THE WMO FIELD INTERCOMPARISON OF RAINFALL
INTENSITY (RI) GAUGES in Vigna di Valle (ITALY), October
2007- April 2009: relevant aspects and results.
Sestola (Italy), 23 June 2009
WMO-TECO, Helsinki, 31 Aug – 1 Sept 2010
Vuerich E. (ITALY)
CONTENTS
BACKGROUND AND
OBJECTIVES
PROCEDURES AND METHODS
DATA ANALYSIS AND
RESULTS
THE INTERCOMPARISON TEST SITE
CONCLUSIONS AND
DEVELOPMENTS
RECOMMENDATIONS
ITALIAN MET SERVICE – AIR FORCE (IMS)
CENTRE OF MET EXPERIMENTATIONS (RESMA)
VIGNA DI VALLE
ITALY
BACKGROUND
Following the increased recognition of scientific and practical
issues related to the assessment of possible climatic trends, the
mitigation of natural disasters (e.g. storms and floods), the
hindering of desertification, the attention paid to accuracy and
reliability in rainfall measurements is currently increasing
The WMO recognized these emerging needs and promoted a
couple of mile-stone meetings:
- (1) Bratislava, Slovak Republic, 23-25 April 2001: calibration
of rain gauges and general aspects of measurements (I phase:
WMO Laboratory Intercomparison of RI gauges, 2004-2005)
- (2) Geneva, Switzerland, 5-9 December 2005: operational
aspects of field measurements and achievable accuracy (II
phase: WMO Field Intercomparison of RI gauges, 2007-2009)
BACKGROUND
Previous WMO intercomparisons: accumulated amounts, on
low RI or on snowfall, on qualitative information
The WMO FI-RI gauges started in Vigna di Valle (Italy) at
the IMS Centre of Met Experimentations on the 1st of October
2007 and, after a 6-months extension, it was concluded in April
2009 (first intercomparison for quantitative 1MIN-RI)
Main objective: to assess and compare quantification and
catching errors of both catching and non-catching type of RI
gauges with the emphasis on high rainfall intensity in order to
complain with the WIGOS basic concepts of improving
standardization, data homogeneity, data quality, compatibility,
interoperability.
Other tasks: providing draft recommendations for CIMO and
guidance material for inclusion into the CIMO guide.
PROCEDURES AND METHODS
30 RI gauges (4 working references) capable to measure RI up to
200mm/h at 1min resolution were selected, randomly distributed
(avoid clustering of large gauges) and installed on dedicated ground
platforms at 1 m height (no windshields)
13 additional meteorological sensors were installed in later
positions for monitoring environmental conditions.
PROCEDURES AND METHODS
List of participating rain gauges (26+4 working references)
#
RAIN GAUGES
MEAS. PRINCIPLE
#
RAIN GAUGES
MEAS. PRINCIPLE
1
RIMCO 7499
Tipping bucket
14
Vaisala VRG101
Weighing gauge
2
Paar AP23
Tipping bucket
15
OTT Pluvio
Weighing gauge
3
Precis-Mecanique
Tipping bucket
16
EWS PG200
Weighing gauge
4
Thies PT
Tipping bucket
17/30
GEONOR T-200B
Weighing gauge
5/27
ETG R102
Tipping bucket
18
MPS TRwS
Weighing gauge
6
LSI-LASTEM DQ031
Tipping bucket
19
SA „MIRRAD“ MPA1M
Not Participating
7
SIAP-MICROS
UM7525/I
Tipping bucket
20
Vaisala PWD22
Optical Disdrometer
8/28
CAE PMB2
Tipping bucket
21
OTT Parsivel
Optical Disdrometer
9
Davis Rain Collector
II
Tipping bucket
22
Thies LPT
Optical Disdrometer
10
Lambrecht 15188
Tipping bucket
23
Vaisala WXT510
Acoustic impact
11
MTX PP040
Tipping bucket
24
Eigenbrodt ANS 410
Water pressure
12
Env. Meas. Ltd
ARG100
Tipping bucket
25
KNMI electric
raingauge
Water level
13/29
Meteoservis
MRW500
Weighing gauge
26
PVK ATTEX “DROP”
Doppler Radar
PROCEDURES AND METHODS
4 Reference gauges, the “Composite Working Reference ” (C.W.R.)
were inserted in Reference Rain Gauge Pits (R.R.G.P.) at the centre of
the Intercomparison site (collectors at ground level) – Minimization
of weather related catching errors (e.g. Jevons effect, 1861)
Standard adopted: ISO/EN-13798: “Specification for a reference
rain gauge pit”, recently revised in 2010
CWR (Recom. 3 CIMOXIV):
corrected
tipping
bucket rain gauges (TBRG)
and weighing gauges (WG)
with the shortest step
response and the lowest
uncertainty according to
Results of WMO Laboratory
Intercomparison of RI 20042005.
Meteoservis
MRW500
CAE-PMB2
ETG-R102
GEONOR
T200B
PROCEDURES AND METHODS
20 catching type gauges were calibrated in the laboratory before
the Field Intercomparison. The WMO recognized laboratory at the
University of Genoa performed the calibration based on the
generation of a constant water flow from a reference hydraulic
device (tests according to Recom.2 CIMO-XIV + statistics based on 1
min evaluation of relative errors )
Objectives: to single out the quantification errors associated with
each instrument; measurement uncertainty and understanding of
field results
Reference hydraulic
device
The Qualification Module
for RI Measurement
Instruments developed
at the University of
Genova (Ur(95%)=0,45%)
PROCEDURES AND METHODS
Lab tests: reference flow rates at 2, 20, 50, 90, 130, 170, 200 mm/h at
1MIN resolution for a variable duration, arithmetic mean of relative
errors and spreading of data around it, providing correction curves
(to be used beyond the intercomp. time)
Results: the constant flow response plot, gauge relative error
plotted versus Lab reference RI (superimposed box-plots)
PROCEDURES AND METHODS
Further lab tests: investigation of the dynamic performance gauges
at 1MIN resolution through the evaluation of the step response to a
step input (time constant) which is a measure of the instrument
stability and ability to detect rapid changes of the input signal.
Results: step response plot, ratio of measured RI / Lab Reference
RI versus time.
PROCEDURES AND METHODS
Quality assurance:
Field Tests-Calibration Device
Raw data are processed by the
Automatic Quality Control (AQC) to
provide quality checked 1-minute
data
Periodic field tests: Field
Calibration Device (developed at
DICAT Laboratory – Univ. of Genoa)
– Metrological confirmation
Periodic maintenance, daily visual
checks, cleaning of instruments,
calibration status checks
Meeting of Participants and local
staff (Vigna di Valle, 21-22 MAY 2008)
– HMEI participated and reported
QA reports by the Site Manager to
the ET.
Meeting of Participants
DATA ANALYSIS AND RESULTS
Analysis dataset: 1MIN-QC checked RI data obtained through the
reduction of the full FI dataset (162) by considering only
synchronized events and events with 2 consecutive minutes and
RI>12mm/h (totally 43 events, approx. 740 data).
No. 2 events with RI > 150mm/h; No. 5 events with RI>100mm/h;
No. 15 events with RI>60mm/h …..
Total availability of
1-min data (rain/no
rain)
1-min valid data (rain/no
Total numbers of
rain): percentage of
precipitation daily
available 1min data that
events
are valid according to QC
T.A. = 95.4%
98.2% of T.A.
162
(Full FI Dataset)
Numbers of
synchronized events
Numbers of events for
reference RI calculations
Number of events
for data analysis
of rain gauges
85
(Reduced FI Dataset)
79
(28 000 1-min data)
43
(740 1-min RI data)
Hail and Mixed
Rain/Hail events
6 events: 13th Jan, 4th
Feb, 7th May, 30th Oct
2008; 1st Jan, 5th Mar
2009
Rainfall accumulated
over the
intercomparison
period
1325 mm
DATA ANALYSIS AND RESULTS
250
4 Nov 2008
200
150
100
50
16
:4
16
:3
8
16
:3
6
16
:3
4
16
:3
2
16
:3
0
16
:2
8
16
:2
6
16
:2
4
16
:2
2
0
16
:2
0
Main objective: performing
the best inter-comparison of
rain gauges in high RI field
conditions
through
the
determination of a reference
value (1MIN RIref)
RIref is the best estimation
of the 1 min RI true value and
it must be derived through a
RI
Composite
Working
Reference value.
DATA ANALYSIS AND RESULTS
Method: weighted average of 1-min
RIi measured by the 4 reference gauges
Weights (µi, working reference
gauge i) were the most challenging
issue, since they must take into account
effects related both to dynamical
characteristics and possible lack of
synchronization on 1 min time base
Si are 3 statistical parameters
calculated throughout the database of
all precipitation events
F = 0,1: “gross” parameter, for each
working reference and for each event
(F=1 if the working ref is not affected
by 1-min lack of synchronization or
high dynamic oscillation)
  RI


i
RIref
i
i
i
i
S i1  Fi
i 
1
S
 i  Fi
S i   ik
k i
(k≠i)
i
N
 ik 
2
i
k 2
(
RI

RI
 j j)
j 1
F=0,1
N
µR102
µPMB2
0.27
0.25
µMRW500 µT200B
0.23
0.25
DATA ANALYSIS AND RESULTS
GEONOR- T200B
CAE-PMB2
100
RD[%]
M-S MRW500
50
0
-50
RI ref(t)-RI ref(t-1) [mm/h]
-100
-100
-50
0
50
100
ETG-R102
DATA ANALYSIS AND RESULTS
Synchronization: to compare the 1-min RI data of all instruments, a
synchronization of ±10 s was required, in other words the internal clock of
the instrument should be within ±10 s compared to the DAQ system
timestamp (nominal timestamp). If the difference/delay between the
instrument’s data output time and the nominal timestamp exceeds the
required ±10 s time interval, the result cannot be correctly compared to
synchronized gauges.
70
gauge A
gauge B
gauge C
60
50
RI [mm/h]
Gauge A and C are
synchronized with the
DAQ system clock; gauge
B has a delay exceeding 10
seconds. Arrows indicate
sample points of B with
large difference due to
non-synchronized
data
points of gauge B.
40
30
20
10
0
12.30
12.34
12.38
12.42
12.46
Time
12.50
12.54
12.58
13.02
DATA ANALYSIS AND RESULTS
Uncertainty of the reference: assuming a normal distribution of the
deviations of pit gauges RI, the standard deviation of the distribution with
respect to the reference intensity is calculated according (σ=[∑(RIRIref)2/N]1/2); thus U95(RIref)=2 σ=4.3mm/h
Relative uncertainty: ur(RIref)= (U(RIref) / RIref )∙100 [%] (green in plot)
60
R102 ETG
T200B GEONOR
PMB2 CAE
reference uncertainty
40
RD [%]
20
0
-20
-40
Pit gauges Relative difference Weighted RI reference
-60
10
30
50
70
90
110
130
RI reference [mm/h]
150
170
190
210
DATA ANALYSIS AND RESULTS
To show the general results of the intercomparison, the intensities
measured by the rain gauges are plotted versus 1MIN Riref (CWR) and
data are fitted with a power law trend curve
RI=a x RIrefb
(corresponding bet fit equation reported on graphs)
To assess the accuracy of field measurements compared to the
reference, a tolerance region (composed of the WMO required 5%
uncertainty and of the uncertainty of the reference, calculated as
[urel(RIref)2+52]1/2 [%] ) is represented in dashed lines on each plot.
For easier comparison, the instruments have been divided into seven
groups according to the measurement principle, as indicated in the title of
each plot. WG instruments are split in two groups for easier presentation
of results.
Specific results and plots: Data Sheets included in the Final Report
To assess the impact of wind losses (Jevons effect) on catching errors,
the ratio RIiOUT/ RIi is plotted versus wind speed (being RIiOUT and RIi the
measured intensities by two identical gauges, one in the pit and the other
one in the corresponding open field platform)
DATA ANALYSIS AND RESULTS
TBRG
220
yMcVan = 1.31x
200
yPAAR = 1.15x
0.90
0.96
PP040-MTX
2
R = 0.68
AP23-PAAR
2
R = 0.85
DQA031-LSI LASTEM
180
yLASTEM = 1.06x
0.96
2
R = 0.72
160
yDAVIS = 1.16x0.92 R2 = 0.73
140
yMTX = 0.96x1.0 R2 = 0.79
RI [mm/h]
ARG100-EML
yEML = 1.21x
120
0.92
2
R = 0.75
RIM7499020-McVan
100
80
Rain Collector II-DAVIS
60
40
tolerance region
20
0
0
20
40
60
80
100
120
RI reference [mm/h]
140
160
180
200
220
DATA ANALYSIS AND RESULTS
TBRG-SC
220
UMB7525/I/SIAP-MICROS
200
PMB2-CAE
tolerance region
180
R102-ETG
160
RI [mm/h]
140
120
100
80
y ETG= 1.01x
0.99
2
R = 0.88
60
yCAE = 0.78x1.05 R2 = 0.87
40
ySIAP-MICROS = 0.92x1.02 R2 = 0.73
20
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
TBRG-PC
220
200
tolerance region
LB-15188-LAMBRECHT
180
160
PT 5.4032.35.008-THIES
RI [mm/h]
140
120
100
80
60
y THIES= 1.01x
0.99
2
R = 0.85
40
y LAMBRECHT = 1.21x0.96 R2 = 0.81
20
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
TBRG-MC & LRG
220
tolerance region
200
180
160
Electrical raingauge-KNMI
RI [mm/h]
140
120
100
R013070-PRECIS MECANIQUE
80
60
y Precis Mecanique= 1.08x
40
y KNMI= 1.05x
0.97
0.95
2
R = 0.77
2
R = 0.82
20
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
WG (1)
220
PLUVIO-OTT
tolerance region
200
180
160
PG200-EWS
RI [mm/h]
140
T200B-GEONOR
120
100
80
60
yPLUVIO-OTT = 0.98x
40
yEWS = 0.98x
20
1.00
1.00
2
R = 0.90
2
R = 0.81
y GEONOR= 0.96x
1.00
2
R = 0.89
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
WG (2)
220
MRW500 -METEOSERVIS
200
tolerance region
180
yMETEOSERVIS= 1.01x
y MPS= 1.09x
0.95
0.98
2
R = 0.74
2
R = 0.59
160
y VRG101-VAISALA= 1.12x0.75 R2 = 0.12
RI [mm/h]
140
yEIGENBRODT = 1.09x
0.96
2
R = 0.67
120
TRwS-MPS
ANS 410/H-EIGENBRODT
100
80
60
40
20
VRG101-VAISALA
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
NON CATCHING TYPE RAIN GAUGES
220
PARSIVEL-OTT
tolerance region
200
LPM-THIES
180
yLPM-THIES = 0.93x
160
yPWD22-VAISALA = 0.81x0.94 R2 = 0.51
1.07
2
R = 0.80
yPARSIVEL-OTT = 0.82x1.10 R2 = 0.77
RI [mm/h]
140
WXT510-VAISALA
y WXT510-VAISALA= 1.72x0.91 R2 = 0.74
120
y PVK ATTEX= 1.43x
0.82
2
R = 0.53
100
80
LCR-PVK ATTEX
60
PWD22-VAISALA
40
20
0
0
20
40
60
80
100
120
140
RI reference [mm/h]
160
180
200
220
DATA ANALYSIS AND RESULTS
Measurement accuracy of all gauges by relative deviations from RIref
DATA ANALYSIS AND RESULTS
2.0
yR102 = -0.008x + 0.985
1.8
y T200B= -0.009x + 1.003
1.6
RIout/RIpit
1.4
1.2
1.0
0.8
0.6
Gauge R102 - RIout/RI_pit
Gauge T200B - RIout/RI_pit
0.4
Linear (Gauge R102 - RIout/RIpit)
0.2
Linear (Gauge T200B - RIout/RIpit)
0.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Wind speed [m/s]
A relevant effect of the wind did not appear in this intercomparison, we
are able to affirm that the wind is not affecting in a significant way the
outer instruments compared to those installed in the pit.
CONCLUSIONS AND DEVELOPMENTS
This comparison at 1MIN time scale in field conditions demonstrates
the possibility to evaluate the performance of RI gauges, identifying
directions where research and technological development should be
oriented.
The achievable accuracy of WG can be improved in field conditions by
means of the reduction of the response time below 1-minute and by
appropriate filtering methods
With regard to TBRG, the method applied by TBRG-SC confirms the
possibility to improve the 1-min RI resolution and to provide accurate field
measurements
With regard to TBRG, the method applied by TBRG-PC revealed the
possibility to provide accurate field measurements at higher RI, even if the
performance is limited by their resolution at lower RI.
With regard to non catching type rain gauges, despite the need of very
low maintenance and the possibility to determine the type of precipitation
(not for all), none of them agreed well with the reference. We are confident
that their performance will be improved in the next future, considering the
possibility of improving calibration methods to reduce systematic errors.
CONCLUSIONS AND DEVELOPMENTS
Disdrometers tended to overestimate the RI. Despite their very different
calibration procedures, they agreed better to each other than to the
reference, as indication of a good degree of precision but they were not as
accurate as conventional gauges
For the best quality instruments, the achievable measurement
uncertainty in laboratory, under constant flow conditions, was found to be
5% above 2 mm/h and 2% above 10 mm/h.
One of the most challenging aspects of the Intercomparison was the
definition of a 1-minute field reference intensity. The procedure adopted
confirmed the suitability of R102-ETG, PMB2-CAE and T200B-GEONOR for
the calculation of the reference.
The rainfall intensity is highly variable from minute to minute. Therefore,
the time synchronization of the instruments is crucial to inter-compare their
measurements and to design the measurement systems.
The results of the intercomparison confirmed the feasibility to measure
1MIN RI and provided information on the achievable measurement
uncertainties, responding to the need of accuracy, homogeneity and
standardization of RI measurements, as required by WIGOS.
CONCLUSIONS AND DEVELOPMENTS
In field conditions, the uncertainty of the RI CWR was evaluated to be
4.3 mm/h. Consequently, the relative uncertainty of the reference was
found to be below 5% only for intensities above 90 mm/h. Below 90
mm/h, the relative uncertainty of the reference values was higher than the
5% required measurement uncertainty provided in the CIMO Guide.
A number of standardization activities at national and European level
(CEN- TC “Hydrometry”)) has been promoted by the Italian Met Service,
such as a national standard on the classification of rain gauges according to
accuracy, the revision of the standard “Specification for a reference rain
gauge” (ISO/EN13798:2010), a pre-standard (CEN technical report) on RI
measurements (a possible WMO/ISO common standard in future)
The IMS proposed the establishment of a CIMO Lead Centre on
precipitation intensity (Univ. Genova, Vigna di Valle, M.Cimone).
RECOMMENDATIONS (3 of 18)
The 1-min RI is highly variable from minute to minute. Therefore, it is
recommended that 1-min RI should only be measured in a station and used
for further analysis if the following conditions are met: (a) All 1-min data
must be transmitted and used (a single 1-min RI value is not representative
of a longer period of time; (b) A very good time synchronization, better than
10 s, must be achieved, both between the reference time and the different
instruments of the observing station.
Changes be made to the CIMO Guide Table:
Precipitation intensity (liquid):
Achievable measurement uncertainties:
-Under constant flow conditions in laboratory, 5% above 2 mm/h, 2%
above 10 mm/h.
-In field conditions, 5 mm/h, and 5% above 100 mm/h
It is recommended that RI measurements be further standardized based
on the advances in knowledge obtained from this intercomparison to allow
the users to obtain homogeneous data sets (based on the achievable RI
measurement performance (accuracy) rather than on measuring principle or
gauge design/technical solutions)
ACKNOLEDGEMENTS
The ITALIAN MET SERVICE – AIR FORCE
The WMO-CIMO (Dr Ondras, Dr Ruedi)
The colleagues, local staff of the intercomparison site (Vigna di Valle)
Dr Claudia Monesi – Univ. of Rome TRE (data analyst)
Dr Eckhard Lanzinger – DWD (The project Leader)
Dr Bruce Baker –USA (ET member)
Dr Michel Leroy – Meteo France (ET chair)
And
Prof. Luca Lanza and Eng. Luigi Stagi – Univ. of Genova