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JMA/WMO Workshop on Quality Management of Surface Observations RA II WIGOS Project Tokyo, Japan, 19-23 March 2018 Accuracy of precipitation measurements, instrument calibration and techniques for data correction and interpretation

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Accuracy of precipitation measurements, instrument calibration and techniques for data correction and interpretation

University of Genova - DICCA Dept of Civil, Chemical and Environmental Engineering WMO/CIMO Lead Centre “B. Castelli”

  • n Precipitation Intensity

Mattia Stagnaro Luca G. Lanza Arianna Cauteruccio Tokyo, 22 March 2018

JMA/WMO Workshop on Quality Management of Surface Observations RA II WIGOS Project Tokyo, Japan, 19-23 March 2018

WMO

Catching-type Rain Gauges: Standards and Performance

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University of Genova WMO/CIMO LC-PrIn

  • Rain Gauges
  • WMO/CIMO guide
  • Satandards (Catching-type Gauges)
  • Calibration techniques (lab and field)
  • Tipping-Bucket Rain Gauges
  • Weighing Gauges
  • Drop Counter Rain Gauges
  • References
  • Contents
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University of Genova WMO/CIMO LC-PrIn

 Tipping Bucket Rain Gauge (TBRG)  Weighing Gauge (WG)  Drop Counter

  • Rain Gauges

 Disdrometer  Optical  Radar  Impact Catching type Rain Gauges NON-Catching type Rain Gauges

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University of Genova WMO/CIMO LC-PrIn

  • WMO/CIMO guide

Guide to Meteorological Instruments and Methods of Observation: (CIMO guide) WMO- No. 8 (2014 – updated 2017) CHAPTER 6. MEASUREMENT OF PRECIPITATION Annex 6.C: Provides principles and requirements for the rain gauges calibration system and calibration procedures. In addition, describes the procedure for data interpretation of the results in terms of relative error, and the indication of the ±5% limit on the graph to highlights the WMO requirements

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BS 7843–3:2012: Acquisition and management of meteorological precipitation data from a gauge network. Part 3: Code of practice for the design and manufacture of storage and automatic collecting rain gauges. A standard on the UK reference storage daily rain gauge and related aspects. Calibration concepts are based on the CEN/TR 16469. A classification is introduced only on Tipping Bucket gauges:

University of Genova WMO/CIMO LC-PrIn

  • Standards (Catching-Type)

Class Maximum Acceptable Deviation A ±5% dynamically calibrated according to CEN/TR 16469:2012 B 10-15 mm/h static calibration only

Weighing rain gauges should be calibrated accurately by the manufacturer

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University of Genova WMO/CIMO LC-PrIn

  • Standards (Catching-Type)

UNI 11452:2012 HYDROMETRY – MEASUREMENT OF RAINFALL INTENSITY (LIQUID PRECIPITATION) : METROLOGICAL REQUIREMENTS AND TEST METHODS FOR CATCHING TYPE GAUGES This standard defines the metrological requirements for rainfall intensity (liquid precipitation) gauges and establishes classification criteria based on the evaluation of measurement accuracy. This standard is applicable irrespective of the measurement principle (i.e. physical principle on which measurement is based) and the technical and technological characteristics of the specific gauge. The description of test procedures and equipment for calibration and metrological confirmation both in the laboratory and on-site, under steady flow conditions are provided for the catching type gauges only.

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University of Genova WMO/CIMO LC-PrIn

  • Standards (Catching-Type)

Class Maximum Acceptable Deviation Step response time (*) A ±3% < 1 min B ±5% < 1 min C ±5% ±10% ≥ 1 min < 1 min (*) Relevant for weighing gauges only

The various Classes shall be attributed to each rainfall intensity gauge, for the interval of rain intensity values where attribution of the Class is requested, according to the following requirements: 𝑓𝑠𝑓𝑚 % = 𝑆𝐽𝑛𝑓𝑏𝑡 − 𝑆𝐽𝑠𝑓𝑔 𝑆𝐽𝑠𝑓𝑔 ∙ 100 𝑆𝐽𝑛𝑓𝑏𝑡 Measured Rainfall Intensity 𝑆𝐽𝑠𝑓𝑔 Reference Rainfall Intensity

TC 318 – Hydrometry WG 12 – Measurement of rainfall intensity

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University of Genova WMO/CIMO LC-PrIn

  • Calibration techniques (lab and field)

RI simulator performance evaluation:

  • Calibration of the generated RI

(average of multiple realizations).

  • Evaluation of the RI repeatability.
  • Evaluation of the instrumental

delays in executing the commands (start, stop, RI change) Operational range: 20 - 450 ml/min Trueness estimation: max eavg= 1 % evaluated at 60 ml/min Precision estimation: max CV = 0.4% evaluated at 20 ml/min Time resolution: ∆tmin= 15 sec (characteristic time < 200 ms)

VARIABLE RI SIMULATOR (Laboratory)

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University of Genova WMO/CIMO LC-PrIn

  • Calibration techniques (lab and field)

Man-portable device for calibration verification in field Stagi L. and Lanza, L.G. (2006)

VARIABLE RI SIMULATOR (Field)

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University of Genova WMO/CIMO LC-PrIn

  • Calibration techniques (lab and field)

Stagi L. and Lanza, L.G. (2006)

  • Verify the operational status of raingauges
  • According to the raingauge collector size

and the value of rainfall intensity chosen for the calibration, the suitable combination of air intakes and nozzles should be selected to generate the desired constant flow

VARIABLE RI SIMULATOR (Field)

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University of Genova WMO/CIMO LC-PrIn

  • Tipping-Bucket Rain Gauges

Main source of errors: (a) The loss of water during the tipping action in heavy rain SISTEMATIC MECHANICAL ERROR (b) The discontinuous nature of the record may not provide satisfactory data during light drizzle or very light rain. SAMPLING ERROR

Boxplot Class A (±3%) Class B (±3%) Class C (±10%) Boxplot

  • Regr. Curve
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University of Genova WMO/CIMO LC-PrIn

  • Tipping-Bucket Rain Gauges

Colli, M., Lanza, L.G., Chan, P.W., (2013)

1-min aggregation Traditional TBR interpretation Inter-tip algorithm Smoothing algor. Real-world RI

THE SAMPLING ISSUE Development of algorithms aimed at the improvement of the rainfall intensity sampling performed by tipping-buckets type gayges. The investigations was carried out by means of numerical simulations of real-world events measured by the Hong Kong Observatory drop counter located at the HK International Airport. The TBR sensitivity were simulated in

  • rder to isolate the effect of sampling

limitations from other typical uncertainties factors (mechanical errors, wetting looses, etc.): hn=0.5 mm The tested algorithms consist of:

  • a traditional interpretation of the

volumetric sampling.

  • an improved algorithm based on the

inter-tip times.

  • statistical disaggregation of version n.

2 (smoothing algorithm).

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University of Genova WMO/CIMO LC-PrIn

RIref [mm/h] 1 10 100 e smoothed[%]

  • 100
  • 50

50 100 150 200 hn=0.5 mm

Colli, M., Lanza, L.G., Chan, P.W., (2013)

erough: errors using a traditional interpretation algorithm esmoothed: errors using the improved algorithm Box plots of the one-minute RI measurements errors obtained by an ideal TBR with sensitivity hn=0.5 mm.

THE SAMPLING ISSUE

  • Tipping-Bucket Rain Gauges
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University of Genova WMO/CIMO LC-PrIn

THE DOUBLE LAYER TBRG - Shangai SL3

  • Tipping-Bucket Rain Gauges

Mean Relative error - Count of tip

RImeas (mm/h)

50 100 150 200 250

erel (%)

  • 10

10 20 mean error Class A (± 3%) Class B (± 5%) Class C (±10%)

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University of Genova WMO/CIMO LC-PrIn

  • Tipping-Bucket Rain Gauges

Relative error - Count of tip

RImeas (mm/h)

50 100 150 200 250

erel (%)

  • 10

10 20 Boxplot mean error Class A (± 3%) Class B (± 5%) Class C (±10%)

THE DOUBLE LAYER TBRG - Shangai SL3

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University of Genova WMO/CIMO LC-PrIn

  • Tipping-Bucket Rain Gauges

Relative error - Inter-tip algorithm

RImeas (mm/h)

50 100 150 200 250

erel (%)

  • 10

10 20 Boxplot mean error Class A (± 3%) Class B (± 5%) Class C (±10%)

THE DOUBLE LAYER TBRG - Shangai SL3

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University of Genova WMO/CIMO LC-PrIn

  • Weighing Gauges

0.0 0.2 0.4 0.6 0.8 1.0 1.2 60 120 180 240 RI [-] t [sec] RI ref RI step 1 RI step 2 RI tot

𝑧 𝜐 = 1 − 𝑓−𝑢

𝜐

Correction methodology for WGs based

  • n the similarity with a first order

dynamical system. The estimation of WG dynamic behavior has been performed by realizing single, double and multiple steps flow rates with the laboratory RI generator.

ℎ𝐽 𝑛𝑗𝑜 = 𝑒𝑢 − 𝜐 1 − 𝑓

−𝑒𝑢 𝜐

ℎ𝐽𝐽 𝑛𝑗𝑜 = 𝑒𝑢 − 𝜐 1 − 𝑓

−𝑒𝑢 𝜐

1 + 𝛽 + + 𝛽 𝜐 1 − 𝑓

−𝑒𝑢 𝜐 2

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University of Genova WMO/CIMO LC-PrIn

  • Weighing Gauges

Colli, M., Lanza, L.G., La Barbera, P., 2013

Improved estimation of Rainfall Intensity employing the correction methodology:

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University of Genova WMO/CIMO LC-PrIn

  • Drop Counter Rain Gauges

Measuring Principle:

  • It is a catching type Rain gauge.

The funnel collect the rain towards a calibrated nozzle which start to drip.

  • An optical sensor, placed under the

nozzle, detect the drop passage and calculate the drop frequency. The droplet frequency is related to the rainfall intensity.

  • The total volume measured by the

sensor is calculated assuming a constant volume of the droplets.

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University of Genova WMO/CIMO LC-PrIn

  • Drop Counter Rain Gauges

RIref (mm/h)

50 100 150 200

erel (%)

  • 10

10 20 30 40 Class A (± 3%) Class B (± 5%) Class C (±1%)

Performance of Drop Counting Rain Gauge under Dynamic Calibration using a constant drop volume

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University of Genova WMO/CIMO LC-PrIn

  • Drop Counter Rain Gauges

The tests show that the volume of the droplets varies as the drop frequency changes:

Drop Freq. (drops/min)

200 400 600 800 1000

Drop Volume (g)

0.00 0.02 0.04 0.06 0.08

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University of Genova WMO/CIMO LC-PrIn

  • Drop Counter Rain Gauges

RIref (mm/h)

50 100 150 200

erel (%)

  • 10

10 20 30 40 Class A (± 3 %) Class B (± 5 %) Class C (±10 %)

RIref (mm/h)

50 100 150 200

erel (%)

  • 10

10 20 30 40 Class A (± 3%) Class B (± 5%) Class C (±1%)

Adopting a calibration curve of the drop volume, the instrument performance increase and fulfil the Class A requirements for almost the full operating range.

𝑊

𝑒𝑠𝑝𝑞 = 𝑔 𝐸𝐺

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University of Genova WMO/CIMO LC-PrIn

  • Drop Counter Rain Gauges

Operational limit: Since the instrument measures only frequencies of the droplets, it is not possible to know a-priori the real RI, because it is not possible to know if the limit is exceeded or not. A co-located rain gauge is required to be able to use this gauge

  • perationally.

The operational limit of this kind of instrument is given by the RI at which the water flux from the nozzle starts to be continuous or it can not be considered as a regular drop dispensing flux.

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University of Genova WMO/CIMO LC-PrIn

  • Non-Catching type Gauges

Non-catching type gauges Tests on NON-Catching type instruments : disdrometers

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University of Genova WMO/CIMO LC-PrIn

  • Non-Catching type Gauges

Tests on NON-Catching type instruments : disdrometers The results of the tests highlights that the instruments in some cases attribute the drops to the upper class

  • f diameter, inducing a overestimation of rainfall

intensity.

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University of Genova WMO/CIMO LC-PrIn

  • References
  • BS 7843-3:2012: Acquisition and management of meteorological precipitation data from a gauge

network., Standard, British Standards Institution, 2012.

  • UNI 11452:2012: Hydrometry - Measurement of rainfall intensity (liquid precipitation) - Metrological

requirements and test methods for catching type gauges., Standard, Ente Nazionale Italiano di Unificazione, Milano, IT, 2012.

  • WMO: Guide to Meteorological Instruments and Methods of Observation (CIMO)-No. 8, World

Meteorological Organization, 8th edn., 2014

  • Stagi L. and Lanza, L.G. (2006). Device for the generation of various known and constant liquid flow
  • rates. Patent University of Genoa n. 102006A000868, 7 December 2006.
  • Colli, M., Lanza, L.G. and P.W. Chan (2013). Co-located tipping-bucket and optical drop counter RI

measurements and a simulated correction algorithm. Atmos. Res., 119, 3-12.

  • Colli, M., Lanza, L.G. and P. La Barbera (2013). Performance of a weighing rain gauge under laboratory

simulated time-varying reference rainfall rates, Atmos. Res., 131, 3-12

  • Stagnaro, M., Colli, M., Lanza, L.G. and P.W. Chan (2016). Performance of post-processing algorithms for

rainfall intensity measurements of tipping-bucket rain gauges. J. Atmos. Meas. Techn., 9, 5699–5706.

http://www.precipitation-intensity.it

for further information: mattia.stagnaro@unige.it luca.lanza@unige.it