Relationship between radar-estimated precipitation and synoptic weather patterns in the European Alps

A 9-year (2000-2008) precipitation climatology for the central and western European Alps has been generated from ground-based operational weather radar data provided by the Swiss radar network. The radar-based climatology enables analysis of the relationship 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 localized climates within the radar coverage area. For each region, estimated rainfall rate derived from radar data is analyzed on a seasonal basis for total daily precipitation and frequency of high precipitation rate events. 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. No regional trends toward decreasing precipitation are found in any season. The fall season has the fewest regional trends in precipitation, except for the area south of the Alps where precipitation increases. 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 rainfall rate events during summer qualitatively appear to have increased for 2003-2008 as compared to 2000-2002. Daily synoptic scale weather patterns are associated with total daily precipitation and high precipitation rate events to show that an advective synoptic scale pattern with southerly mid-tropospheric flow results in the highest median and 90th quantile values for total daily precipitation while a convective synoptic scale pattern results in elevated frequency of extreme rainfall rate events. A probability distribution of precipitation occurrence and quantity is developed with resolution of 2x2 km2 and related to synoptic weather patterns. Global climate model (GCM) output for the 21st century is coupled with the observed relationship between precipitation distribution and synoptic patterns to provide expected precipitation for a Ticino river basin sub-catchment (76 km2); thereby obtaining expected precipitation at high resolution from a much coarser resolution GCM.