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Colli M.,University of Genoa | Lanza L.G.,University of Genoa | Lanza L.G.,Wmo Cimo Lead Center stelli On Precipitation Intensity | La Barbera P.,University of Genoa | Chan P.W.,Hong Kong Observatory
Atmospheric Research

The contribution of any single uncertainty factor in the resulting performance of infield rain gauge measurements still has to be comprehensively assessed due to the high number of real world error sources involved, such as the intrinsic variability of rainfall intensity (RI), wind effects, wetting losses, the ambient temperature, etc. In recent years the World Meteorological Organization (WMO) addressed these issues by fostering dedicated investigations, which revealed further difficulties in assessing the actual reference rainfall intensity in the field.This work reports on an extensive assessment of the OTT Pluvio2 weighing gauge accuracy when measuring rainfall intensity under laboratory dynamic conditions (time varying reference flow rates). The results obtained from the weighing rain gauge (WG) were also compared with a MTX tipping-bucket rain gauge (TBR) under the same test conditions. Tests were carried out by simulating various artificial precipitation events, with unsteady rainfall intensity, using a suitable dynamic rainfall generator. Real world rainfall data measured by an Ogawa catching-type drop counter at a field test site located within the Hong Kong International Airport (HKIA) were used as a reference for the artificial rain generation system.Results demonstrate that the differences observed between the laboratory and field performance of catching-type gauges are only partially attributable to the weather and operational conditions in the field. The dynamics of real world precipitation events is responsible for a large part of the measurement errors, which can be accurately assessed in the laboratory under controlled environmental conditions. This allows for new testing methodologies and the development of instruments with enhanced performance in the field. © 2013 Elsevier B.V. Source

Lanza L.G.,University of Genoa | Lanza L.G.,Wmo Cimo Lead Center stelli On Precipitation Intensity | Stagi L.,Wmo Cimo Lead Center stelli On Precipitation Intensity
Water Science and Technology

The analysis of counting and catching errors of both catching and non-catching types of rain intensity gauges was recently possible over a wide variety of measuring principles and instrument design solutions, based on the work performed during the recent Field Intercomparison of Rainfall Intensity Gauges promoted by World Meteorological Organization (WMO). The analysis reported here concerns the assessment of accuracy and precision of various types of instruments based on extensive calibration tests performed in the laboratory during the first phase of this WMO Intercomparison. The non-parametric analysis of relative errors allowed us to conclude that the accuracy of the investigated RI gauges is generally high, after assuming that it should be at least contained within the limits set forth by WMO in this respect. The measuring principle exploited by the instrument is generally not very decisive in obtaining such good results in the laboratory. Rather, the attention paid by the manufacturer to suitably accounting and correcting for systematic errors and time-constant related effects was demonstrated to be influential. The analysis of precision showed that the observed frequency distribution of relative errors around their mean value is not indicative of an underlying Gaussian population, being much more peaked in most cases than can be expected from samples extracted from a Gaussian distribution. The analysis of variance (one-way ANOVA), assuming the instrument model as the only potentially affecting factor, does not confirm the hypothesis of a single common underlying distribution for all instruments. Pair-wise multiple comparison analysis revealed cases in which significant differences could be observed. © IWA Publishing 2012. Source

Colli M.,University of Genoa | Colli M.,Wmo Cimo Lead Center stelli On Precipitation Intensity | Rasmussen R.,U.S. National Center for Atmospheric Research | Theriault J.M.,University of Quebec at Montreal | And 4 more authors.
Journal of Applied Meteorology and Climatology

Recent studies have used numerical models to estimate the collection efficiency of solid precipitation gauges when exposed to the wind in both shielded and unshielded configurations. The models used computational fluid dynamics (CFD) simulations of the airflow pattern generated by the aerodynamic response to the gauge-shield geometry. These are used as initial conditions to perform Lagrangian tracking of solid precipitation particles. Validation of the results against field observations yielded similarities in the overall behavior, but the model output only approximately reproduced the dependence of the experimental collection efficiency on wind speed. This paper presents an improved snowflake trajectory modeling scheme due to the inclusion of a dynamically determined drag coefficient. The drag coefficient was estimated using the local Reynolds number as derived from CFD simulations within a time-independent Reynolds-averaged Navier-Stokes approach. The proposed dynamic model greatly improves the consistency of results with the field observations recently obtained at the Marshall Field winter precipitation test bed in Boulder, Colorado. © 2015 American Meteorological Society. Source

Theriault J.M.,University of Quebec at Montreal | Rasmussen R.,U.S. National Center for Atmospheric Research | Petro E.,Ecole Polytechnique de Montreal | Trepanier J.-Y.,Ecole Polytechnique de Montreal | And 4 more authors.
Journal of Applied Meteorology and Climatology

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. © 2015 American Meteorological Society. Source

Santana M.A.A.,National Institute for Space Research | Guimaraes P.L.O.,National Institute for Space Research | Lanza L.G.,University of Genoa | Lanza L.G.,Wmo Cimo Lead Center stelli On Precipitation Intensity | Vuerich E.,Wmo Cimo Lead Center stelli On Precipitation Intensity
Meteorological Applications

The monitoring of hydro-meteorological variables for operational and research purposes requires accurate measurements. The reliability of such measurements varies depending on the need to meet different requirements in several sectors, including aviation, agriculture, civil protection, entertainment and weather forecast. In the case of rain gauges, many factors and variables affect the measurement of liquid and solid precipitation in the field. Calibration is a primary tool for quality control and requires suitable infrastructure to perform the tests, and is done by using properly evaluated testing methods and procedures. The current study presents the analysis of a typical calibration system for tipping-bucket rain gauges, using the gravimetric method, in accordance with the recommendations and requirements of both meteorology and metrology. As a result, the uncertainty contribution of each component of the system and an assessment of the resulting overall uncertainty budget are obtained. © 2015 Royal Meteorological Society. Source

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