CEN/TR 17993:2023
(Main)Calibration and accuracy of non-catching precipitation measurement instruments
Calibration and accuracy of non-catching precipitation measurement instruments
Non-catching type gauges are the emerging class of in situ precipitation measurement instruments. For these instruments, rigorous testing and calibration are more challenging than for traditional gauges. Hydrometeors’ characteristics like particle size, shape, fall velocity and density need to be reproduced in a controlled environment to provide the reference precipitation, instead of the equivalent water flow used for catching-type gauges. They are generally calibrated by the manufacturers using internal procedures developed for the specific technology employed. No agreed methodology exists, and the adopted procedures are rarely traceable to internationally recognized standards. This document describes calibration and accuracy issues of non-catching instruments used for liquid/solid atmospheric precipitation measurement. An overview of the existing models of non-catching type instruments is included, together with an overview and a description of their working principles and the adopted calibration procedures. The literature and technical manuals disclosed by manufacturers are summarized and discussed, while current limitations and metrological requirements are identified.
Kalibrierung und Genauigkeit von nicht auffangenden Niederschlagsmessgeräten
Étalonnage et précision des pluviomètres sans captage
Kalibracija in točnost instrumentov za merjenje padavin brez njihovega zajemanja
Merilniki brez funkcije zajemanja so nov razred instrumentov za merjenje padavin na mestu uporabe. Strogo preskušanje in kalibracija sta pri teh instrumentih zahtevnejša kot pri tradicionalnih merilnikih. Značilnosti hidrometeorjev, kot so velikost delcev, oblika, hitrost padca in gostota, je namesto enakovrednega pretoka vode, ki se uporablja za merilnike s funkcijo zajemanja, treba reproducirati v nadzorovanem okolju, da se zagotovijo referenčne padavine. Običajno jih kalibrirajo proizvajalci z notranjimi postopki, ki so razviti za specifično uporabljeno tehnologijo. Dogovorjena metodologija ne obstaja, sprejeti postopki pa so redko sledljivi do mednarodno priznanih standardov. Ta dokument opisuje težave v zvezi s kalibracijo in točnostjo instrumentov brez funkcije zajemanja, ki se uporabljajo za merjenje atmosferskih padavin v tekočem/trdnem stanju. Vključuje pregled obstoječih modelov instrumentov brez funkcije zajemanja ter pregled in opis načinov njihovega delovanja ter sprejetih postopkov kalibracije. V dokumentu so povzeti in obravnavani literatura in tehnični priročniki, ki so jih navedli proizvajalci, ter opredelitve trenutnih omejitev in meroslovne zahteve.
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2023
Kalibracija in točnost instrumentov za merjenje padavin brez njihovega zajemanja
Calibration and accuracy of non-catching precipitation measurement instruments
Kalibrierung und Genauigkeit von nicht auffangenden Niederschlagsmessgeräten
Étalonnage et précision des pluviomètres sans captage
Ta slovenski standard je istoveten z: CEN/TR 17993:2023
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
17.120.20 Pretok v odprtih kanalih Flow in open channels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
CEN/TR 17993
TECHNICAL REPORT
RAPPORT TECHNIQUE
October 2023
TECHNISCHER REPORT
ICS 07.060; 17.120.20
English Version
Calibration and accuracy of non-catching precipitation
measurement instruments
Étalonnage et précision des pluviomètres sans captage Kalibrierung und Genauigkeit von nicht auffangenden
Niederschlagsmessgeräten
This Technical Report was approved by CEN on 8 October 2023. It has been drawn up by the Technical Committee CEN/TC 318.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17993:2023 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
Introduction . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Main characteristics of atmospheric precipitation . 6
4.1 Definitions . 6
4.2 Particle size distribution . 7
4.3 Shape of the rain drop . 9
4.4 Terminal fall velocity . 10
5 Non-catching precipitation measurement instruments . 12
5.1 General. 12
5.2 Optical gauges . 12
5.2.1 General. 12
5.2.2 Optical transmission . 12
5.2.3 Optical scattering . 13
5.2.4 Optical imaging . 13
5.3 Impact disdrometers . 14
5.3.1 General. 14
5.3.2 Electro-acoustic devices . 15
5.4 Microwave radar . 15
6 Existing calibration procedures . 17
7 Influence parameters . 20
7.1 Wind. 20
7.2 Contemporary particle crossings . 20
7.3 Drop shape . 21
7.4 Further sources of bias for optical instruments . 21
7.5 Further sources of bias for impact disdrometers . 21
7.6 Further sources of bias for radar disdrometers . 21
Bibliography . 22
European foreword
This document (CEN/TR 17993:2023) has been prepared by Technical Committee CEN/TC 318
“Hydrometry”, the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
The document was prepared following a request for research development submitted by CEN/TC 318 in
October 2017 to EURAMET, the European Association of National Metrology Institutes, through the
cooperation programme between STAIR (the joint CEN-CENELEC strategic Working Group supporting
standardization in research and innovation) and EMPIR (the European Metrology Programme for
Innovation and Research of EURAMET).
This led to the approval and funding of the EURAMET pre-normative project 18NRM-03 "INCIPIT -
Calibration and accuracy of non-catching instruments to measure liquid/solid atmospheric precipitation"
(2019-2021). The project Deliverable D1, "Overview of existing models and working principles of non-
catching precipitation gauges together with test/calibration schemes for different types of non-catching
precipitation gauges" was provided as a supporting document to CEN/TC 318 and is the basis of the
present CEN/TR draft.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
Introduction
The development of highly accurate precipitation gauges for both liquid and solid precipitation is an
increasingly relevant and pressing requirement in the environmental sciences and their applications
(Lanza and Stagi, 2008). Non-catching instruments, which do not use a container to collect the
hydrometeors when approaching the ground, are the emerging class of in-situ precipitation gauges
(Cauteruccio et al., 2021). They detect the microphysical and dynamic characteristics of single or multiple
hydrometeors while these cross a given section, or a volume, of the atmosphere (or directly impact the
sensor) by employing optical, acoustic, and microwave principles.
National Meteorological and Hydrological Services (NMHS) and other organizations, in charge of the
management of monitoring observation networks over large regions, increasingly look at such kind of
instruments as a potential improvement over the more traditional catching-type gauges (typically
tipping-bucket and weighing gauges), notwithstanding the higher lifecycle cost. The reasons are their
potential in reducing the maintenance burden (by eliminating any moving part or containers to be
periodically emptied and serviced), the high temporal resolution, the large number of parameters
provided, and their suitability to be part of a fully automated monitoring observation network.
Drawbacks can be easily identified in the higher complexity of the exploited technology, so that the
capability of the user to correctly manipulate, maintain and calibrate the instrument might be limited.
Non-catching instruments are generally calibrated by the manufacturers, using internal procedures
developed for the specific technology employed. No widely agreed procedure – nor any documentary
standard – exists within national or international institutions. The adopted procedures are rarely
traceable to the International System of Units (SI) and are often not even reproducible. Limited
information is generally provided by the manufacturers about the methodology and instrumentation
adopted for calibration purposes.
Having no funnel to collect the rainwater, traceable calibration and uncertainty evaluation for non-
catching gauges are more difficult than for catching type gauges, and the use of an equivalent, reference
flow rate (see e.g. Colli et al., 2014) is not possible. Rather, for an appropriate metrological
characterization of non-catching instruments, reproducing the actual rain event characteristics is needed,
including particle size distribution, shape, density and fall velocity. A considerable metrological effort is
therefore needed to resolve traceability and uncertainty issues and to support new calibration methods
including the development of standardized laboratory rainfall simulators.
As regards solid precipitation, non-catching instruments were included in the recent WMO SPICE (Solid
Precipitation InterComparison Experiment) and compared with gauge measurements in a DFIR (Double
Fence Intercomparison Reference) at various test sites (Nitu et al., 2018). The study concluded that
further analysis is needed to better understand the behaviour of non-contact type measurement
instruments, especially working with the raw data (drop size and fall speed distribution), and exploiting
the full capacity of such devices, that can provide much more information than the precipitation
accumulation (precipitation type, SYNOP and METAR codes, etc.). Field tests on SPICE reference sites
have been continued in that sense after the official end of the project (Smith et al., 2020) to enhance the
knowledge on the operational use of non-catching type instruments in winter conditions.
For liquid precipitation measurements, the evidence from the last WMO intercomparison of rainfall
intensity gauges in the field (Vuerich et al, 2009) is that, due to calibration issues, caution should be posed
in using the information obtained from non-catching instruments in any real-world application and in
assessing the results of scientific investigations based on such measurements.
The main effort to develop standard procedures for the calibration of precipitation measurement
instruments is presently being performed at the European level. The first experience was the
development of the Italian national standard UNI 11452:2012, and the follow-up extension of such
initiative at the European scale, leading to the publication of the recent standard EN 17277:2019. The
scope of the standard is however limited to catching type gauges, which – due to the presence of the rain
collector – can be calibrated using a known and constant flow rate generated in the laboratory as the
reference (Santana et al., 2015). Traceable instrument calibration for non-catching gauges is the next step
of the ongoing normative effort at the European scale under CEN/TC 318/WG12, but various scientific
and methodological aspects are still open issues.
The project MeteoMet (Merlone et al., 2015), funded under the European Metrology Research
Programme (EMRP), initiated a series of experimental activities in metrology for meteorology, with the
MeteoMet2 specifically addressing the issue of atmospheric precipitation measurements from a
metrological perspective. An associated research grant focused on rainfall measurements using catching
and non-catching gauges. It is under this framework that, to support the ongoing normative effort, the
INCIPIT project “Calibration and accuracy of non-catching instruments to measure liquid/solid
atmospheric precipitation” was initiated in July 2019 (Merlone et al., 2020).
The project aimed at introducing metrological soundness, reproducibility, and standardization in the
calibration of non-catching type instruments, so that an uncertainty budget can be determined, and
measurements made traceable to the SI. A rigorous metrological approach based on modelling the
measurement process and expressing the influence parameters in a model function was implemented,
taking in account different types of rain-gauges and the different calibration schemes. By developing,
characterizing, testing, and comparing different types of rain generators, test calibration of a
representative number of different non-catching rain gauges was performed.
This document provides an overview of the existing models of non-catching instruments with a
description of the w
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