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

Performance of post-processing algorithms for rainfall intensity using measurements from tipping-bucket rain gauges

Abstract. Eight rainfall events recorded from May to September 2013 at Hong Kong International Airport (HKIA) have been selected to investigate the performance of post-processing algorithms used to calculate the rainfall intensity (RI) from tipping-bucket rain gauges (TBRGs). We assumed a drop-counter catching-type gauge as a working reference and compared rainfall intensity measurements with two calibrated TBRGs operated at a time resolution of 1 min. The two TBRGs differ in their internal mechanics, one being a traditional single-layer dual-bucket assembly, while the other has two layers of buckets. The drop-counter gauge operates at a time resolution of 10 s, while the time of tipping is recorded for the two TBRGs. The post-processing algorithms employed for the two TBRGs are based on the assumption that the tip volume is uniformly distributed over the inter-tip period. A series of data of an ideal TBRG is reconstructed using the virtual time of tipping derived from the drop-counter data. From the comparison between the ideal gauge and the measurements from the two real TBRGs, the performances of different post-processing and correction algorithms are statistically evaluated over the set of recorded rain events. The improvement obtained by adopting the inter-tip time algorithm in the calculation of the RI is confirmed. However, by comparing the performance of the real and ideal TBRGs, the beneficial effect of the inter-tip algorithm is shown to be relevant for the mid–low range (6–50 mmh−1) of rainfall intensity values (where the sampling errors prevail), while its role vanishes with increasing RI in the range where the mechanical errors prevail

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

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

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

A field assessment of a novel rain measurement system based on earth-to-satellite microwave links

This work presents the performance of an innovative environmental monitoring system - Smart Rainfall System (SRS) - that estimates rainfall in real-time by means of the analysis of the attenuation of satellite signals (DVB-S in the microwave Ku band). SRS consists in a set of peripheral microwave sensors placed on the field of interest, and connected to a central processing and analysis node. It has been developed jointly by the University of Genoa, with its departments DITEN and DICCA and the University spin-off \u201cArtys Srl\u201d. The rainfall intensity measurements accuracy and sensitivity performance of SRS are discussed, based on preliminary results from a field comparison experiment at the urban scale. The test-bed is composed by a set of preliminary measurement sites established since Autumn 2016 in the Genoa (Italy) municipality and the data collected from the sensors during a selection of rainfall events is studied. Point-scale rainfall intensity measurements made by calibrated tipping-bucket rain gauges constitute the reference for the comparative analysis of the system performance. The dynamic calibration of the reference rain gauges has been carried out at the laboratories of DICCA using an automatic calibration rig and the measurements have been processed taking advantage of smart algorithms to reduce counting errors. Additional information about the spatial distribution of precipitation have been provided by the WSR radar of Monte Settepani. An objective of this investigation is the optimization of the specific attenuation model parameters for rain with respect to those recommended by the International Telecommunication Union standard ITU-R P.838-3. In addition, the experimental set-up allows a fine tuning of the retrieval algorithm and a full characterization of the accuracy of the rainfall intensity estimates from the microwave signal attenuation as a function of different precipitation regimes

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