GPM Dual-frequency Precipitation Radar in the Swiss Alps

Precipitation in mountainous regions is of primary importance for water resources but also for natural hazards (floods, landslide and avalanches are all significantly driven by precipitation). The interactions between atmospheric flow and the complex topography result in strong variability in cloud and precipitation features, with more frequent solid precipitation at altitude. Combined with more difficult monitoring due to remoteness and harsh conditions, mountainous precipitation remains poorly understood and challenging to accurately quantify. Compared to ground-based monitoring networks, spaceborne sensors have the definite advantage of a top-down view over mountainous regions. On the other hand, the typically larger footprint strongly affects the quality of satellite precipitation estimates close to the ground. Moreover, satellite remote sensing is often the only source of information in countries which cannot afford expensive operational precipitation observation networks. In this contribution, the quality and accuracy of the precipitation estimates derived from the Global Precipitation Measurement (GPM) dual-frequency precipitation radar (DPR) are investigated in the Swiss Alps, where the operational precipitation monitoring network of rain gauges and weather radars, complemented by dedicated field campaigns, is dense enough to provide reference data in order to quantitatively evaluate the quality of GPM reflectivity measurements and precipitation estimates. It appears that GPM DPR reflectivity measurements are in good agreement with ground based ones (at both Ku and Ka bands) but they are contaminated by ground clutter up to 1km above ground, leading to significant underestimation of precipitation over the Alps. The limited vertical extent above the terrain and the frequent measurements in ice phase above the freezing level are important factors to explain this underestimation, in particular for winter precipitation.