# 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  
Figure 7 displays the change in radar QPE performance after tilt correction, assessed over the whole campaign period. Precipitation in this region, as calculated using the raingauge network, varies considerably. The wettest catchments are in the west, with the wettest catchment totalling an average of 6562.3 mm of precipitation during the campaign period (equivalent to 3078.6 mm/yr), compared to the driest catchments, in the east, with precipitation totals as low as 1281.8 mm (equivalent to 601.3 mm/yr). This trend is further exaggerated in the radar QPE due to the high radar beam elevation necessary in the east due to the interposing Lake District fells (Figure 6). This results in the radar beam overshooting large portions of the precipitation in this region and consequently produces strongly negative biases and poor MAPE and correlation in the radar QPE regardless of any tilt correction. Because of this, it is often instructive to limit the analysis to those 85 catchments, outlined in bold in Figure 7, with a Lowest Useable Elevation (LUE) of less than 2 km. Here, vertical processes (including melting, orographic enhancement and advection) are less likely to influence the QPE comparisons [28], and they additionally tend to be closer to the radar (due to the radar beam propagating at a positive elevation angle), thus limiting the influences of beam broadening, reduced radar sensitivity and partial beam filling on the QPE comparisons. The percentage biases shown in Figure 7 are negative to a greater or lesser extent for all catchments, including those with LUEs less than 2 km. However, the application of the tilt correction does result in a systematic improvement in bias over the entire radar domain, including a median increase in bias of 11% for catchments with LUEs less than 2 km. This is expected given the majority of the tilt correction periods lead to an increase in the magnitude of the partial beam blockage correction across the entire domain. This is due to all the computed elevation offsets (static + tilt) having a positive sign, which indicates the radar is always pointing at a lower elevation than requested. As the elevation angle is lower than requested, the partial beam blockage correction applied in the first instance is insufficient and must increase in magnitude to correct for the elevation offsets. The absence of positive percentage biases across all catchments in the corrected QPE is a strong indicator that the increase in tilt correction is not an overestimation, which would introduce too much beam blockage correction and hence positive rainfall biases. The benefit of using the tilt correction is also clear in the general trend to lower MAPE. This statistic displays a median reduction of 5% for catchments with LUEs less than 2 km. There are, however, two catchments in Scotland that suffer a slight (less than 3%) increase in MAPE, which highlights the spatially distributed impact of applying a tilt correction that is strongly dependent on local topography. The application of the tilt correction produces a more mixed effect on the correlation coefficient, with some of the catchment with LUEs less than 2 km displaying a modest increase (five display increases \(>0.05\)), some displaying slight decreases (three display decreases \(<-0.02\) ) and the majority displaying little change, particularly among those with acceptable correlation coefficients to begin with. Given the tilt correction changes temporally, this suggests that the variability it creates from period to period is not the dominant control for the correlation of the QPE with surface observations, and that the correction is not causing additional decorrelation across the dataset. Figure 8 summarises the performance of radar QPEs both before and after tilt correction for each of the correction periods in turn. In this case, only the 85 catchments with LUEs below 2 km are included.