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Ground-based digital imager systems in the visible and near infrared region of the solar spectrum have the potential to nicely complement existing instruments and observation networks of National Weather Services with very accurate, high spatial and temporal resolution, 2D and 3D macroscopic cloud data such as cloud amount, cloud-base height and 3D cloud-base motion. This paper discusses two current approaches to ground-based cloud sensing: the prototype instrument used at ETH/MeteoSwiss within Cloudmap and Cloudmap2 for stereoscopy tests, and a Daylight Visible/NIR Whole Sky Imager (WSI) system developed and fielded by the Scripps Institution of Oceanography (SIO). The article includes descriptions of the radiometric and geometric calibration methods. Cloud amount, cloud-base height and cloud-base motion results from two ETH/ MeteoSwiss measurement campaigns and from the operational WSI use at the German Weather Service (DWD) are shown. Finally, a case study with coincident satellite and ground data illustrates that ground-based digital imager systems are an interesting technique to validate satellite-based cloud-top heights and cloud-top motion winds of vertically thin clouds.

This paper summarizes the findings of seven years of research on föhn conducted within the project 'Föhn in the Rhine Valley during MAP' (FORM) of the Mesoscale Alpine Programme (MAP). It starts with a brief historical review of föhn research in the Alps, reaching back to the middle of the 19th century. Afterwards, it provides an overview of the experimental and numerical challenges identified before the MAP field experiment and summarizes the key findings made during MAP in observation, simulation and theory. We specifically address the role of the upstream and cross-Alpine flow structure on föhn at a local scale and the processes driving föhn propagation in the Rhine Valley. The crucial importance of interactions between the föhn and cold-air pools frequently filling the lower Rhine Valley is highlighted. In addition, the dynamics of a low-level flow splitting occurring at a valley bifurcation between the Rhine Valley and the Seez Valley are examined. The advances in numerical modelling and forecasting of föhn events in the Rhine Valley are also underlined. Finally, we discuss the main differences between föhn dynamics in the Rhine Valley area and in the Wipp/Inn Valley region and point out some open research questions needing further investigation.

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The Rhine valley, which stretches from the main Alpine crest to the Lake of Constance, was chosen as the target area to study unstationary aspects of foehn during the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP). This large valley is up to 10?km wide and has some of the highest foehn frequencies in the European Alps. The MAP subprogram FORM (FOehn in the Rhine valley during MAP) was designed to investigate various aspects of the foehn including the interaction of foehn flow with the boundary layer and the processes that remove the cold air pool. The subprogram was also focused on improving the understanding and forecasting of foehn-related phenomena such as waves and turbulence. A large number of in-situ and remote sensing observing systems were deployed to take measurements during the field phase of MAP. Among them were about 50 surface stations, up to 9 radiosonde stations, 2 wind profilers, 4 Doppler sodars, 2 scintillometers, 1 scanning and 1 backscatter lidar and different research aircraft. This paper gives an overview of the objectives of FORM, describes the target area and its instrumentation, and provides a detailed synoptic description of the 12 foehn cases observed during the MAP SOP

Abstract. Snow cover plays a vital role in the SwissAlps and therefore it is of major interest to determine and understand its variability on different spatiotemporal scales. Within the activities of the National Climate Observing System (GCOS Switzerland) inter-annual variations of snow days over Switzerland were derived from 2000 to 2010 based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite. To minimize the impact of cloud cover on the MODIS snow product MOD10C1, we implemented a post-processing technique based on a forward and backward gap-filling approach. Using the proposed methodology it was possible to determine the total number of annual snow days over Switzerland from 2000 to 2010 (SCD MODIS ). The accuracy of the calculated snow days per year were quantitatively evaluated against three in situ snow observation sites representing different climatological regimes (SCD in situ ). Various statistical indices were computed and analysed over the entire period. The overall accuracy between SCD MODIS and SCD in situ on a daily basis over 10 yr is 88 % to 94 %, depending on the regional characteristics of each validation site. Differences between SCD MODIS and SCD in situ vary during the snow accumulation period in autumn and smaller differences after spring, in particularly for the Central Alps.