Improving operational measurement of precipitation using radar in mountainous terrain-Part I: methods

Four major rainfall events that affected the Western Alps between autumn 1994 and autumn 2000 are analyzed to assess the bias between the radar rainfall estimates at rain gauge locations and the gauge amounts. The aim of this study is to demonstrate the importance of: 1) bias adjustment; 2) the training procedure used to train various adjustment methods by means of independent data; and 3) a quality check of the radar-gauge couples that were used for the training itself. A first adjustment method is simply based on a single "bias-correction" coefficient. A weighted multiple regression (WMR) is well worth the additional effort of determining three additional coefficients, which give a spatial distribution of the adjustment factor rather than a constant one for the whole domain as the output. The independent dataset that was used to train the gauge-adjustment techniques consists of daily radar/gauge amounts accumulated during the first day of each event. The following days are used for an independent verification that is dealt with in a companion letter, which will validate the methods and illustrate the improvements and the feasibility of a real-time application during intense events. The WMR technique tries to correct not only the overall bias but also the beam-broadening, visibility, and orography influences. The training procedure of both the bulk- and WMR-adjustment methods highlighted a considerable radar underestimation, which is certainly not surprising in mountainous terrain. The WMR-derived coefficients also clearly show that the radar underestimates precipitation for higher sampling volumes and longer distances. Since the WMR is fast and simple to use, it represents an alternative to more sophisticated methods and seems to be particularly useful for operational services.