Today's rain-gauge networks in the Alpine region provide the spatially densest regular in-situ observations of a meteorological parameter in the area. Using such data, an analysis was performed of some basic elements of the current Alpine precipitation climate. Here we describe aspects of interest for the further planning of the MAP field phase.
The underlying dataset has been established by combining observations supplied by several national services (see the acknowledgements below). It comprises daily rain-gauge totals in the period 1973-1992 at more than 3000 stations (Fig. 1). Additional data for Italy and France is highly desirable to improve the analyses. Daily precipitation fields with a resolution of 28 km serve as a basis for the annual and monthly climatologies (Frei et al. 1995).
Figure 1. Location of rain-gauge stations from which data is
used for the climatology.
Figure 2. Annual mean precipitation [mm/day] for the period 1973-1992.
The main features of the annual precipitation (Fig. 2) are a wet band extending along the northern rim of the ridge, three wet zones to the south of the main crest (Mont Blanc massif, Ticino and Friaul area) and dryer conditions at inner Alpine areas (Wallis and Aosta valley, Tirol, Kärnten). Finer scale wet anomalies are found in connection with major mountain massifs and adjacent hill ranges (Black Forest, Jura, Appenines). The basic pattern agrees well with existing regional climatologies. However there is some uncertainty for the southern and western portions of the analysis domain, due to comparatively poor data coverage in these regions (see also Fliri 1974).
An analysis of the frequency of strong precipitation events was conducted at hand of the occurrence of daily totals larger or equal 20 mm. Here the frequencies were determined at individual stations and subsequently analysed. High activity is found mostly during the summer half year. During the tentative MAP special observing period an increased occurrence can be expected in particular in August and October (Fig. 3). In late summer such events appear to be associated mostly with thunderstrom activity, affecting in particular the northern Alpine rim and the southern wet zones. In October strong precipitation is frequent along the southern Alps and the Po valley, and often seems to be linked to orographically influenced systems associated with southerly flow situations.
The occurrence of strong precipitation is subject to considerable year-to-year variations. Fig. 4 displays the situation for two years with extreme high and low activity.
Figure 3. Frequency of days with strong precipitation (daily total \xb3 20 mm) for August (left) and October (right).
Figure 4. Frequency of days with strong precipitation from August till November in 1982 (left) and 1989 (right).
Between August and November 1982 strong precipitation has occured at a high rate (every 8 to 15 days) over the Po valley and the southern Alps. In contrast during fall 1989 this region has experienced very little occurrences (typically one strong event per 20 to 40 days). Higher activity is found, however, along the northern rim of the Alps.
The results of the analysis support the timing of the MAP field phase between late summer and late fall. Flexibility with respect to the location of special observing systems may help to reduce the adverse effects in case a year with rare interesting events is encountered.
Frei C., M. Widmann and C. Schär, 1995: Climatological database of daily precipitation in the Alpine region. Document and figures can be accessed via anonymous ftp on bach.ethz.ch (directory pub/rr_clim).
Fliri F., 1974: Niederschlag und Lufttemperatur im Alpenraum. Wissenschafl. Alpenvereinshefte, 24.
We are indepted to the following Institutes for providing access
to daily precipitation data: German Weather Service (Offenbach),
Hydrographisches Zentralbüro and ZAMG (Wien), (data status
January 1995), Meteo France (Toulouse), MeteoSwiss
(Zürich).
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