4.4 Summary

The conclusions of the full-scale driving rain gauge comparison test are listed here (and were partially presented in [Högberg et al. 1999]):

• Driving rain intensities on façade positions P4 and P5 are considered to be identical (figures 4.6a and 4.6b). Temporal fluctuations between these positions were not investigated, because the test did not include measurements with identical driving rain gauges at P4, P5 and P7, which are located adjacent to each other. We suggest such an experiment for a further study.

• The monthly driving rain amounts of the CTH, DTU and TUE-II gauges deviate within 30% from each other (table 4.1). Gauge TUE-I registers approximately half of the monthly driving rain amount.

  On much smaller time bases, such as 10-min intervals, gauge responses can deviate significantly (figures 4.1-4.4). This applies especially to small time bases of the used tipping-bucket driving rain gauge (CTH): during a 10 min period it tips only once at a driving rain intensity of 0.18 mm h$^{-1}$. Therefore we suggest that for short-time intervals one should apply a continuous measuring principle instead of the tipping-bucket principle.

• The effect of size and shape of the catchment area cannot clearly be deduced from the experiments. A comparison of the CTH gauge (0.032 m$^{2}$) and the TUE-I gauge (0.5 m$^{2}$) does not give a straightforward conclusion, because of the difference in measuring principle of the collected rain flux, as pointed out in the previous item. However, the results of gauges TUE-I and TUE-II suggest that for large catchment areas (e.g. 0.5 m$^{2}$) a wiper is necessary.

  Another difficulty in interpreting the effect of the size of the catchment area results from a comparison between the TUE-I, TUE-Ib, TUE-II and CTH gauges. While the CTH and TUE-II gauges (two very different catchment areas and measurement principles) yield comparable results (at least over long periods of time), the TUE-I and TUE-Ib gauges (same measurement principle but very different catchment areas) register about 47% and 32% of the driving rain amount of the TUE-II gauge, respectively. One would expect that the TUE-Ib gauge would suffer less from raindrop evaporation than the TUE-I gauge because of its smaller catchment area, and consequently measure higher driving rain intensities. An explanation may be the fact that the drainage path length of the TUE-Ib gauge is much larger than the CTH gauge.

• Teflon coating for a smooth, hydrophobic collector surface is on itself not completely satisfactory. This has also been concluded from laboratory tests at the CTH. Teflon gets dirty, like other finishes (e.g. perspex). A wiper can serve to keep the surface clean, and to improve coagulation and dripping-down of collected raindrops.

• Readings of the DTU gauge (compared to the TUE-II readings) are sensitive to driving rain intensity (figures 4.1b-4.4b). Lower driving rain intensities are overestimated by the DTU gauge. The scatter in the DTU/TUE-II correlations is larger than in the TUE-I/TUE-II correlations. This is probably due to the noise caused by the wind acting on the freely suspended collector. The applied signal-processing method (see section 3.4.6) was kept simple and can perhaps be improved.

• Given every aspect of the driving rain comparison test, one concludes that the TUE-II gauge registers driving rain intensities well. It has a good resolution for shorter time intervals (e.g. 10-min periods). Its wiper keeps the surface clean and forces impinged raindrops to coagulate and drip down (hence less evaporation). Moreover, it is not sensitive to wind.

The comparison test gives the following design rules for driving rain gauges:

• a large catchment area ($\sim$0.5 m$^{2}$) is useful for a better resolution for the water flux gauge,
• one should prevent drops from remaining stuck onto the collector surface. A rotating wiper is a simple solution for this problem,
• the water flux gauge, which measures the water flux coming from the collector, should be adapted to the desired time basis and the catchment area of the collector. For a resolution of $R$ mm h$^{-1}$ in $t$ min, a water flux gauge with a collector of $A_{\text{catch}}$ m$^{2}$ should be able to detect $(R t A_{\text{catch}})/60$ kg. When designing a driving rain gauge it is important to have a fairly good estimate of $R$. As an example of full-scale driving rain measurements, one can consider the distribution of 5-min driving rain intensities measured at the Main Building in the next chapter (figure 5.15). Maximum values of $R_{\text {f}}$ at the Main Building are listed in tables 5.5 and 5.6,
• a smooth surface (e.g. of perspex or teflon) will enable easy cleaning of the driving rain gauge. A rotating wiper also helps in this respect.

Some problems were not solved by the full-scale experiments:

• the influence of shape and size of the collector on the reading of the driving rain gauge is still unclear,
• the possibility of splashing and the effects of protruding rims and other projections were not investigated,
• the influence of the raindrop spectrum on the readings of the gauges was not investigated due to lack of sufficient data.