold air channelling close to the Alps. This involves the modification of the prefrontal air by the warm foehn air and of the postfrontal air by blocking effects resulting in an increase in precipitation. Additionally the influence of a sloping surface in the vicinity of the orography is considered. The problems are discussed in term of a north-south-oriented cold front behaving as an atmospheric gravity current propagating along the east-west oriented Alps.
A significant number of summertime cold fronts in southeast Australia have gravitycurrent-like features (Garratt et al., 1985), including those occurring along the east coast of Australia referred to as the Southerly Buster. Similar types of cold fronts have been observed in the vicinity of the Rocky Mountains (Shapiro et al., 1985). In Europe, many cold fronts change their propagation speed signific@y as soon as they approach the Alps with eastward-moving fronts being deformed, retarded or even diminished in intensity by the Alps. Some fronts propagate faster in the vicinity of the Alps than elsewhere (Figure 1). A rough estimate shows that close to the Alps the front in Fiiure 1 propagates at a speed of about 10 m s -' (between 06 and 12 UTC) whereas north of the Danube river the front moves with about half this speed. Observations suggest that at least some of these fronts have the local character of a gravity-current, but not all claims to this effect seem to be well-founded.
A foehn is a warm, dry, and gusty wind descending in the lee of a mountain range. North of the Alps, in the prefrontal region, there frequently is a foehn which leads to less cloud formation in the lower troposphere than usually expected in prefrontal areas. The warm foehn air originating from the south-westerly regions replaces the rest of the cold air which has been transported towards the Alps. The foehn air is very dry up to 3 to 4 km, and cannot support cloud formation or precipitation. This foehn effect can sometimes be observed up to a distance of 200 km north of the Alps, and is frequently noticed up to 50 km (Hoinka and Rosier, 1987). After the breakdown of the foehn, the polar air mass is advected towards the Alps behind the front and is blocked by the Alps, leading to frontal precipitation augmented by the orography, presumably because of uplift.
In this note, we show that frontal distortion through orographic channelling can occur under the influence of orography arising only from the thermodynamic effects of the