## 3 Results
### QPE Improvement
#### 3.2.3 Runway Condition  
Section 3.1 demonstrated the practical applicability of the _qRad_-based radar composites for pilots and air traffic managers. In addition to the _qRad_ software, the _qPrec_ package also offers clear benefits for aviation, namely, through an estimation of the precipitation amounts (QPE) directly on the runways. Assessment of the runway condition and optimization of the airport operation management are key issues in the research of air traffic management, particularly in the SESAR (Single Europe Sky Air Traffic Management Research) program [32]. The amount of water, regardless of its phase, is a crucial factor for the safety of aircraft landing. On a wet or contaminated runway, the risk of adverse effects on the plane's braking performance increases. During the cold season, snow cover and/or ice may significantly decrease the plane's braking performance, whereas, during the warm season, the thickness of the water film on the runway affects the risk of aquaplaning. In our experiment, we focused on the observation of the height of the water film on the runway surface and attempted to estimate this on the basis of radar measurements. The mini-experiment was only performed for the warm season (more precisely, the summer period of June/July/August), mostly for the following reasons. First, to perform a distant estimation of the amount of precipitation in a solid phase is a complicated task mostly due to the effect of a number of further physical and thermodynamic processes (e.g., melting, freezing, snow compacting, and wind effects). Second, climate change scenarios for the Central Europe generally foresee changes in the intra-annual distribution of the precipitation: one can expect more frequent and more intensive precipitation events in all seasons, whereas the mean precipitation is supposed to increase (decrease) in winter (in summer) [33; 34; 35]. The experiment was performed at the International Airport Poprad-Tatry (Slovakia; ICAO: LZTT; IATA: TAT). The location of the airport, with respect to the network of four meteorological radars of SHMU is depicted in Figure 9. This reveals that the coverage of the target area by meteorological radars is sufficient: the height of the layer that remains invisible for radars is approximately 300 m, which is acceptable from the point of view of detection of storms. The height of the water film on the runway surface is measured at three locations of the runway (with a length of 2600 m and oriented in the 09/27 direction): near its both ends, and approximately in the middle of the runway. The intelligent road sensors Lufft IRS31 Pro-UMB provide measurement of the water film height with a temporal resolution of 1 min. On the other hand, the amount of precipitation on the runway was also estimated by means of two methods of QPE on the basis of the CMAX radar product: in a traditional way, using a simple Marshall-Palmer relationship (Equation (8)) with no QI included (termed simply as CMAX), and using the _qPrec_ package with the QI included (termed as _qPrec_). Corresponding to the location of each sensor, the maximum precipitation amount was estimated approximately within a 1-km radius and within an X-minute time window (where X \(\geq\) 4). Finally, for each datum, the maximum of the three maximum values was selected for the analysis. We propose that the selected options (the spatial and temporal resolution and the method of maxima) ensures that the QPE can capture the rainfall even in the case when the core of the rainstorm does not hit exactly the runway. The results of the analysis are demonstrated in Figure 10. This presents the scatter plots of the measured water film heights vs. the estimated rainfall amounts at the runway for the summer months of the years 2019 and 2020 for both of the QPE methods considered and for the rainfall accumulation time of 4 min. The linear relationships between the water film heights and the rainfall amounts corresponding to the particular QPE method were weak in both cases.