## 2 Methods
### Specific Measures of the Radar Data Quality  
Any measurement of a meteorological radar is influenced by a variety of factors. Some of these originate from the radar hardware and the settings of the measurement parameters (e.g., the power, polarity, beam-width, noise, detection range, and various filters on the hardware side). Other factors are caused by the surrounding environment (e.g., beam height above the terrain, beam-blockage, sea-clutter, and interference with other electronic devices) or by the actual meteorological situation (e.g., beam propagation, partial beam-filling, bright-band, overshooting of the low clouds, and second-trip echoes). The effect of the majority of these quality factors can be expressed in a mathematical form, or we can estimate their probability. The most convenient way of using the quality factors (quality indices, QI) is in a form of dimensionless numbers between 0.0 and 1.0, where the value 0.0 (1.0) stands for the lowest (highest) quality. The _qRad_ software estimates the quality of each measured bin in the radar volume data using different quality indices. The current version of the _qRad_ software is based on a combination of nine quality indices. These evaluate the following factors:  
* The distance from the radar;
* The beam height above the terrain;
* The beam blockage by the terrain;
* The similarity of the target bin to the surrounding ones;
* The time quality;
* The cloud type quality;
* The cloud top height;
* The average quality;
* The constant quality. The listed QIs can be divided into two groups based on the principle of whether the same or a similar concept of the given QI has already appeared relatively frequently in previous studies (the first five QIs above), or the use of the particular QI is less frequent and/or it represents a novel concept of the authors (the last four QIs). The definition, the mathematical formulation (if extant) and the conceptual background of the particular QIs together with the references to formerly published studies are summarized in Appendix A. Here, we provide a formal definition of the individual QIs in the form of shadowed masks in Figure 1 with short comments to them in the following paragraph. An example of a PPI scan (plan position indicator) made by the radar located in Brzuchania (south of Poland) is shown (Figure 1a) along with the masks of all the QIs discussed herein (Figure 1b-j). The distance QI that expresses the degradation of the radar signal quality with the distance is clearly radially symmetrical (Figure 1b). The beam height above the terrain has an important role in rainfall estimation: the closer the target bin to the surface, the more precise the estimation. In Figure 1c, the beam height QI is of an acceptable quality only within a radius of about 100 km from the radar. The beam blockage QI (Figure 1d) apparently reflects the roughness of the surrounding terrain that is characteristic for the particular radar. The similarity QI evaluates the similarity of the target bin with those within a 3 \(\times\)3 window centered on it. As demonstrated by Figure 1e, the similarity QI practically copies the edges of the objects detected by the radar. The time QI expresses the temporal degradation of the radar signal within a single antenna rotation. Figure 1f indicates that the most recent information on the scan (i.e., higher values of the time QI) comes from the north-to-north-eastern sector, while the largest time differences (i.e., lower time QI) are assigned to the bins in the north-west-to-northern sector. The dark spots on the mask of the cloud type QI (Figure 1g) indicate echoes where no clouds were identified by the NWC SAF product (Nowcasting Satellite Application Facilities, [25]); however, radar detected some backscattered signal, perhaps due to non-meteorological targets, electromagnetic disturbance, or ground clutter. The cloud top height QI (Figure 1h) conveys similar information but with the focus on the altitudes over the clouds.