The following text is partly taken from the scientific objectives of the Mesoscale Alpine Programme (MAP):
The effect of the Alps on the precipitation distribution is well attested and evident in all seasons. In effect the orography creates specific patterns of ascending and descending air, which enhance or reduce precipitation. In the case of synoptic scale cyclonic activity, the orography in general enhances precipitation upstream and induces a drying effect downstream. In situations of weak synoptic activity convection in the Alps is quite common, as for example during the summer season when convection becomes the main source of precipitation. It is usually triggered via thermal circulations or other orographic effects, or may be prompted by upper-air anomalies advected into the area.
As regards precipitation forecast, there is increasing evidence that the correct specification of the surface moisture budget is a key aspect for the numerical prediction of the rain intensity. Thus, substantial effort is required to improve the representation of land-use, soil properties, soil moisture and snow cover in mesoscale models of the Alpine region. Especially in high mountainous areas, the exchange of moisture and heat with the atmosphere is highly influenced by the presence of snow cover
This can be clearly seen on the above infrared satellite image. The image has been taken in the afternoon, at 15 UTC, which is normally the time of maximum convective activity in the Alps. Synoptic activity on this day in the Alpine region is weak, but convective activity is very pronounced (also in the mountainous areas of Italy and former Yugoslavia). Looking closer at the situation in the Alps, numerous showers and thunderstorms appear on the northern and southern slopes of the Alps while the central parts, still covered to a large extent by snow, are not affected by convection. In fact, during the months of April and May, thunderstorm probability in the Pre-Alps and the surrounding plains is much higher than in the inner parts of the Alps.
This is also reflected in the precipitation climatology of the Alps, as elaborated by the group of Prof. Schär (ETH Zürich) within the project "Alpine Precipitation Analyses from High-Resolution Rain-Gauge Observations". This precipitation climatology covers the Alps at a resolution of about 25 km based on observations at one of the densest rain gauge networks over complex topography worldwide, embracing more than 6600 stations from the high-resolution networks of the Alpine countries. As an example, below you can find the climatology for the occurrence of strong precipitation (percentage of days with precipitation more than 20 mm) for the months of May and July. For the month of May, the number of days with strong (convective) precipitation in the inner Alps is clearly lower than in the Pre-Alps. In July, when the snow cover in the Central Alps is much reduced, strong convective rain also occurs in the inner Alpine areas.
of days (in %) in May for which the daily total precipitation exceeds the
threshold 20 mm (20 years reference period 1971-1990).The frequencies have
been determined individually for the time series and subsequently analysed
on the grid.
of days (in %) in July for which the daily total precipitation exceeds
the threshold 20 mm (20 years reference period 1971-1990).The frequencies
have been determined individually for the time series and subsequently
analysed on the grid.