Precipitation Measurement Instruments: Calibration, Accuracy and Performance

Though ranking high among the relevant environmental variables (due to the well-known significant interactions with the everyday human life and economic activities), atmospheric precipitation is not yet measured operationally with neither the degree of accuracy that would meet the most demanding applications nor any rigorous standardization framework [...

On the aggregation of wind/snow data when using a transfer function to account for wind-induced errors

Since solid precipitation records, and the associated wind speed data, are commonly stored with a quite coarse resolution in time (30 or 60 minutes), we investigated the impact of the aggregation scale on the accuracy of data corrected by using the transfer functions. We used data from the WMO SPICE (Solid Precipitation Intercomparison Experiment) field campaign, observed at the Marshall field test site (Colorado, USA) during the winter seasons from 2013 to 2015. The snowfall rates were recorded by three Geonor weighing gauges with different configurations: unshielded (UN), SA shielded and a DFIR to serve as the reference. Both precipitation and wind speed data are quality controlled and provided with the time resolution of 1 minute

The Collection Efficiency of Shielded and Unshielded Precipitation Gauges. Part II: Modeling Particle Trajectories

The use of windshields to reduce the impact of wind on snow measurements is common. This paper investigates the catching performance of shielded and unshielded gauges using numerical simulations. In Part II, the role of the windshield and gauge aerodynamics, as well as the varying flow field due to the turbulence generated by the shield–gauge configuration, in reducing the catch efficiency is investigated. This builds on the computational fluid dynamics results obtained in Part I, where the airflow patterns in the proximity of an unshielded and single Alter shielded Geonor T-200B gauge are obtained using both time-independent [Reynolds-averaged Navier–Stokes (RANS)] and time-dependent [large-eddy simulation (LES)] approaches. A Lagrangian trajectory model is used to track different types of snowflakes (wet and dry snow) and to assess the variation of the resulting gauge catching performance with the wind speed. The collection efficiency obtained with the LES approach is generally lower than the one obtained with the RANS approach. This is because of the impact of the LES-resolved turbulence above the gauge orifice rim. The comparison between the collection efficiency values obtained in case of shielded and unshielded gauge validates the choice of installing a single Alter shield in a windy environment. However, time-dependent simulations show that the propagating turbulent structures produced by the aerodynamic response of the upwind single Alter blades have an impact on the collection efficiency. Comparison with field observations provides the validation background for the model results

Wind tunnel validation of the aerodynamic performance of rain gauges simulated using a CFD approach.

Wind is recognized as the primary cause for the undercatch of solid and liquid precipitation as experienced by catching type gauges. The airflow pattern above the collector, modified by the presence of the gauge body, influences the particle trajectories and reduces the collection of precipitation. Windshields are employed in the field to reduce the impact of wind. As an alternative, measured data are corrected in post-processing using correction functions derived from field data or numerical simulations. Aerodynamic rain gauges have been also developed, with their outer shape designed to reduce the aerodynamic impact of the gauge body on the surrounding airflow. In a previous work, CFD simulations of aerodynamic gauges were performed and the performance of different shapes were compared. The aim of this work is to validate the airflow pattern around the gaugeas predicted by improved CFD simulations by performing wind tunnel tests both in laminar and turbulent base-flow conditions. The airflow in the proximity of the gauge was simulated using the Unsteady Reynolds Average Navier-Stokes (URANS) equations approach. Advantages of the URANS method include the possibility of describing accurate time-varying patterns of the turbulent air velocity field while maintaining acceptable computational requirements. The simulations were performed under two different turbulence conditions in order to assess the role of the base-flow turbulence on the calculated flow pattern. In the first case, the free stream velocity profile is assumed steady and uniform. Under these conditions the time varying pattern of the airflow around the rain gauge collector is due to the instrument aero-dynamics alone. The second case includes a free-stream turbulence intensity approximately equal to 13%, generated by introducing a fixed solid fence upstream the gauge. Validation of the CFD results was provided by realizing the same airflow conditions in the DICCA wind tunnel and measuring the air velocity components in different fixed positions around the collector of the gauge. Results are presented in comparative terms, based on the time-averaged air velocity, the amplitude of the oscillating components and the turbulent kinetic energy

WITHDRAWN: Experimental evidence of the wind-induced bias of precipitation gauges using Particle Image Velocimetry and particle tracking in the wind tunnel

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Laboratory assessment of two catching type drop-counting rain gauges.

This study reports the results of laboratory tests performed to assess the performance of three drop counting rain gauges of the catching type , and to propose suitable correction so as to make them compliant with the specifications of the World Meteorological Organisation (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. Using a suitable correction algorithm, based on calibration curves as obtained from the tests performed in the laboratory, it is possible to improve the accuracy of the three instruments and to obtain results that are fully compatible with the WMO required measurement uncertainty provided in the CIMO guide (WMO, Pub. No 8), although only within the acceptable measurement ranges. The laboratory tests were performed under known and constant flow rates in closely controlled conditions, according to the recommended procedures developed during the WMO Laboratory Intercomparison of RI gauges and recommended by WMO. The performance in the field may be lower than those observed in the laboratory, due to errors induced by the atmospheric conditions, installation, status of maintenance, etc

Impact of Wind Direction, Wind Speed, and Particle Characteristics on the Collection Efficiency of the Double Fence Intercomparison Reference

The accurate measurement of snowfall is important in various fields of study such as climate variability, transportation, and water resources. A major concern is that snowfall measurements are difficult and can result in significant errors. For example, collection efficiency of most gauge–shield configurations generally decreases with increasing wind speed. In addition, much scatter is observed for a given wind speed, which is thought to be caused by the type of snowflake. Furthermore, the collection efficiency depends strongly on the reference used to correct the data, which is often the Double Fence Intercomparison Reference (DFIR) recommended by the World Meteorological Organization. The goal of this study is to assess the impact of weather conditions on the collection efficiency of the DFIR. Note that the DFIR is defined as a manual gauge placed in a double fence. In this study, however, only the double fence is being investigated while still being called DFIR. To address this issue, a detailed analysis of the flow field in the vicinity of the DFIR is conducted using computational fluid dynamics. Particle trajectories are obtained to compute the collection efficiency associated with different precipitation types for varying wind speed. The results show that the precipitation reaching the center of the DFIR can exceed 100% of the actual precipitation, and it depends on the snowflake type, wind speed, and direction. Overall, this study contributes to a better understanding of the sources of uncertainty associated with the use of the DFIR as a reference gauge to measure snowfall

An Improved Trajectory Model to Evaluate the Collection Performance of Snow Gauges

Recent studies have used numerical models to estimate the collection efficiency\ud of solid precipitation gauges when exposed to the wind, in both\ud shielded and unshielded configurations. The models used computational fluid\ud dynamics (CFD) simulations of the airflow pattern generated by the aerodynamic\ud response to the gauge/shield geometry. These are used as initial conditions\ud to perform Lagrangian tracking of solid precipitation particles. Validation\ud of the results against field observations yielded similarities in the overall\ud behavior, but the model output only approximately reproduced the dependence\ud of the experimental collection efficiency on wind speed. This paper\ud presents an improved snowflake trajectory modeling scheme due to the inclusion\ud of a dynamically-determined drag coefficient. The drag coefficient\ud was estimated using the local Reynolds number as derived from CFD simulations\ud within a time-independent Reynolds Averaged Navier-Stokes (RANS)\ud approach. The proposed dynamic model greatly improves the consistency of\ud results with the field observations recently obtained at the Marshall, CO Winter\ud Precipitation Testbed

Wind-tunnel measurements of the airflow pattern above the collector of different shielded and unshielded precipitation gauges.

Wind is the first environmental source of precipitation undercatch for catching-type precipitation gauge. This work presents an aerodynamic investigation on different precipitation gauge geometries and on a wind shield by means of wind tunnel tests. Experiments have been jointly performed by University of Genoa, DICCA, and Politecnico di Milano within the Italian project PRIN 20154WX5NA \u201cReconciling precipitation with runoff: the role of understated measurement biases in the modelling of hydrological processes\u201d. The airflow, around precipitation gauges, was measured employing two different experimental techniques: a traversing system equipped with \u201cCobra\u201d multi hole pressure probes and the Particles Image Velocimetry PIV. Cobra probes allow to measure the three components of the local flow velocity in the measuring points, while PIV technique provides two-dimensional velocity fields on the investigated planes. The airflow velocity and direction were investigated for different wind speed values and different precipitation gauge geometries: the \u201cchimney\u201d, the \u201ccylindrical\u201d and the \u201cinverted conical\u201d shapes. The effect of a traditional Single Alter windshield was also assessed on the cylindrical shape. These experiments allow to detect qualitatively and quantitatively the main features of the flow, speed-up and updraft, above the collector which influence the particle trajectories and their collection. Results confirm the dependency of the airflow disturbance on the gauge geometry, especially in terms of maximum local velocity and distribution of the upward and downward components of the vertical velocity. PIV velocity fields and Cobra velocity profiles show the expected attenuation of the flow velocity above a gauge located inside the windshield due to the break of the flow induced by the shield slats. The experimental campaign provided a wide dataset suitable for the validation of numerical Computational Fluid Dynamics simulations. This work is propaedeutic to the quantification of the precipitation undercatch and the elaboration of correction curves to obtain the actual precipitation in windy conditions

First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy Dipole Resonance in 68Ni

The excitation of the Pygmy Dipole Resonance (PDR) in the 68Ni nucleus, above the neutron emission threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a 68Ni beam, obtained by the fragmentation of a 70Zn primary beam at INFN-LNS, impinged on a 12C target. The γ-ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The 68Ni isotope as well as other heavy ion fragments were detected using the FARCOS array. The population of the PDR was evidenced by comparing the detected γ-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68Ni channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative γ-ray angular distribution. The measured γ-ray angular distribution confirms the E1 character of the transition. The γ decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error