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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-174
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-174
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  22 Jun 2020

22 Jun 2020

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This preprint is currently under review for the journal AMT.

Something fishy going on? Evaluating the Poisson hypothesis for rainfall estimation using intervalometers: results from an experiment in Tanzania

Didier de Villiers1, Marc Schleiss2, Marie-Claire ten Veldhuis1, Rolf Hut1, and Nick van de Giesen1 Didier de Villiers et al.
  • 1Department of Water Management, Faculty of Civil Engineering, Delft University of Technology
  • 2Department of Geoscience & Remote Sensing, Faculty of Civil Engineering, Delft University of Technology

Abstract. A new type of rainfall sensor (the intervalometer), which counts the arrival of raindrops at a piezo electric element, is implemented during the Tanzanian monsoon season alongside tipping bucket rain gauges and an impact disdrometer. The aim is to test the validity of the Poisson hypothesis underlying the estimation of rainfall rates using Marshall and Palmer’s (1948) exponential raindrop size distribution parameterisation. The latter is defined independently of the scale of observation and therefore implicitly assumes that rainfall is a homogeneous Poisson process. Our results show that 28.3 % of the total observed rainfall patches can reasonably be considered Poisson-distributed and that the main reasons for Poisson deviations of the remaining 71.7 % are non-compliance with the stationarity criterion (45.9 %), the presence of correlations between drop counts (7.0 %), particularly at higher arrival rates (ρa > 500 m−2.s−1) and failing a chi squared goodness of fit test for a Poisson distribution (17.7 %). Our results show that whilst the Poisson hypothesis is likely not strictly true for rainfall that contributes most to the total rainfall amount it is quite useful in practice and may hold under certain rainfall conditions. Despite the non-compliance with the Poisson hypothesis, estimates of total rainfall amount over the entire observational period derived from disdrometer drop counts are within 2 % of co-located tipping bucket measurements. Uncorrected intervalometer estimates of total rainfall amount overestimate the co-located tipping bucket measurements by a factor of approximately 3. The overestimate is most likely due to poor calibration of the minimum detectable drop size (Dmin). Intervalometer estimates of total rainfall when corrected for minimum drop size are within 1 % of co-located tipping bucket measurements. The intervalometer principle shows good potential for use as a rainfall measurement instrument and for determining rough estimates of mean drop size.

Didier de Villiers et al.

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Latest update: 27 Nov 2020
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Short summary
A new inexpensive rainfall measuring instrument (the intervalometer) is tested. It is reliant on an accurate parameterisation of the drop size distribution (DSD) to determine rainfall rates. Statistical homogeneity is a fundamental assumption of the most widely used current DSD parameterisations. This study shows that whilst this assumption is most likely not strictly true for rainfall it is useful in deriving accurate rainfall rates. The intervalometer shows good potential for deployment.
A new inexpensive rainfall measuring instrument (the intervalometer) is tested. It is reliant on...
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