WORLD METEOROLOGICAL ORGANIZATION
____________________
Distr.: RESTRICTED
CBS/OPAG-IOS (ET AWS-1)/Doc. 7
COMMISSION FOR BASIC SYSTEMS
OPEN PROGRAMME AREA GROUP
ON INTEGRATED OBSERVING SYSTEMS
EXPERT TEAM ON REQUIREMENTS OF DATA FROM
AUTOMATIC WEATHER STATIONS
FIRST SESSION
(1.IX.2002)
_________
ITEM: 5.
Original: ENGLISH ONLY
GENEVA, SWITZERLAND, 2-6 SEPTEMBER 2002
Review and Refine Practices for Reporting the Instantaneous Precipitation
Intensity, Total Cloud Amount and Cloud Height
(Submitted by Jitze P. van der Meulen, the Netherlands)
________________________________________________________________
Summary and Purpose of Document
This document contains information and considerations on reporting precipitation
intensity, cloudiness and cloud height. Reference is made to the
recommendations by a recent expert meeting on precipitation intensity. In an
annex considerations are given for further discussing the typical difference
between data directly obtained from instruments measurements and
observational data to be presented to the users as source of information.
________________________________________________________________
ACTION PROPOSED
The meeting is invited to review the considerations on reporting precipitation
intensity, cloudiness and cloud height and to prepare a draft recommendation on
enhanced reporting of these variables.
References:
[1] Report of Expert Meeting on Rainfall Intensity Measurements, Bratislava, Slovakia, 23 to
25 April 2001
[2] Report of WG on Surface Measurements, Geneva, 27-31 August 2001.
[3] Report on Expert Meeting on Requirements and Representation of Data from Automatic
Weather Stations, De Bilt, Netherlands, 19 - 23 April 1999
ET/AWS, Doc. 7, p. 2
1
Background
For a number of variables, traditionally observed at weather stations, such as
precipitation intensity, measure of cloud overcast and cloud base, instrument measurements
do not provide in some way an optimal performance in line with the expected requirements.
Research and development on enhanced measurement technologies will improve this
performance, but on the other hand a change in requesting alternative variables, to be
measured easily should be considered as well. In particular the availability of data obtained
by satellite born and ground based remote sensing should be taken into account when
stating any requirements for observing variables at point locations such as weather stations.
2
Reporting precipitation intensity
Although the variable (total amount of) precipitation is defined already for a long time,
it is based on a total cumulative amount of precipitation, fallen during a specific period (24 h
or 60 min). No definition and corresponding accuracy requirements on precipitation intensity
(PI) are stated or recommended by WMO. Since it was recognized by WMO Members that
precipitation intensity is of highly interest, especially for nowcasting, an Expert Meeting on
Rainfall Intensity Measurements was organized in (Bratislava, Slovakia, 23 to 25 April
2001)1. This expert meeting recommended a number of present and future requirements for
rainfall intensity (ri) measurements, inclusive enhanced calibration methods and the start of
an instruments intercomparison, based on a laboratory test. Moreover a measuring range
and uncertainty requirements for rainfall intensity where defined (see Annex 1) for adoption
by CIMO-XIII (Sept. 2002).
Since PI can be measured by traditional instruments quantitatively, the qualifiers
'light, moderate and heavy' have to be avoided because of subjectivity (see
recommendations of the Expert Meeting on Requirements and Representation of Data from
Automatic Weather Stations, De Bilt, Netherlands, 19 - 23 April 1999 and CBS-XII, 6.1.45).
These qualifiers are only for interest as part of information on request by specific users or the
general public, but not as measurements values.
Point of concern for precipitation measurements are the 'standard errors' in the
measurement, larger than the required accuracy. These errors are typically based on wind
impacts (small (m) to median (hm) scale), for which appropriate corrections should be made
before reporting. In this report, the corrected value, the measure of correction and which
used (WMO recommended) method should be encoded.
It is recommended to measure continuously a flow rate of collected rain. This method
includes the use of appropriate reduction techniques of the wind impact (e.g. by using a pit
hole) and instantaneous measurements. A traditional instrument, still in practice, is the socalled tipping-bucket gauge. This device suffers from unacceptable non-linear behaviour. A
WMO intercomparison of calibration measurements of tipping-bucket gauges will be started
up soon and will produce practices for the appropriate correction figures.
Traditional measurements cannot instantaneous measure the falling rate of solid
precipitation, like snow or hail. To avoid unacceptable evaporation, melting of snow takes an
unacceptable amount of time. A recently introduced instrument, measuring the increment of
the mass of falling precipitation (liquid or solid) does not suffer from the melting requirement.
An alternative type of observation is based on the measurement of falling particles by light
scattering (optical raingauges). This method is also used in combined MOR/PW observing
systems, but the wind impact, non-linearities and undefined calibration procedures cause
unpredictable uncertainties.
3
Reporting overcast (cloud amount)
In the SYNOP code the measure over overcast or cloud amount is indicated by a
simple parameter N= 1 to 8, indicating the range 'clear sky' to 'total overcast'. It refers to a
measure of 1/8 to 8/8 overcast and has a reporting accuracy of 1/8. It refers to the measure
of cloudiness observed by visual means but observed from a single point. Taking into
account new types of observing methods, typically based on remote sensing technology,
providing cloud information on a frequent base, we have to reconsider the need to observe
1
Report available on www.wmo.ch
ET/AWS, Doc. 7, p. 3
separately cloudiness or sky conditions ('clear sky', 'broken clouds', 'overcastted'). Although
the state of the sky is highly importance for the radiation budget the measurement
uncertainty of N (also by visual means), missing higher clouds like cirrus (also by visual
means, especially at night), this parameter seems to become a less relevant parameter.
Especially the use of cloud maps, generated from satellite observations on a frequent base
(< 30 min), is of more interest than N. Nevertheless the measurement of cloudiness in terms
of measure of 'broken clouds' can be of interest for a surface weather station (frequency and
intervals of clouds detected above a station). Taken this into account, the use of a LIDAR
system, today common practice in ceilometers, to detect the variability of cloud layers in
zenith seems to be a better alternative. It should be considered to recommend CBS to
choose for these parameters and abandon N.
4
Reporting Cloud Height
Traditionally, the altitudes of cloud are reported in terms of the altitude of the ceiling of
clouds. Not only because visual observations were only able to determine the cloud base,
but also because this parameter is of interest for the meteorologist as additional information
on the atmospheric profile and layers and for aeronautical purposes to inform pilots when
landing. Taking this use into account, cloud height measurements (ceiling, top of clouds) is
part of the set of upper air measurements. For these particular measurement use is made of
in-situ measurements (radio sonde) and ground based remote sensing techniques like with
the radar based profilers (e.g. RASS). Using these technologies the atmospheric profile for
wind, temperature and humidity is determined. Other techniques, like the optical based
LIDAR and also other RADAR based techniques can provide a profile of the optical extinction
coefficient, from which other parameters such as cloud bases can be derived (as is done
already in LIDAR based ceilometers, taking significant changes in the extinction coefficient
as measure). Using satellite observations clouds can be detected and classified based on
the measurement cloud top temperatures and sizes. This latter techniques is outmost helpful
since the ground based remote sensing signals cannot penetrate clouds for more than the
optical (or electromagnetic) depth which prevents the measurements of upper layers.
As a consequence it should be recommended to replace the typically reports on cloud
base height with a reports containing a profile of the optical extinction coefficient. This profile
will provide the user to determine cloud bases if he desires, based on a standard algorithm
recommended by WMO. The format of such a profile should be in line with those profiles, as
recommended for radio sonde or wind/temperature profiler data from RASS systems.
5
Some general considerations on reporting data based on measurements.
There still is a tendency to try to replace visual observing techniques with alternative
instrument techniques, which should determine identical variables. However, it is only
relevant to determine only those parameters, which are required in terms of information data
for the public in general or specific users dealing with the weather. For that reason, it is in
most cases not relevant to have specific (point) location related data, but data representative
for and area (local weather information for e.g. airports or harbours are specific local user
requirements and not of interest for the meteorology in general).
Information on the weather is expressed in variables (parameters) or by phenomena
(expressed as occurrences). Definitions of variables and phenomena have to confirm to
international approved definitions to prevent confusion or non-information. Principally the
type of instrument or method of observation is not relevant for this information taking into
account that the required uncertainties are met. If not, the user should be informed
specifically in terms of metadata information.
ET/AWS, Doc. 7, p. 4
ANNEX 1
Expert Meeting on Rainfall Intensity Measurements (Bratislava, Slovakia, 23 to 25 April
2001):
Recommendation 3: Measuring range and uncertainty requirements
The Expert Meeting recommended that the following measuring range and uncertainty
requirements should be considered for adoption:
(a)
(b)
Minimum time resolution:
t = 1 min (data transmission all 10 min)
Required measuring range(s) and related uncertainties (RI)2
(referred to the above measuring period of 1 minute)
Total range: 0.02 to 2000 mm/h with the following distinction in uncertainty:
0.02 to 0.2 mm/h (trace – see CIMO Guide (WMO-No. 8))
yes/no information (for RI > 0) as precipitation
indication (mainly used for present weather
observations, such as for road meteorology)
0.2 to 2 mm/h
2 to 2000 mm/h
RI = 0.1 mm/h
RI = 5 %
The certainty if these accuracy requirements are fulfilled is based primarily to
laboratory calibrations. Due to additionally induced errors in the field, such as related to
siting and exposure (representativity) and environment induced instrumental errors, will
reduce the accuracy of measurement. In practice such issues are not very well defined or
well known quantitatively and "operational achievable accuracy" will be a common measure
for uncertainty of measurement.
2
Note, SI Units are used, such as mm/h and not mm/min!
Additional comment: One may also note the related information contained in general summary of CBSXII, 6.1.45: "As regarded the recommendation to use the physical units mm h -1 and kg m-2 s-1 to express the
intensity of all type of precipitation, the Commission was pleased to note that the appropriate proposal was
already included for adoption in the proposals submitted under agenda item 6.2" (i.e. BUFR code proposal)