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 [...

Thermo-fluid dynamic simulation of the Hotplate precipitation gauge.

The present study addresses the aerodynamic response of the recently developed "Hotplate" liquid/solid precipitation gauge when exposed to the wind. The Hotplate gauge employs two heated thin 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, falling on the instrument and the precipitation rate. However, 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. After an initial analysis of real-world "Hotplate" measurements from a field test site located in Marshall, CO (USA) and the comparison with more traditional measurements obtained from a co-located, shielded reference gauge, the role of wind-induced errors is highlighted. The main approach used in this work is based on the numerical simulation of the airflow field around the gauge, using Computational Fluid Dynamics (CFD) to identify areas where the wind-induced updraft, local acceleration and turbulence are significant. The performed CFD airflow simulations use the URANS SST k - \u3c9 modelling scheme, and are the first 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, which measures the heat flux needed to evaporate the collected water amount under a constant plate surface temperature, thermo-fluid dynamic simulations are addressed as well. Dedicated tests have been performed in the wind tunnel facility available at DICCA, University of Genoa to validate simulation results. Results indicate that the presence of wind is a relevant source of systematic bias when using the "Hotplate" gauge for the measurement of precipitation, and its effect must be corrected by adopting suitable correction curves as a function of the wind velocity. The magnitude of the correction can be derived from numerical thermo-fluid dynamic simulations and an assessment of the airflow patterns developing around the gauge at various wind velocity regimes is provided in this work. Wind tunnel tests allowed for a substantial validation of the numerical results, and possible improvements of the model are highlighted and proposed for future developments

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

The benefits of one-dimensional detectors for high-pressure powder X-ray diffraction

High-pressure powder X-ray diffraction is a fundamental technique for investigating structural responses to externally applied force. Synchrotron sources and two-dimensional detectors are required. In contrast to this conventional setup, high-resolution beamlines equipped with one-dimensional detectors could offer much better resolved peaks but cannot deliver accurate structure factors because they only sample a small portion of the Debye rings, which are usually inhomogeneous and spotty because of the small amount of sample. In this study, a simple method to overcome this problem is presented and successfully applied to solving the structure of an L-serine polymorph from powder data. A comparison of the obtained high-resolution high-pressure data with conventional data shows that this technique, providing up to ten times better angular resolution, can be of advantage for indexing, for lattice parameter refinement, and even for structure refinement and solution in special cases

Combined Approach of Mechanochemistry and Electron Crystallography for the Discovery of 1D and 2D Coordination Polymers

Mechanochemical synthesis is an attractive preparative method that combines a green approach with versatility, efficiency, and rapidity of reaction. However, it often yields microcrystalline materials, and their small crystal size is a major hindrance to structure elucidation with conventional single-crystal or powder X-ray diffraction methods. This work presents the novel approach of combining mechanochemistry with electron diffraction techniques to elucidate the crystal structure of metal−organic compounds of zinc(II) and copper(II) with 2,6-pyridinedicarboxylic acid and 4,4′-bipyridine

Extended “orbital molecules” and magnetic phase separation in Bi0.68Ca0.32MnO3

The low-temperature structure of Bi0.68Ca0.32MnO3 has been solved from electron and neutron diffraction data. The quantitative simultaneous refinement indicates an ordering of the Mn cations in a “stripe/chess”-like pattern. The ordering is accompanied by the formation of short Mn—Mn distances and the rearrangement of the Mn—O bonds indicating the development of complex extended “orbital molecules.” The primary order parameter breaks inversion symmetry and allows the generation of a spontaneous electrical polarization as the secondary order parameter. The neutron data at low temperature indicate the coexistence of a pseudo-CE long-range-ordered structure with a strongly reduced moment and short-range ferromagnetic correlations. These results indicate an intricate competition between the charge, orbital, and magnetic degrees of freedom and the Bi3+ stereoactivity in this manganite system