The Royal Netherlands Meteorological Institute (KNMI) operates two dual-polarization C-band weather radars in Simultaneous Transmission And Reception (STAR; i.e., horizontally and vertically polarized pulses are transmitted simultaneously) mode, providing 2-D radar rainfall products. Despite the application of Doppler and speckle filtering, remaining non-meteorological echoes (especially sea clutter) mainly due to anomalous propagation still pose a problem. This calls for additional filtering algorithms, which can be realized by means of polarimetry. Here we explore the effectiveness of the open-source ωradlib fuzzy echo classification and clutter identification based on polarimetric moments. Based on our study, this has recently been extended with the depolarization ratio and clutter phase alignment as new decision variables. Optimal values for weights of the different membership functions and threshold are determined employing a 4-h calibration dataset from one radar. The method is applied to a full year of volumetric data from the two radars in the Dutch temperate climate. The verification focuses on the presence of remaining non-meteorological echoes by mapping the number of exceedances of radar reflectivity factors for given thresholds. Moreover, accumulated rainfall maps are obtained to detect unrealistically large rainfall depths. The results are compared to those for which no further filtering has been applied. Verification against rain gauge data reveals that only a little precipitation is removed. Because the fuzzy logic algorithm removes many non-meteorological echoes, the practice to composite data from both radars in logarithmic space to hide these echoes is abandoned and replaced by linearly averaging reflectivities.

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Hydrology and Quantitative Water Management Group, Wageningen University & Research, Wageningen, the Netherlands.