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Introduction Rationale The test system Method Results Conclusions

WMO TECHNICAL CONFERENCE ON METEOROLOGICAL AND ENVIRONMENTAL INSTRUMENTS AND METHODS OF OBSERVATION TECO-2012 Brussels, Belgium, 16-18 October 2012. ENVIRONMENTAL CHAMBER ASSESSMENT OF THE ACCURACY OF WEIGHING GAUGES UNDER LOW INTENSITY REFERENCE FLOW RATES M. Colli, E. Vuerich and L.G. Lanza

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Introduction Rationale The test system Method Results Conclusions

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  1. WMO TECHNICAL CONFERENCE ON METEOROLOGICAL AND ENVIRONMENTAL INSTRUMENTS AND METHODS OF OBSERVATION TECO-2012 Brussels, Belgium, 16-18 October 2012 ENVIRONMENTAL CHAMBER ASSESSMENT OF THE ACCURACY OF WEIGHING GAUGES UNDER LOW INTENSITY REFERENCE FLOW RATES M. Colli, E. Vuerich and L.G. Lanza WMO Lead Centre on Precipitation Intensity (Italy)

  2. OUTLINE • Introduction • Rationale • The test system • Method • Results • Conclusions TECO Brussels (Belgium) - 16-18 October 2012

  3. Introduction The WMO/CIMO Lead Centre on Precipitation Intensity performs laboratory and in-field investigations and intercomparison campaigns in order to assess and improve the performance of precipitation intensity gauges. Genoa lab Genoa lab Vigna di Valle test site TECO Brussels (Belgium) - 16-18 October 2012

  4. Rationale The Lead Centre was involved in the pre-SPICE session of measurements with the objective of performing laboratory tests on two potential candidate reference instruments, the Geonor T-200B (3-wires) and the OTT Pluvio2 weighing gauges. Relevant issues within SPICE are the possibility of actually sampling snowfall data at a high resolution in time (one minute) and to ensure consistency between precipitation depth and intensity measurements. Laboratory experiments at very low precipitation rates and controlled environmental testing conditions have been performed in order to demonstrate the good performance of the candidate reference instruments and to make available some reliable quantitative assessment of their accuracy and precision in measuring precipitation intensity. This information, together with additional testing of the gauges under non-steady reference flow rate conditions and the assessment of the dynamic behavior of the reference instruments, will be useful to better interpret discrepancies between any instrument under test and the co-located reference instrument during the next SPICE winter campaign. TECO Brussels (Belgium) - 16-18 October 2012

  5. Rationale The SPICE reference snowfall gauges GEONOR T-200B OTT PLUVIO2 Type: vibrating wires gauge Collector area: 200 cm2 Resolution: 0.05 mm Range of measure: not provided Capacity 600 mm Type: weighing gauge Collector area: 200 cm2 Resolution: 0.1 mm Range of measure : not provided Capacity: 1500 mm TECO Brussels (Belgium) - 16-18 October 2012

  6. The test system Flow rate generator • Constant head aspiration reservoir for the pumping device • Flow rate generator based on a ISMATEC REGLO-CPF Digital piston pump. range: 0.1 to 45 ml/min accuracy: < 1% CV: <= 0.17 % TECO Brussels (Belgium) - 16-18 October 2012

  7. The test system Environmental chamber (Pratica di Mare – Rome) • Real time monitoring system of the environmental chamber conditions (temperature, RH, etc.) • METTLER TOLEDO Densito 0PX Temperature and density meter for the testing fluid sample (range: 0 to 2 g/cm3, resolution: 0.0001 g/cm3) • METTLER TOLEDOPB4002-S/FACT Precision scale for validation of generated flow rates range: 0 to 4000 g resolution: 0.01g TECO Brussels (Belgium) - 16-18 October 2012

  8. The test system Measured residual error of the flow rate generator 1.1 mm/h 54 mm/h Testing fluid: Ethylene glycol dimethyl ether density (liquid state at snowing temperature) TECO Brussels (Belgium) - 16-18 October 2012

  9. Method 4 different combination of equivalent precipitation intensities (RI) and durations These generation conditions were realized to test the OTT PLUVIO 2 and the GEONOR T-200B gauges at the following environmental temperatures: -5, +0 and +5°C Precise evaluation of the evaporation in terms of the negative contribution to precipitation intensity (RIevap) was also performed by weighing the content of a sample ethylene glycol dimethyl ether flask at the beginning and at the end of each test. TECO Brussels (Belgium) - 16-18 October 2012

  10. Method Level 0 indication: Vibrating wires average frequency values converted into net water depth by means of a manufacturer provided function GEONOR T-200B OTT PLUVIO2 Level 0 indication: “Real Time Bucket” output string, the unfiltered gross water depth measured by the weighing gauge (the bucket contribution is included) The water depth measurements were retrieved with a 6-seconds time resolution. TECO Brussels (Belgium) - 16-18 October 2012

  11. Method Precipitation intensity values used for the rain gauge performance evaluation are averaged over a 2-minute time span, providing the RI2min measurement. GEONOR T-200B OTT PLUVIO2 The data analysis is carried out by calculating the percentage relative errors e between RI2min measurements and the reference value previously validated by weighing the actual fluid depth generated by the rainfall simulation system. TECO Brussels (Belgium) - 16-18 October 2012

  12. Results Measurement accuracy in terms of average percentage relative errors (eavg) with varying the environmental temperature (T) and the simulated precipitation intensity (RIref) Measurement precision in terms of standard deviation of the percentage relative errors (dev.st(eavg)) when varying the environmental temperature (T) and the simulated precipitation intensity (RIref) TECO Brussels (Belgium) - 16-18 October 2012

  13. Results Rain gauges performance at T = -5 °C TECO Brussels (Belgium) - 16-18 October 2012

  14. Results Rain gauges performance at T = 0 °C TECO Brussels (Belgium) - 16-18 October 2012

  15. Results Rain gauges performance at T = +5 °C TECO Brussels (Belgium) - 16-18 October 2012

  16. Conclusions • Steady-state tests demonstrate that precision and accuracy (trueness) deteriorate with the decrease of the precipitation intensity for both instruments. Precision, in particular, decreases with reducing the intensity, with relevant differences between the two gauges. • The inherent resolution of the instruments in measuring precipitation intensity also affects the performance at low precipitation rates. • Dynamic tests performed at ambient temperature (not reported here) show that both gauges are able to reproduce likely real world events with quite comparable performance. • The results from laboratory tests open the way for possible improvements of their accuracy and precision. • The Lead Centre is continuing to perform laboratory tests under both static and dynamic conditions and with varying temperature as well. TECO Brussels (Belgium) - 16-18 October 2012

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