1,720,966 research outputs found
Calibration of non-catching type rain gauges: preliminary tests on an optical disdrometer
No full textThe last WMO Field Intercomparison on Precipitation Intensity highlights that non-catching type rain gauges show limited performance, in terms of precision and accuracy of the measurements, when compared to the traditional catching type rain gauges.
Moreover, the performance of these instruments differ significantly from each other, this fact suggests that the lack of a standardized calibration procedure leads to different behaviour in field measurements.
Building on these results and in order to understand the causes of this behaviour, the CIMO Lead Centre on Precipitation Intensity has developed a rain drop generator, able to reproduce drops with various diameters.
The Thies LPM disdrometer has been tested using the rain drop generator and the results are presented in terms of total number of droplets detected by the instrument, droplets per diameter class, total accumulation volume and measured rainfall intensity.
Preliminary results show that the disdrometer detects a larger number of droplets than those actually generated. Most of them are attributed by the instrument to the lower diameter class, and have a low impact in the total volume account or rainfall intensity estimation, but could affect the shape of the Drop Size Distribution (DSD) provided by the instrument. Moreover, in many cases it has been found that the disdrometer assigns most of the real droplets to the upper diameter class, therefore resulting in a significant overestimation of the rainfall amount and intensity.
These results, although still preliminary, reveal that a standardized and rigorous calibration procedure is needed for the non-catching type rain gauges to foster more reliable and comparable measurements
The role of free-stream turbulence on the collection performance of catching type precipitation gauges
No full textWIND TUNNEL TESTS TO EVALUATE THE RAIN DROPS DEVIATIONS AROUND DIFFERENT TYPES OF PRECIPITATION GAUGES
No full textThe wind-induced bias of the Thies Laser Precipitation Monitor obtained using CFD and a Lagrangian particle tracking model
No full textWIND TUNNEL VALIDATION OF A PARTICLE TRACKING MODEL TO ASSESS THE WIND-INDUCED UNDERCATCH OF RAINFALL GAUGES
No full textThermo-fluid dynamic simulations of the Hotplate precipitation gauge and wind tunnel experiments
No full textThe present study addresses the aerodynamic response of the recently developed “Hotplate” precipitation gauge when exposed to the wind. The Hotplate gauge employs two heated plates to provide a reliable method of precipitation measurement. The measuring principle is based on an algorithm to associate the latent heat needed to evaporate the snow, or the rain, and the precipitation rate. The presence of the instrument body immersed in a wind field is expected to induce significant deformations of the airflow pattern near the gauge, with an impact on the associated catching efficiency. Indeed, the fall trajectories of the hydrometeors when approaching the gauge can be deviated away from the collecting plate resulting, in general, in some underestimation of the precipitation rate.
This work is based on Computational Fluid Dynamics (CFD) simulation of the airflow field around the gauge for different wind speeds, to identify areas where the wind-induced updraft, local acceleration and turbulence are significant. The performed CFD airflow simulations use the URANS SST k-ω approach, and are the initial modelling step to quantify the associated undercatch. These will be possibly coupled in future developments with particle tracking models to derive suitable correction curves for operational purposes. Due to the specific measurement principle exploited by the “Hotplate” gauge, thermo-fluid dynamic simulations are addressed as well. Dedicated tests have been performed in the wind tunnel facility available at the Department of Civil, Chemical and Environmental engineering DICCA, University of Genoa to validate simulation results.
Numerical results indicate that the presence of wind is a relevant source of systematic bias and its effect must be corrected by adopting suitable correction curves as a function of the wind velocity. An assessment of the airflow patterns developing around the gauge at various wind velocity regimes is provided in this work and wind tunnel tests allowed for a substantial validation of the numerical results
Bluff body aerodynamics of the Thies Laser Precipitation Monitor investigated using CFD and wind tunnel measurements
No full textOptical disdrometers are among the non-catching type instruments used to measure liquid and solid precipitation. The increasing use of such instruments in operational observations is due to their capability to provide additional information than the precipitation rate alone, like e.g. the
particle size distribution and the fall velocity of hydrometeors. Furthermore, they are well suited to operate in unattended, automatic weather stations. Having no collector to catch the approaching hydrometeors, their outer shape strongly depends on the measuring principle exploited. The impact of wind on the measurement is therefore different from the typical undercatch that is expected from more traditional catching type precipitation gauges. In general, they are not axisymmetric and base the identification and classification of hydrometeors on the coupling of
particle size and fall velocity characteristics, which can be affected by the wind and by the airflow deformation and turbulence produced by their bluff-body aerodynamic response. The focus of this work is the Thies Laser Precipitation Monitor (LPM), which uses an optical sensor to detect the obstruction of an infrared laser beam caused by the crossing hydrometeors. The reduction in the sensor output voltage is proportional to the drop dimension, while the duration of the reduction is proportional to the drop falling speed. This instrument presents a very complex and not
axisymmetric outer shape that makes it difficult to qualitatively predict the flow pattern and requires to consider multiple wind directions and wind speeds. The airflow field was obtained with a Computational Fluid Dynamics (CFD) approach, by numerically solving the Reynolds Averaged Navier-Stokes equations with the k-ω SST turbulence closure model. Results are validated through local flow velocity measurements obtained in the DICCA wind tunnel. The Thies LPM® was placed in the measuring chamber of the wind tunnel (1.7 x 1.35 x 8.8 m) on to a rotating plate and the airflow velocity was sampled at multiple positions around the instrument. The measurements were obtained using a traversing system equipped with a “Cobra” multi hole pressure probe, that provides the three velocity components of the local flow. Different orientation angles of the gauge with respect to the incoming flow direction were tested. Based on the simulations and wind tunnel tests performed, the less impacting configuration of the instrument relative to the main wind direction is obtained. The information can be useful to design effective solutions to minimise the impact of wind and turbulence on the measurements (e.g. windshields) and to derive suitable correction curves to improve the measurement accuracy. This work is funded as part of the activities of the EURAMET – Normative project “INCIPIT – Calibration and Accuracy of Non-Catching Instruments to measure liquid/solid atmospheric precipitation”
Using the measured Particle Size Distribution to assess the wind-induced bias of catching-type raingauges
No full textDynamic calibration of two catching type drop-counting rain gauges
No full textThis study reports the results of laboratory tests performed to assess the performance of two drop
counting rain gauges of the catching type, and to propose suitable correction to make them compliant
with the specifications of the World Meteorological Organization (WMO) at one-minute time
resolution for Rainfall Intensity (RI) measurements. The tests were limited to the steady state
conditions, with known and constant flow rates provided to the instrument at various reference
intensities for a sufficient period of time, in order to compare the measures provided by the gauge
with the reference figures (which is known as dynamic calibration).
The instruments investigated are manufactured by Ogawa Seiki Co. Ltd (Japan) and the Chilbolton
RAL (UK). They are designed as high-sensitivity drop counter type rain gauges.
The dynamic calibration adopting a constant volume of the droplets declared by manufacturers
reveals that these instruments do not fulfil the WMO requirements. A calibration curve of the drop
volume with respect to the measured drop frequency has been derived for each instrument. Using a
suitable correction algorithm, based on calibration curves as obtained from the tests performed in the
laboratory, the precision of the instruments is improved and the results are fully compatible with the
WMO required maximum admissible error (WMO, Pub. No 8).
Laboratory tests also reveal the operational limit of this kind of instrument that is given by the
rainfall intensity at which the water flux from the nozzle start to be continuous, therefore a standalone
installation is discouraged
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