Large sample evaluation of cumulative rainfall amounts in the Alps using a network of three radars

Quantitative precipitation estimation in complex terrain is surely a challenge. Many papers have been published on the use of radar measurements to estimate surface rainfall during intense events lasting a few days, but little work has been dedicated to large data sets spanning several months and tens of thousands of km2. This paper presents the analysis of 2 years of radar and gauge data measured in Switzerland (from December 2000 to November 2002). The analysis is based on the operational MeteoSwiss radar product RAIN, which combines radar measurements from a total of 20 elevations, to obtain the best estimate of surface precipitation in real time. The resolution is 5 min and 1 km. The data processing includes automatic calibration, 7-step clutter elimination, correction for partial shielding and profile effects, as well as long-term radar-gauge adjustment. The root mean square areal difference between the radar and in situ measurements, rms(AD), is of the order of 1700 mm (the average gauge total of the 2-year period is 3031 mm). This figure is an average of a 39,500 km2 area (427 gauges) that includes mountainous areas with bad radar visibility. A bulk adjustment reduces the rms(AD) to ∼900 mm. If an adjustment based on a non-linear weighted multiple regression (WMR) is used, the rms(AD) decreases to ∼700 mm. A modified form of the WMR is able to further reduce the rms(AD) to ∼400 mm. The results of this and other studies have been used to modify the RAIN algorithm in March 2003 and February 2004, and thus better radar-gauge agreement is expected for the year 2004.