# Correcting for Mobile X-Band Weather Radar Tilt Using Solar Interference
## 5 Pointing Correction Verification
### Radar QPE and Verification with External Data
#### 5.2.3 QPE Correction Results
The limited influence of tilt correction on the correlation  
coefficient found for the full campaign is repeated during each of the periods. It is also noticeable that the correlation during each period is higher than that for the dataset as a whole, indicating the temporal variability of radar QPE performance, particularly when using a fixed Z-R relationship. Considerably larger changes are observed in the percentage bias and the MAPE, both of which display improvement for all periods except Period 1. Period 1's worsening performance is driven by an increasingly negative percentage bias, perhaps suggesting that the positive changes to radar elevation pointing lead to an under-correction of partial beam blockage. Period 1 is, however, only seven days long, and the correction is based on only 40 solar hits, suggesting a limitation of the methodology, particularly when it indicates significant changes to pointing are required. Additionally, seven days is a very limited period over which to assess the data. Results for Period 5, only 13 days long, should also be treated with caution. The performance clearly varies across the time periods, with a possible decline in performance seen later in the project (periods 6 to 8b, in particular), with an increasingly negative percentage bias, an increase in MAPE and a reduction in correlation. While the tilt  
Figure 8: Boxplots summarising catchment average performance statistics calculated using radar QPEs without the tilt correction (light green bars) and with the tilt correction (darker green bars) for the 85 catchments with mean lowest useable elevations of the radar beam of less than 2 km. The coloured box displays the interquartile range, the median is shown as the horizontal black line therein, and the typical range of the statistic is shown as black dashed lines extending to a maximum of 1.5 times the interquartile range, with outliers beyond this range shown as hollow circles. An outlier for a single catchment in Period 5 lies beyond the range of the boxplot showing MAPE. correction has a positive effect on these results, it does not fully restore performance. This is likely due to the increasing occurrence of total beam blockage that cannot be corrected. Figure 6 clearly shows total beam blockage occurring at 0.5\({}^{\circ}\) elevation to the east of the radar where there is high topography close to the radar location. This total beam blockage occurs more frequently as the elevation offset (both fixed and due to tilt) increases because the radar's actual elevation angle is lower than the one requested (a nominal elevation angle of 1.0\({}^{\circ}\) is actually scanning at 0.5\({}^{\circ}\) or lower in some cases). To mitigate this error, one option would be to introduce dynamic elevation selection into the processing, which may recover more information from the dataset; however, this would come from higher elevations, which then have a greater uncertainty due to the impacts of horizontal advection, phase changes and vertical profiles of reflectivity. This highlights the requirement to monitor tilt in real-time, as it can have a fundamental, uncorrectable impact on radar QPE quality depending on its magnitude and the local topography.