A radar-based climatology of high precipitation events in the European Alps: 2000-2007

Characterizing and quantifying precipitation on temporal and spatial scales in mountainous regions is essential for better understanding of the processes that modulate precipitation intensity, distribution, and type. Knowledge about precipitation variability on annual, decadal, and centennial time scales and the linkage to large-scale atmospheric circulation sets the basis for assessing the impact of climate change on alpine precipitation and validating regional and global climate models. To better understand regional precipitation variability and trends in the European Alps, an 8 year (2000-2007) precipitation climatology has been generated from ground-based operational weather radar data provided by the Swiss radar network. As compared to a rain gauge based precipitation climatology, the radar-based climatology has the advantage of providing finer spatial and temporal resolution as well as vertical information. The radar-based climatology that is developed has resolution of 2 x 2 km² and enables analysis of the relationships between synoptic scale flow and mesoscale precipitation patterns over complex alpine terrain. The analysis divides the Alps into six regions (each approximately 200 x 200 km² in size), one on the Northern, two each on the Western and Southern side of the Alps, and one in the Massif Central, representing various orographic aspects and regional climates within the radar coverage area. For each region, estimated rainfall rate derived from radar data is analyzed for duration, extent, and frequency of high precipitation events on an annual and seasonal basis. The summer season shows the greatest frequency and geographic extent of high precipitation events for all regions in the study with winter having the longest interval between high precipitation events. The region south of the Alps was found to have the greatest frequency of high precipitation events. The fine spatial scale of the radar coverage also enables correlation of high precipitation events to topographic aspect and elevation. Future work will analyze vertical structure for variation in snow/rain ratio: a precipitation characteristic with particular importance for hydrological parameters such as runoff, water storage, and glacier retreat. In addition, further study is planned to correlate topography and upstream conditions at the synoptic, meso-, and convective scale to precipitation patterns and characteristics within each region.