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. A new type of rainfall sensor (the intervalometer), which counts the arrivalof raindrops at a piezo electric element, is implemented during the Tanzanianmonsoon season alongside tipping bucket rain gauges and an impactdisdrometer. The aim is to test the validity of the Poisson hypothesisunderlying the estimation of rainfall rates using an experimentally determinedraindrop size distribution parameterisation based onMarshall and Palmer (1948)'s exponential one. These parameterisations are definedindependently of the scale of observation and therefore implicitly assume thatrainfall is a homogeneous Poisson process. The results show that28.3 % of the total intervalometer observed rainfall patches canreasonably be considered Poisson distributed and that the main reasons forPoisson deviations of the remaining 71.7 % are non-compliance withthe stationarity criterion (45.9 %), the presence of correlationsbetween drop counts (7.0 %), particularly at higher arrival rates(ρa&gt;500 m-2s-1), and failing a χ2goodness-of-fit test for a Poisson distribution (17.7 %). Our resultsshow that whilst the Poisson hypothesis is likely not strictly true forrainfall that contributes most to the total rainfall amount, it is quite usefulin practice and may hold under certain rainfall conditions. Theparameterisation that uses an experimentally determined power law relationbetween N0 and rainfall rate results in the best estimates of rainfallamount compared to co-located tipping bucket measurements. Despite thenon-compliance with the Poisson hypothesis, estimates of total rainfall amountover the entire observational period derived from disdrometer drop counts arewithin 4 % of co-located tipping bucket measurements. Intervalometerestimates of total rainfall amount overestimate the co-located tipping bucketmeasurement by 12 %. The intervalometer principle shows potential foruse as a rainfall measurement instrument.

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