Investigation of the Wind-Induced Airflow Pattern Near the Thies LPM Precipitation Gauge

Abstract

The airflow velocity pattern generated by a widely used non-catching precipitation gauge (the Thies laser precipitation monitor or LPM) when immersed in a wind field is investigated using computational fluid dynamics (CFD). The simulation numerically solves the unsteady Reynolds-averaged Navier–Stokes (URANS) equations and the setup is validated against dedicated wind tunnel measurements. The adopted k-ω shear stress transport (SST) turbulence model closely reproduces the flow pattern generated by the complex, non-axisymmetric outer geometry of the instrument. The airflow pattern near the measuring area varies with the wind direction, the most intense recirculating flow and turbulence being observed when the wind blows from the back of the instrument. Quantitative parameters are used to discuss the magnitude of the airflow perturbations with respect to the ideal configuration where the instrument is transparent to the wind. The generated airflow pattern is expected to induce some bias in operational measurements, especially in strong wind conditions. The proposed numerical simulation framework provides a basis to develop correction curves for the wind-induced bias of non-catching gauges, as a function of the undisturbed wind speed and direction.

Influence of Wind-Induced Effects on Laser Disdrometer Measurements: Analysis and Compensation Strategies

“…As evident from this figure, the spectrograph for the WD1 sector, due to local vertical orographic forcing in a high turbulence environment, shows relevant growth in the number of particles characterized by a very small diameter (<0.4 mm) and relatively high terminal velocity than what happens for WD2. This finding highlights the importance of the orientation of the disdrometer with respect to the prevailing wind, as pointed out in [9,16], as well as the local orography of the installation site. Consequently, it would be highly recommended to have collocated wind and disdrometer measurement, as well as to accurately evaluate local orographic effects.…”

“…This could happen for two reasons. The first one is caused by the outer frame of the instrument, which exerts an air resistance to the external wind then produces a horizontal and a vertical small-scale airflow modification within the sensitive volume of the disdrometer [9]. The second reason is due to the drop trajectory deviation caused by an external horizontal wind.…”

“…According to the achievements of this study, increasing wind speed values determine an underestimation in precipitation intensity estimated by the disdrometer. The wind-induced airflow near the Thies disdrometer has been recently analyzed and discussed in [9]. Starting from an ideal scenario, in which the device is free of obstacles and has no wind-protection shield, this work simulated the impact of the instrument itself on the wind field around the sensor trough computational fluid dynamics models.…”

Error investigation of rain retrievals from disdrometer data using triple colocation

“…Their measurements are accessible in Fehlmann et al (2020b) and refer to a dataset consisting of about 20,000 observations of rain precipitation sampled each minute, collected during the period from 1 November 2017 to 30 June 2019 in a pre‐alpine site in Switzerland for a total of 607 days. The measuring site was characterized by very low wind conditions (values of mean, standard deviation, and maximum value during rain events are 0.4 m s −1 , 0.5 m s −1 , and 4.7 m s −1 , respectively), thus allowing to minimize the detrimental wind effects (Chinchella et al, 2021; Colli et al, 2018; Kruger & Krajewski, 2002).…”

“…Particle sizes and vertical speeds are subdivided into 22 and 20 classes, respectively and, in each class ( j , k ), the count of the falling particles ( N j , k ) is stored every minute together with the precipitation intensity, its amount and type (Clima, 2015). Thies Clima provides a more complete characterization of the precipitation with respect to traditional devices, although its measurements may be affected by some detection issues related to wind effects, double or partial drop detection (binning effect) and splashing (Capozzi et al, 2021; Chinchella et al, 2021). In order to remove spurious measurements, the same velocity filter used for the 2DVD is applied.…”