Automated precipitation monitoring with the Thies disdrometer: biases and ways for improvement

Abstract

Abstract. The intensity and phase of precipitation at the ground surface can have important implications not only for meteorological and hydrological situations but also in terms of hazards and risks. In the field, Thies disdrometers are sometimes used to monitor the quantity and nature of precipitation with high temporal resolution and very low maintenance and thus provide valuable information for the management of meteorological and hydrological risks. Here, we evaluate the Thies disdrometer with respect to precipitation detection, as well as the estimation of precipitation intensity and phase at a pre-alpine site in Switzerland (1060 m a.s.l.), using a weighing precipitation gauge (OTT pluviometer) and a two-dimensional video disdrometer (2DVD) as a reference. We show that the Thies disdrometer is well suited to detect even light precipitation, reaching a hit rate of around 95 %. However, the instrument tends to systematically underestimate rainfall intensities by 16.5 %, which can be related to a systematic underestimation of the number of raindrops with diameters between 0.5 and 3.5 mm. During snowfall episodes, a similar underestimation is observed in the particle size distribution (PSD), which is, however, not reflected in intensity estimates, probably due to a compensation by snow density assumptions. To improve intensity estimates, we test PSD adjustments (to the 2DVD) and direct adjustments of the resulting intensity estimates (to the OTT pluviometer), the latter of which are able to successfully reduce the systematic deviations during rainfall in the validation period. For snowfall, the combination of the 2DVD and the OTT pluviometer seems promising as it allows for improvement of snow density estimates, which poses a challenge to all optical precipitation measurements. Finally, we show that the Thies disdrometer and the 2DVD agree well insofar as the distinction between rain and snowfall is concerned, such that an important prerequisite for the proposed correction methods is fulfilled. Uncertainties mainly persist during mixed-phase precipitation or low precipitation intensities, where the assignment of precipitation phase is technically challenging, but less relevant for practical applications. We conclude that the Thies disdrometer is suitable not only to estimate precipitation intensity but also to distinguish between rain and snowfall. The Thies disdrometer therefore seems promising for the improvement of precipitation monitoring and the nowcasting of discharge in pre-alpine areas, where considerable uncertainties with respect to these quantities are still posing a challenge to decision-making.

Corrections of Precipitation Particle Size Distribution Measured by a Parsivel OTT2 Disdrometer under Windy Conditions in the Antisana Massif, Ecuador

“…The hydrometeors-type employed have been commonly used to describe the composition of DSDs [24,27,52,56]. Indeed, by superimposing these fall velocity-diameter relationships over the measured DSDs, almost 88% of the particles matched the envelope formed between snow and rain curves.…”

“…First, the fall velocitydiameter relationship was calculated for raindrops (R), snow (S), wet snow (WS), graupel (G), lump graupel (LG), soft hail (SH), fresh hail (FH), and lump hail (LH). The relationships and diameters selected (Table 2) were based on the most common formulations for these hydrometeors found in the literature [24,[53][54][55][56]. A correction factor was applied to the fall velocity values in order to consider the velocity increase caused by the decrease in air density [57].…”

Laser attenuation in falling snow correlated with measurements of snow particle size distribution

“…7,8 The correlation between the precipitation rate and both the measured transmission signal and the data from the visibility sensor, are in agreement with these assessments. However, as the work by Fehlmann et al 15 showed, the operating principle of the laser disdrometer, relying on hydrometeor mass density estimations, will result in uncertainties when transforming the raw data to a precipitation rate in the case of falling snow. Therefore, it is difficult to assess if our observed large variations in transmission with respect to precipitation rate is due to inherent variations in the precipitation, or variations introduced by the measurement equipment, or both.…”

“…Recently, Fehlmann et al evaluated a Thies laser disdrometer similar to the one used in the reported campaign with respect to monitoring rain and snowfall events. 15 In the experiments, they compared the results from the laser disdrometer with the results from a pluviometer from OTT and a 2DVD video disdrometer from Joanneum research. The pluviometer was used as a reference for precipitation rate and the 2DVD disdrometer was the reference for PSD measurements.…”

“…The development and refinement of equipment for precipitation measurements, e.g., disdrometers that perform in-situ measurements of hydrometeor size and velocity, [12][13][14][15] have opened up possibilities for detailed correlation of precipitation characteristics with laser transmission measurements. Examples of the use of knowledge of the PSD when investigating transmission in precipitation are Brazda and Fiser, 16 and Peckhaus et al 17 Brazda and Fiser investigated the impact of rain on an 850-nm FSO-link, and Peckhaus et al investigated the effect of rain and falling snow on a high power laser beam centered around 1 μm.…”