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. 2021 Sep 21;21(18):6321.
doi: 10.3390/s21186321.

On the Use of Dynamic Calibration to Correct Drop Counter Rain Gauge Measurements

Mattia Stagnaro  1 , Arianna Cauteruccio  1 , Luca G Lanza  1   2 , Pak-Wai Chan  3
Affiliations

On the Use of Dynamic Calibration to Correct Drop Counter Rain Gauge Measurements

Mattia Stagnaro et al. Sensors (Basel). .

Abstract

Dynamic calibration was performed in the laboratory on two catching-type drop counter rain gauges manufactured as high-sensitivity and fast response instruments by Ogawa Seiki Co. Ltd. (Japan) and the Chilbolton Rutherford Appleton Laboratory (UK). Adjustment procedures were developed to meet the recommendations of the World Meteorological Organization (WMO) for rainfall intensity measurements at the one-minute time resolution. A dynamic calibration curve was derived for each instrument to provide the drop volume variation as a function of the measured drop releasing frequency. The trueness of measurements was improved using a post-processing adjustment algorithm and made compatible with the WMO recommended maximum admissible error. The impact of dynamic calibration on the rainfall amount measured in the field at the annual and the event scale was calculated for instruments operating at two experimental sites. The rainfall climatology at the site is found to be crucial in determining the magnitude of the measurement bias, with a predominant overestimation at the low to intermediate rainfall intensity range.

Keywords: drop counter; dynamic calibration; measurement accuracy; rain gauge.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The Drop Counter rain gauges are manufactured by the Ogawa Seiki Co. Ltd. (OSC) (a) and the Chilbolton RAL (CRAL) (b).
Figure 2
Figure 2
Example of the relationship between the measured drop frequency and RIref. Results refer to the OSC gauge with a funnel area of 127 cm2 (left-hand axis) and 314 cm2 (right-hand axis).
Figure 3
Figure 3
Dynamic calibration results for (a) the OSC and (b) the CRAL gauges, assuming a constant volume for the generated drops. Horizontal lines indicate the limit of the European standard classification and the WMO recommendation.
Figure 4
Figure 4
Calibration curves (black lines) and experimental data (black dots) of the generated drop volume as a function of the drop frequency for (a) the OSC gauge and (b) the CRAL gauge.
Figure 5
Figure 5
Performance assessment after bias correction according to the calibration curves obtained from dynamic calibration by assuming the actual volume for the generated drops, for (a) the OSC gauge and (b) the CRAL gauge. Horizontal lines indicate the limit of the European standard classification and the WMO recommendation.
Figure 6
Figure 6
Non-parametric distribution of (a) the average value and (b) the standard deviation of the one-minute RI calculated for each precipitation event for the two OSC gauges and the CRAL gauge.
Figure 7
Figure 7
Non-parametric distribution of the relative difference between the total rainfall event amounts (RAdiff) obtained under the constant and variable drop volume assumption, as a function of RImean for (left) the OSC gauges (at HKIA) and (right) the CRAL gauge (at CFARR).
See this image and copyright information in PMC

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