(project leader)

Results of the WMO Laboratory Intercomparisonof rainfall intensity gauges

Luca G. LanzaUniversity of Genoa

WMO

(Project Leader)

DIAMUNIGE

September 2004 – September 2005

BackgroundMethods

Results & Discussion

WMO

Tipping-bucket rain gauges

RIMCO

HS-TB3

PAAR

Meteoservis MR3H

SIAP

CAE

ETG

Lambrecht

Casella

Waterlog

Yokogawa

IMD

0

100

200

300

0 100 200 300

I reference [mm/h]

I m

ea

su

red

[m

m/h

]

600 mm/h

720 mm/h

635 mm/h

2000 mm/h



WMO

0.02 to 2000 mmh-1 full range 0.02 to 0.2 mmh-1 rep. as

“trace” Output averaging time: 1 minute

Maximum error in RI measurements:• 0.2 to 2 mmh-1: 0.1 mmh-1

• 2 to 2000 mmh-1: 5 %

Renewed user requirements in meteorology:- meteo-hydrological warnings- interfacing meteorological and hydrological models- flood forecasting- etc.

Unusual variable: rainfall intensity (RI)(lack of knowledge, practice, standardisation,

recommendations, measurement instruments, etc.)

Expert Meeting on Rainfall Intensity Measurements Bratislava (Slovakia), April 2001

Intercomparison RI Measurement Instruments Laboratory tests first, then in the Field

Amount of precipitationcollected per unit time interval



WMO

Previous related WMO Intercomparisons:. International Comparison of National Precipitation Gauges with a Reference Pit Gauge

(Sevruk et al., 1984). . WMO Solid Precipitation Measurement Intercomparison (Goodison et al., 1998). Precipitation intensity was investigated for the first time in the assessment of present weather systems: . WMO Intercomparison of Present Weather Sensors/Systems (Leroy et al., 1998). but only for qualitative information (light, moderate, heavy)

focus on rainfall accumulationlow intensity rainfall (snow)overall effect of counting and catching errors

Catching errors = The errors due to the weather conditions at the collector, as well as those related to wetting, splashing and evaporation processes. They indicate the ability of the instrument to collect the exact amount of water that applies from the definition of precipitation at the ground, i.e. the total water falling over the projection of the collector’s area over the ground.

Counting errors = Counting errors are on the other hand related to the ability of the instrument to “sense” correctly the amount of water that is collected by the instrument. They can be experienced both in catching and non-catching type of instruments, although in the latter case the assessment of such errors is very difficult, and is hard to be performed in laboratory conditions.



WMO

Laboratory controlled conditionsconstant flow rate

reference flow ratecounting errors

Drawbacks:no real rainfall (variability, etc.)no catching errorsno real operating conditions

Follow-up in the fieldWMO Field Intercomparison

of Rainfall Intensity GaugesVigna di Valle (Rome)

To start in late Spring 2007

The main objective of the intercomparison was to test the performances of catchment type rainfall intensity gauges of different measuring principles under documented conditions.

Laboratory vs. Field Tests



WMO

Genova, DIAm

MeteoFrance

De Bilt, KNMI

Different testing devices in the three laboratories



WMO

List of participating instruments



WMO

Tipping Bucket

Each calibration was performed at least at seven reference flow rates with the following rules :• Seven reference intensities are fixed at 2, 20, 50, 90, 130, 170, 200 mm/h;• If the maximum declared intensity is less or equal to 500 mm·h-1, further reference intensities are determined at 300 and 500 mm·h-1.• Otherwise, three further reference intensities are determined within the remaining range of operation of the instruments by dividing it logarithmically from 200 mm·h-1 up to the maximum declared intensity.

The reference intensity has been obtained within the following limits:

1.5 – 4 mm·h-1 at 2 mm·h-1

15 – 25 mm·h-1 at 20 mm·h-1

and within a limit of 10% at higher intensities.



WMOStep response test

0

50

100

150

200

250

300

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

time [sec]

Ir , Im [mm/h]

Weighing gauges

In addition to measurements based on constant flow rates, the step response of each instrument was checked based on the devices developed by each laboratory.

The step response of the weighing gauges was measured by switching between two different constant flows, namely from 0 mm·h-1 to 200 mm·h-1 and back to 0 mm·h-1. The constant flow was applied until the output signal of the weighing rain gauge was stabilized. The time resolution of the measurement was higher than 1 minute, e.g. 10 seconds, and the possible delay was evaluated by determining the first time interval when the measure is stabilized, within a maximum period of 10 minutes.

Attention was paid in particular to assess the effects of vibrations and to reduce them in order that their impact on the measurement is < 1%.

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90 100 110 120 130

time/duration [min]

Ir [

mm

/h]

- Im

[m

m/h

]



WMO

Presentation of the results

The results are presented in the form of an average error curve that is derived as follows:• The error is evaluated per each reference flow rate as:

where Im is the intensity measured by the

instrument and Ir the actual reference intensity

provided to the instrument;• Five calibration tests are performed per each set of reference intensities, so that five error curves are associated with each instrument;• An average error curve is obtained by discarding the minimum and maximum error value obtained per each reference flow rate, and evaluating the arithmetic mean of the three remaining error and reference values.Results have been presented in the (Ir, Im) space by fitting data with a power law curve:

with and two suitable numeric coefficients.

%100

r

rm

I

IIe

mr II

CAE PMB2 #14897

-20%

-10%

0%

10%

20%

0 50 100 150 200 250 300

I reference (mm/ h)

Err

or

(%)

"before correction"

after correction

ETG R102 #1535

Im = 0.9834 Ir 1.0023

R2 = 0.99960

100

200

300

400

0 100 200 300 400

I reference (mm/ h)

I m

easu

red

(mm

/h)

DIAM

MétéoFrance

Power law trend



WMO

Tipping-bucket rain gauges

RIMCO

HS-TB3

PAAR

Meteoservis MR3H

SIAP

CAE

ETG

Lambrecht

Casella

Waterlog

Yokogawa

IMD

0

100

200

300

0 100 200 300

I reference [mm/h]

I m

easu

red

[m

m/h

]

600 mm/h

720 mm/h

635 mm/h

2000 mm/h



WMO Tipping-bucket rain gauges

RIMCO HS-TB3

PAAR

IMS

SIAP

Lambrecht

Casella

0

100

200

300

0 100 200 300

I reference [mm/h]

I m

easu

red

[m

m/h

]

Tipping-bucket rain gauges with correction

Meteoservis MR3H

ETG

Yokogawa

0

100

200

300

0 100 200 300

I reference [mm/h]

I m

easu

red

[m

m/h

]

CAE

Waterlog

Weighing gauges

0

100

200

300

0 100 200 300

I reference [mm/h]

I mea

sure

d [m

m/h

]

Water level gauges

0

100

200

300

0 100 200 300

I reference [mm/h]

I m

easu

red

[m

m/h

]

Alluvion

Serosi

Per

form

ance

s o

f ea

ch i

nd

ivid

ual

gau

ge



WMO

Synthesis of the results



WMO Coefficients of the error curves



WMOAverage relative error

0

50

100

150

200

250

300

0 50 100 150 200 250 300

I reference [mm/h]

I mea

sure

d [m

m/h

]

A r

A m

eAVG = (A m - A r) / A r

RIMCO

HS TB-3

MR3H-FC MPS

Serosi

Vaisala

Geonor

IMS

PAAR

SIAP

Casella

Lambrecht

MRW500

YokogawaETG

CAE

OTT

Waterlog

Alluvion

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.02

-0.02

Overall intercomparisonof the participating types of

Rainfall Intensity Gauges

Intercomparison results



WMOConclusions

The tipping-bucket rain gauges that were equipped with proper correction software provided good quality rainfall intensity measurements. The gauges where no correction was applied had larger errors. In some cases problems of water storage in the funnel occurred that could limit the usable range for rain intensity measurement.

The uncertainty of the rainfall intensity is generally less for weighing gauges than for the tipping-bucket rain gauges under constant flow rate conditions, provided there is a sufficient time to stabilize the instrument. The measurement of rainfall intensity is affected by the response time of the acquisition system. Significant delays were observed in “sensing” the variation in time of the rain intensity. The delay is the result of the internal software which is intended to filter the noise. Only one instrument had a delay that met the WMO 1-minute rainfall intensity requirement.

The two gauges using a conductivity measurement for determining water level showed good performances in terms of uncertainty under these controlled laboratory conditions. Siphoning problems for one gauge limits its ability to measure a wide range of rainfall intensity. For the other one, a limitation is related to the emptying mechanism, in which case 2-minute delay was observed. These gauges are potentially sensitive to the water conductivity, but with no demonstrated problems during the laboratories’ tests.

The laboratory tests were performed under controlled conditions and constant flow rates (rain intensities) so as to determine the intrinsic counting errors. It must be considered that rainfall intensity is highly variable in time. Furthermore, catching errors may have a strong influence on the overall uncertainty of the measurement.

(project leader)

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