AMOSSG/2-SN No. 9
29/01/01
AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)
SECOND MEETING
De Bilt , 20 to 23 February 2001
Agenda Item 5: Review of the requirements for meteorological observations and reports contained
in Chapter 4 to Annex 3
REVIEW OF REPORTABLE PRESENT WEATHER PHENOMENA
(Presented by E. Robinson)
(Prepared by E. Robinson, J. St-Coeur and W. van Dijk)
1. INTRODUCTION
2.
1.3
The task of this sub-group of AMOSSG was to undertake a preliminary review of the list of
weather phenomena in the International Standards and Recommended Practices (Meteorological Service
For International Air Navigation) Annex3 (4.8.4-4.8.7 & 4.12.3) taking into account the operational
requirements and the work undertaken by the WMO Expert Group on Codes in 1994.
1.4
1.5
This review considered the following issues:
f)
trends in coding based on studies by the WMO Expert Group on Codes;
g)
operational aviation perspectives regarding use and scope;
h)
forecasters perspectives regarding use and scope;
i)
automation limitations and emerging possibilities; and
j)
recommendations.
2. TRENDS IN INTERNATIONAL CODES
3.
2.4
The WMO trend towards element reporting is crystal clear. The continued emergence of BUFR
(Binary Universal Form for the Representation of meteorological data) and CREX (Character form for the
Representation and EXchange of data) codes as the preferred methods is well documented and
recommended. BUFR is a binary (machine to machine) code whereas CREX is a human readable character
form of a similar element based reporting code. BUFR has the added benefit that it allows packing and
quality checks.
2.5
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2.6
The benefits of table driven codes are not in dispute but the implications of implementation on a
global level are huge. It was stated in the “WMO Table Driven Codes: The 21 st Century Universal
Observation Codes” that “the global application of BUFR and CREX can probably not be done within the
next decade so the current operationally used alpha-numeric codes must continue to be applied within the
next decade.” This implies that BUFR and CREX should be implemented now but in parallel with
currently accepted codes. Hence the transition to the full benefits of these codes will take time to realize.
2.1
BUFR and CREX codes offer great advantages in comparison with the traditional alphanumeric
codes. To be specific, “the main features of these codes are self-description, flexibility and expandability,
which are fundamental in times of fast scientific and technical evolution.”
2.2
2.3
BUFR and CREX codes are very versatile and apply to wide varieties of data including satellite
imagery, radar data, surface observations and atmospheric soundings, to name a few. In this paper, we are
limiting our review to present weather phenomena of surface observations.
2.4
2.5
BUFR contains a large capacity for expansion so any future definitions of present weather
phenomena could easily be added and existing definitions could be enhanced, changed or deleted with no
serious implications. As well the current table definitions include much more information with the
potential, especially in light of automation, of categorizing individual types in a much more thorough
manner for ingest into intelligent displays, processors and telecommunications devices of the future. For
example, the intensity of precipitation can be in mm/hr or in kgm-2s-1 depending on table entry.
2.6
2.7
BUFR uses CODE Tables and FLAG Tables for definitions, both binary. CODE Tables are used
to explicitly define a condition whereas FLAG Tables are used to describe combinations of conditions. For
example, multiple precipitation types can be communicated, parsed and processed simply by setting
selected bits in a type FLAG Table but the character of precipitation would be described in a CODE Table.
Tables 1 and 2 illustrate these flexibility’s, as a reference, for precipitation.
2.8
2.9 Table 1. FLAG Table — Type of Precipitation
2.10
2.11
1
precipitation - unknown type
2.12
2
Liquid precipitation, not freezing
2.13
3
Liquid freezing precipitation
2.14
4
Drizzle
2.15
5
Rain
2.16
6
Solid precipitation
2.17
7
Snow
2.18
8
Snow Grains
2.19
9
Snow Pellets
2.20
10
Ice Pellets
2.21
11
Ice Crystals
2.22
12
Diamond dust
2.23
13
Small hail
2.24
14
Hail
2.25
15
Glaze
2.26
16
Rime
2.27
17
Hoar frost
2.28
18
Dew
19-20 Reserved
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21
Missing value
Table 2. CODE Table — Character of Precipitation
0
1
No Precipitation
Continuous
2
Intermittent
3
Shower
4
Not reaching ground
5
Deposition
6-14 Reserved
15
Missing value
2.1
BUFR or CREX codes will not limit future definitions or changes to definitions of present
weather phenomena and will have enough capacity to deal with any direction that Annex 3 will likely take
in this area.
2.2
2.3
For the record, an observed benefit of element reporting is that the dogma of current reporting
methods will not limit the interpretation of real data. For example, if ice accretion was detected within
snow, both conditions would be thoroughly reported and informed users would interpret this correctly as a
potential hazard to aviation. This would allow different mix of weather codes to be reported. (e.g. using the
existing METAR code, a report of FZSN does not exist.)
2.4
2.5
6. OPERATIONAL AVIATION PERSPECTIVES
7.
3.8
We must take into account that other means such as weather radar, satellite pictures, lightning
detection systems, nowcasting models and the synoptic networks will give data streams that together with
the traditional measured data will generate the product that will replace the manual observations.
3.1
Initiatives were proposed to reduce the number of reportable present weather types. Specifically
the possible grouping of types into liquid precipitation and solid precipitation, liquid obscuration, dry
obscuration and other phenomena was considered by the sub-group. As applied to Annex 3, 4.8.4, the
following summary is presented for discussion:
3.2
c)
Grouping rain RA and drizzle DZ is supported since runway state (aquaplaning)
is important. Visibility is more important than drizzle. Intensities should be based on water
liquid content of the precipitation (WMO/CIMO and adopted by the CAeM) and not the
visibility.
a)
Grouping snow SN, small hail/snow pellet GS would be possible. Other users
felt that SG (no influence on visibility), PL and IC (too small, too light) were of no
operational importance and could be deleted.
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a)
Hail should be separately identified. No consensus on size reporting.
a)
Grouping sand SA, dust DU, haze HZ and smoke FU has no consensus. Volcanic
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ash VA should continue to be separately identified.
a)
The grouping of dust/sand whirls PO, squall SQ, duststorm DS and sandstorm
SS was also not supported.
a)
The removal of showers SH was supported if the beginning and ending time
were reported.
a)
The removal of low drifting DR was not supported.
a)
Grouping fog FG and mist BR was partially supported.
a)
It was suggested that funnel cloud +FC for tornado or waterspout be identified
separately. (e.g. Tornado TO and waterspout WS).
3.3
Views of the sub-group on Annex 3, 4.8.5 are presented here for discussion:
a)
Shallow MI and Patches BC are enough to report. Some interest was expressed
for dropping Partial PR and Vicinity VC.
a)
Thunderstorm: report thunderstorm without the other artificial and forced
combinations and only in the climb-out and approach area. Multi weather groups to be
reported in the METAR independently. The definition of thunderstorm has to be redefined
due to the automated systems.
a)
Shower: No combinations with snow pellets etc. Shower is more important for the
pilot in tropical areas, so the element “shower” should be kept.
a)
Blowing: no change, except “partial”, this should be deleted. (partial has nearly the
same meaning as patches). The height distinction for low drifting and shallow does not make
sense, so, to be deleted.
3.4
3.5
Views of the sub-group on Annex 3, 4.8.6 are presented for discussion:
f)
intensity.
a)
a)
For precipitation the terms “light, moderate and heavy” will be replaced by
Moderate and heavy to be kept for sand and dust, but these are not useful for low
drifting and shallow.
Should the frequency of thunderstorms be reported?
1. FORECASTER PERSPECTIVES
2.
4.3
In discussions with forecasters, there was general consensus that forecaster’s needs for reportable
weather phenomena are generally met by the current list in Annex3 although some interest for change was
stated.
4.4
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4.5
However, there was concern among some Canadian forecasters that the METAR codes have lead
to a reduction in useful data in some cases. This is not a consensus view. For example, the loss of Ice Fog
in METAR has created a gap between dry (non-accretion) conditions and super cooled droplets below 0
degrees which may represent an icing hazard to aviation.
4.6
4.7
For severe weather forecasting, although the codes exist in Annex3, the ability to report some
phenomena is a continuing limitation and needs improvement. For example, better precipitation detection
and more accurate typing on hail, small hail, ice pellets, snow grains and lightning is required. These are
critical to discerning the atmospheric processes which are in progress.
4.8
4.9
For automatic stations it was recognized that the detection and reporting of present weather
phenomena is an emerging technology. However, some forecasters recommended that automatic stations
report the probabilities of measured types to them as best they can rather than just try to report the types.
This would permit the forecaster to make judgmental decisions based on likely events and other sources of
information. Ice pellets and snow grains are of importance.
4.10
4.11
The introduction of lightning networks has been a direct benefit to forecasters for the tracking,
monitoring and forecasting of severe weather.
4.12
4.13
Obstructions to vision are very important and this tends to be a weak area in automatic stations
which have replaced observers. This needs improvement in detection and classification technologies for
these systems. Blowing Snow BLSN and Drifting Snow DRSN were identified as important to forecast a
potential hazard and/or a pending restriction to aviation. Fog FG and mist BR should be kept. Shallow MI
and patches BC should be kept.
4.14
4.15
Freezing conditions were also important to report in support of potential aircraft icing and for
salting or sanding actions for road safety.
4.16
4.17
Forecasters felt that higher temporal resolution data would be a significant improvement. For
example, rather than a message once per hour which represents current conditions it would be better to
send an hour’s data which represents the conditions minute by minute for that hour which includes current
conditions. Their view was that there is a strong need to process and display these data as well since they
could not handle METAR (text) data in this manner. This view was expanded by the request to send them
everything that we can measure and that it is the forecaster’s responsibility to find ways to process, display
and interpret the data.
4.18
4.19
Squall SQ reporting is required. This can be a snow squall which implies visibility and/or ceiling
limits or wind squall where a frontal passage or outflow from thunderstorm cells is occurring. For
automatic stations it was recommended that software algorithms be developed to better detect and report
these conditions.
4.20
4.21
There was some concern about SPECI reports. In general some of the measured elements in a
normal METAR are not included in a SPECI such as temperature or pressure. Forecasters prefer that the
SPECI includes all the normal parameters of a METAR.
4.22
4.23
There is a desire to add precipitation amounts to the METAR when present weather phenomena
is occurring.
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4.24
4.25
Forecasters would like to add other parameters to their observation reports such as surface
temperatures, surface moisture, soil temperature, soil moisture, sunshine, solar radiation among others.
This would help to improve aviation forecasts and better predict risks and produce flash freeze warnings as
well as other products. (such as agriculture warnings and advisories).
4.26
4.27
As a coding issue, some forecasters have suggested that METAR codes provide placeholders for
parameters that are only reported on occurrence. This applies to the present weather group. For example,
on automatic stations the placeholder could inform them whether that parameter is measured at the site,
whether it is suppressed, missing due to failure or whether it is valid.
4.28
4.29
The forecasters did not have much objection to grouping smoke, haze and dust but some
indicated that, although it occurs infrequently, when it does it may be important to know the differences.
Volcanic ash (VA) is important.
4.30
4.31
32. EMERGING AUTOMATION POSSIBILITIES
33.
5.34
METAR is a character based code which has grown out of telecommunication bandwidth
limitation perceptions. These are real limitations in some areas of the world but evolving world
telecommunications capabilities are rapidly making this an historical problem. Strong efforts should be
made to move toward plain language presentations (all languages) in the future if agreement could be
achieved on methods. Graphic presentation can infer a lot of information quickly to aviation users. The
abbreviated form of presentation in METAR continues to be a source of confusion and interpretation error.
5.35
5.36
It was noted that the current capacities of automatic weather stations vary significantly around the
world. In some cases these systems are used as human aids and in other cases they are used for
replacement of human observers. The later is much more critical for the measurement of present weather
phenomena due to the lack of human verification.
5.37
5.38
Current technologies for present weather detection use either optical, microwave and/or acoustic
techniques. These systems perform very well for some types of present weather phenomena such as rain
RA and snow SN but have varying levels of success for many other phenomena such as drizzle DZ, ice
pellets PL, snow grains SG and others. As technologies continue to emerge the success in these other areas
will improve.
5.39
5.40
Two major efforts need to be undertaken to move these successes forward. First, the
investigation, development and verification of multiparameter algorithms is providing significant
enhancements in performance for these sensors. Multiparameter algorithms are based on inputs from
present weather detection sensors and other meteorological parameters such as temperature and humidity.
In some cases these algorithms are implemented within a present weather sensor directly. In other cases,
these algorithms are performed within automatic weather stations. This is usually driven by the sector
which creates to product. There are benefits to both.
5.41
5.42
Some present weather phenomena cannot be detected and reported by existing systems (but their
effects on the visibility may be reported). Examples include dust DU, haze HZ, smoke FU, sand SA,
volcanic ash VA and others. The reason for this is not necessarily due to the limits of technology but is
likely due to the low probabilities of these events relative to most users, hence investments have not been
made to resolve the problems.
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5.43
5.44
Since shorter wavelengths can detect smaller particles, the emergence of millimeter wave
devices, ultrasonic acoustic devices, low cost laser technologies and very powerful, low cost digital signal
processing (DSP) technologies will make detection and algorithmic solutions much more possible in the
next few years. Meteorological agencies, departments and private sector companies will need to seriously
invest to produce growth in these areas and make solutions probable.
5.45
5.46
Automatic present weather systems have unique and different performance characteristics which
will permit the detection and classification of precipitation types and obscuring phenomena. The ability to
detect a type (probability of detection) and the implication of classifying a type incorrectly (false alarm
ratio) are linked. For example, if snow grains SG, ice pellets PL and snow SN are reported, the user should
be aware that the system is detecting and classifying to the best of it’s ability and reporting based on the
specific technology. Each type will have an associated measurement probability, none of which is 100%.
As an example, a report of SN, SG and PL is much less useful than a report of SN9, SG5, PL1 which, in
this example, represent a 90% measurement probability of SN, a 50% measurement probability of SG and
a 10% measurement probability of PL. This is consistent with the forecasters requests in 4.4).
5.47
5.48
6. ACTION BY THE GROUP
6.1
The group is invited to consider the following recommendations by the sub-group:
a)
future reporting methods shall move toward element reporting;
b)
BUFR code shall be adopted universally for this reporting;
c) standard reports shall move toward higher temporal resolution with automated
systems;
d) for the forecasters, present weather phenomena from automated systems shall have
type probabilities attached to type information;
e) with evolving lightning detection technologies it should be encouraged to develop
practical methods to report lightning at other than specific sites. (e.g. at en route or other
off route locations);
f)
SPECI reports should include the normal METAR parameters from all automated
systems;
g)
precipitation amounts should be added to METARs;
h) for the forecasters, standard reports should include other data groups than those in
current METAR parameters;
i)
automation of present weather phenomena should be encouraged and further
developed;
j)
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funnel cloud +FC for tornado or waterspout should be identified separately in the
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METAR. (e.g. tornado TO and waterspout WS);
k)
efforts to define and move towards future plain language and graphical presentations to
aviation users rather than METAR reports should be encouraged to capture the benefits
of evolving technologies.
— END —