Precipitation Trends in the European Alps – Current and Future Effects on Swiss River Basins

Katja Friedrich, Department of Atmospheric and Oceanic Sciences, U. of Colorado at Boulder

A 9-year (2000-2008) precipitation climatology (partially extended to 2011) for the central and western European Alps has been generated from ground-based operational weather radar data provided by the Swiss radar network. The research intent was to utilize operational radar data to analyze precipitation distribution in the European Alps with three main objectives: i) develop a horizontally distributed precipitation climatology for the years 2000-2008 and relate it to synoptic weather patterns, ii) leverage the relationship between precipitation distribution and synoptic patterns to downscale precipitation from global climate models (GCMs), and iii) determine the three-dimensional structure of precipitation between 2000-2011 including vertical structure, fraction of liquid to solid precipitation, and estimating trends on seasonal and decadal precipitation frequency and intensity. The analysis divides the Alps into six regions (each approximately 200 x 200 km2 in size) representing various orographic aspects and localized climates within the radar coverage area. The summer season has the highest total daily precipitation for all regions in the study with median values of daily precipitation in winter less than half of median daily precipitation for summer. Summer also has the strongest trend over all regions towards increasing total precipitation. High precipitation rate events occur most frequently in the summer, and the south and northwest Alpine regions have the greatest frequency of summertime high precipitation rate events. In all regions high precipitation rate events during summer qualitatively appear to have increased for 2003-2008 as compared to 2000-2002. Swiss river basins located in the northern and western Alps receive more precipitation when moisture is advected from the north or west, and moisture advected from the south increases precipitation totals for basins in southern Switzerland. Convective synoptic patterns, defined by a minimal surface pressure gradient, consistently follow the same trend for expected precipitation across all basins with an anticyclonic pattern resulting in the least amount of precipitation and the cyclonic pattern producing the most precipitation. It is the difference in precipitation received from the various convective patterns that has the greatest implication for future precipitation in Swiss river basins. Leveraging the relationship between precipitation distribution and synoptic patterns to downscale precipitation from GCM models indicate that the probability of heavy precipitation does not appear to be affected by dynamic changes over the 21st century for IPCC climate scenario A1B. Therefore, based on the dynamic changes total precipitation is expected to decrease, but heavy precipitation events account for a larger proportion of the total. Comparing the different climate scenarios (SRESA1B, SRESA2, SRESB1, Commit), the scenario with the strongest predicted increase in temperature (SRESA2) shows the strongest decrease in precipitation in all Swiss river basins based on the dynamic changes.

Thursday, 1 March 2012, 3:30 PM

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February 17, 2012 to March 1, 2012