The idea behind the full-scale comparison test of the driving rain gauges is to register the readings of the different driving rain gauges while they are exposed to the same, natural driving rain for a reasonable period of time. In order to expose the gauges to the same driving rain, they were mounted together as closely as possible on the west façade of the Main Building of the TUE (positions P4, P5 and P7; P4/5/7 for short). Figure 3.8 shows a photograph of the situation. The spatial positions of the gauges are within 2.5 m horizontally and within 0.7 m vertically. The exact operation periods and the exact positions of the mounted driving rain gauges are listed in table 3.3 and described in section 3.2. The periods during which the gauges were operational, are also visible at a glance in table 4.1, in which monthly driving rain amounts of each gauge at P4/5/7 are listed. This table also lists the percentage of clock periods with available data (i.e. when the installed devices worked properly).
The TUE-II gauge will often be used as a reference for the comparisons. It is the gauge which was operational for the whole 24-month measurement period. Moreover, as we will see in the following, it is one of the gauges which functions well. As monthly driving rain amounts do not give many conclusive details on the performance of the different gauges, we mention the monthly results of a gauge compared to the TUE-II gauge only concisely here:
In [Högberg et al. 1999], the readings of the four gauges CTH, DTU, TUE-I and TUE-II were compared for the period of 1-10-1998 to 28-2-1999. For these five months, the CTH, DTU and TUE-I gauges, respectively, measured 94%, 84% and 51% of the driving rain amount registered by the TUE-II gauge. Compared to the TUE-II gauge the DTU gauge measured less during this five-month period (84%) than during the 16-month period (115%). Also the CTH gauge seems to have a different performance after February 1999. It is impossible to check whether the performance of the gauges really changed over time. In the following two sections (sections 4.2 and 4.3) we will therefore compare the driving rain registrations on a smaller time basis, namely 10-min clock periods. On these results our explanation of the differences will be based. In general, differences in reading arise from:
Aspects a, b and c were not investigated during the comparison test, although at designing and mounting of the gauges these aspects were taken into account. Aspects e, f and g were also taken into account at the design, whereas during the test, cleanness of the collector surface and the motion of drops through the drainage system of the gauges were inspected visually whenever possible. Differences due to aspects d, e and h-k are investigated by correlating the readings of the different gauges to each other, and by relating them with the reference rain intensity ( ) and wind speed (). Of course, the raindrop spectrum is an important factor for all the mentioned aspects where rain intensity plays a role. Raindrop spectra were measured from October 1999 to January 2000. However, they are not included in this chapter, because during this period the amount of driving rain was unfortunately unsufficient for a meaningful correlation between raindrop spectrum data and driving rain data.
© 2002 Fabien J.R. van Mook