Preparación de la base y análisis de datos

The total rainfall accumulation for all valid COPE scans are shown in Figure 7.2 at the full range of the radar and Figure 7.3 for the area within 50 km of the radar. Progressing from Panel A to Panel B in both these figures shows the reduction in accumulation over the Dartmoor area in particular, with peak rainfall accumulation from the raw reflectivity measured at 27426.8 mm and from the filtered reflectivity at 333.3 mm which is a far more reasonable estimate but still high enough to suggest contamination from ground clutter. As this maximum accumulation is within 3 km of the radar to the north west it strongly suggests the cause of such a large accumulation is ground clutter contamination of the rainfall intensities in this area. The improvement between Zm and ZQC is also

shown by the rainfall accumulations over the St Clether rain gauge (83◦azimuth, 5.7 km range), where accumulations from Zm are 143.1 mm compared to 6.6 mm when using

ZQC which compare to a rain gauge total accumulation of 19.0 mm. The raw reflectivity

measurements produce significant over estimates in rainfall accumulation as a result of ground clutter in this area due to a combination of topography and wind turbines. The post QC data largely removes this erroneous accumulation but at the expense of valid rainfall during the field campaign contributing to an underestimation of 65% for this gauge site.

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Figure 7.2: Total rainfall accumulation for the COPE field campaign as measured by

the NCAS mobile weather radar at an elevation of 0.5◦. Panel A shows the rainfall as derived from Zm, B the accumulation fromZQC, C the accumulation from ZA and D

the accumulation from ZC. Each panel is a 300 km square centred on the radar and

contains the accumulation from 1131 valid scans across the field campaign.

Between Panels B and C there is an increase in the rainfall accumulations in the lines of rainfall to the north of the radar and to the south east of the radar as a result of attenuation correction. This is most noticeable in the accumulations on the west facing north Cornwall coast around Bude where rainfall accumulations increase from 25 mm to over 40 mm. This region is most noticeable because the convective line which generated this rainfall aligned with the direction of the radar beams causing high attenuation along this direction as the beam travelled through the majority of the convective line. The change from Panel C to Panel D is greatest in overall intensity and the blocked sectors to the south west, south east and north-north-east are well corrected in the near field region however the blocked region to the south east is still visible at longer ranges. This occurs as only the rainfall measured by the radar can be corrected for beam blockage

Figure 7.3: Total rainfall accumulation for the COPE field campaign as measured by

the NCAS mobile weather radar at an elevation of 0.5◦. Panel A shows the rainfall as derived from Zm, B the accumulation fromZQC, C the accumulation from ZA and D

the accumulation from ZC. Each panel is a 100 km square centred on the radar and

contains the accumulation from 1131 valid scans across the field campaign.

and if the rainfall is at a lower intensity and/or affected by attenuation as is the case here then it is not recoverable by beam blockage correction techniques. This has a greater impact at longer range as the radar’s minimum detectable reflectivity increases with range, further decreasing the signals available for correction. For this reason the blockages to the south of the radar are still clearly visible at most ranges as the beam blockage is in excess of 20 dBZ although the rainfall accumulations in this region (average of 5 mm beyond 15 km) are still significantly higher than in the original rainfall estimates from Zm (average of 0.5 mm beyond 15 km). The sector between 305◦and 310◦blocked

by the radar control tower has been removed from the accumulations due to extreme overcorrection and reflection effects. Figure 7.4 compares the total accumulations at each of the EA rain gauges across the peninsula with rainfall accumulations from each

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of the four reflectivity estimates. For each of the estimates it is clear that the radar underestimates total accumulation when using reflectivity as the rainfall estimator, with the fully corrected reflectivity showing a significant improvement when compared to the other stages of the processing chain. Linear regression shows the measured reflectivity has no correlation with the rain gauge data (R2= 0.05) and R2 increases through each of the processing steps, with the final corrected reflectivity having an R2 of 0.36, a gradient of 0.58 and an offset of 3.23 mm. Clearly the offset between gauge and radar should be zero and a linear regression with fixed offset instead shows an improvement in R2 _from -0.01 to 0.34 with a gradient change from 0.45 to 0.72 through the processing (ZM toZC).

This improvement is also shown in the mean absolute percentage differences (MAPD) for the gauges, which decreases from 86% to 31% throughout the processing, with a final mean percentage difference (MPD) of 26% indicating the systematic under measurement by the radar when compared to the rain gauges.

Often the rainfall calculated from higher elevations is used in preference to the lowest elevation to avoid beam blockage and ground clutter, with many radar processing chains incorporating the idea of a lowest usable elevation angle for each location (Harrison et al., 2012; Tabary, 2007). Generating rainfall accumulations with data from an elevation angle of 1.5◦for the COPE campaign shows the increased elevation increases the gradient of the relationship between radar and rain gauge data in all but the fully corrected case and R2 increases from 0.0 to 0.26 for ZQC and from 0.05 to 0.39 for ZA. However for

ZC the correlation decreases from 0.34 to 0.15 when comparing the two elevations. The

reduction in R2 from ZA toZC at 1.5◦elevation is indicative of the uncertainty within

the PBB correction at this elevation. The beam blockage correction calculations for 1.5◦contain fewer valid data points than the equivalent calculations for 0.5◦, leading to greater variability in the results. Despite the decrease in correlation between the observations the PBB correction does improve the MAPD from 36% (ZA) to 28% (ZC)

suggesting the PBB correction improves the rainfall estimates on average, despite greater variability between locations. This is a marginal improvement on the MAPD obtained when using fully corrected 0.5◦elevation reflectivity (31%) which could be attributed to the different number of comparisons in each dataset (1.5◦scans did not feature in several of the COPE scan strategies, see Section 3.2), or could be a result of St Clether being much better represented by the higher elevation data due to the reduction in ground clutter returns and mixed signal echoes for this gauge site.

Figure 7.4: Comparison of rain gauge total rainfall accumulation for the COPE field

campaign with rainfall as measured by the NCAS mobile weather radar at an elevation of 0.5◦. The y-axis of panel A shows the rainfall accumulation fromZm, B the accumulation

from ZQC, C the accumulation fromZA and D the accumulation from ZC. Each blue

cross represents the total for each of the EA rain gauge sites. Panel A contains one less data point than the other panels as the St Clether gauge has been omitted due to its

extreme raw rainfall total. The dashed line in each panel is the one to one line.

Figure 7.5 shows the total rainfall accumulations from the 1.5◦elevation scans across a 100 km square centred on the radar, which is spatially similar to Figure 7.3 but with some notable differences. The rainfall accumulations over Dartmoor are much lower, and spatially continuous which suggests they are better representations of the true rainfall in this area, while the accumulations to the north are much lower ( 20 mm compared to 40 mm at 0.5◦). It is much harder to validate this region, the closest rain gauge is Tamar Lakes which is located at a radar azimuth of 28◦and a range of 29.1 km however this lies on the edge of the intense region of rainfall as seen at 0.5◦. The radar accumulation at this location was 9.6 mm at 0.5◦and 8.22 mm at 1.5◦compared to 13.40 mm for the gauge, which suggests the lower elevation is more representative but this result can’t be reliably

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Figure 7.5: Total rainfall accumulation for the COPE field campaign as measured by

the NCAS mobile weather radar at an elevation of 1.5◦. Panel A shows the rainfall as derived from Zm, B the accumulation fromZQC, C the accumulation from ZA and D

the accumulation from ZC. Each panel is a 100 km square centred on the radar and

contains the accumulation from 1131 valid scans across the field campaign.

extrapolated to the north west to cover the higher accumulations observed in this area highlighting the difficulty of validating radar rainfall estimates even in a location that is covered by a widely distributed rain gauge network.

Overall the total rainfall accumulations suggest that unless correction for partial beam blockage is possible it is better to use data from a higher elevation than use blocked low elevation data, but once correction is possible the reduced height of the beam at the low elevation generates more representative rainfall estimates. They also indicate regions of persistent ground clutter still present problems for the radar’s QPE despite the QC processing and more work needs to be done to handle these regions to obtain accurate QPE. Rainfall accumulations from the next available elevation indicates this could be

achieved by merging in data from a height above the ground clutter, subject to a lack of representatively due to increasing while another solution could be to selectively merge in the velocity filtered reflectivity data from the radar while avoiding the inherent issues previously observed in this field. These options will be explored more in the future and are touched on briefly in Chapter 8.

7.2.2 Widespread stratiform rainfall of low intensity - an example of