Mountains have a profound influence on our weather and hence climate. They act as a natural barrier on which weather systems can be deflected, modified, intensified, and newly build.  Figure 1 shows rain fall rate over the Alps measured by the Swiss weather radars. In the afternoon, convective cells indicate prior to the arrival of a cold front. During the night and in the early morning, the cold front arrived and precipitation is strongly modified.

Often local circulation pattern which we investigated is called Alpine pumping (Fig. 2) which is hypothesized to play a fundamental role in the vertical transport of pollutant. During the day a thermal circulation develops which transports boundary layer air and pollutant form Munich towards the Alps, and through the development of thunderstorms at higher altitudes. A sequence of this event is shown in Fig. 3 and can also be seen in the NE corner of Fig. 1.

Meteorological observations within mountains are challenging. The accuracy rain fall rate estimations using weather radars are often effected by the reflection of radar signals from obstacles (Fig. 4).  In the next several years, large numbers of polarimetric weather radars will be installed in mountainous terrain. Polarimentric measurements require very accurate measurements. Up to this point the influence of mountain returns on the accuracy of polarimentric measurements has hardly been investigated. The project Radar de France-Compté aimed in investigating this aspect and evaluating the added benefit of polarimetry in mountains. Investigations revealed that the quality of polarimetric measurements will be decreased even when the intensity of ground clutter is lower than that of rain. In order to meet the accuracy requirements for instance for rainfall rate estimation, data need to be corrected. Otherwise, the size of observational area close to the ground level will strongly decrease (Fig. 5).