AMOFSG/9-SN No. 17
5/7/11
AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
NINTH MEETING
Montréal, 26 to 30 September 2011
Agenda Item 5: Observing and forecasting at the aerodrome and in the terminal area
5.1: Observations
AUTOMATED CLOUD REPORTING WITH THUNDERSTORM IN THE VICINITY
(Presented by Ossi Korhonen)
SUMMARY
This paper informs the group of possible different interpretations of automated
could reporting practices when reporting thunderstorm in the vicinity without
actual CB detection.
1.
INTRODUCTION
1.1
Currently there are no economically practical means to have cloud type detection in the
automatic systems and so define the cloud type in the automated aerodrome routine meteorological report
(in meteorological code form) (METAR) and aerodrome special meteorological report (in meteorological
code form) (SPECI) reports and local routine and SPECIAL reports. Anyway the system may include
lightning information and it would be possible to observe the CBs indirectly when there's lightning
detected. However the insufficient information of cloud type and thunderstorm in automated systems lead
to insufficient reporting or contradictions in present weather and cloud groups. Guidance is needed for
those situations.
2.
DISCUSSION
2.1
In the current fully automated systems cloud coverage is typically detected with one or
several ceilometers. This means that only cloud height can be measured and from a very limited area.
Cloud amount estimation is based on historical data and it is dependable on wind speed and direction at
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cloud layer heights. Cloud type cannot be detected. ICAO Annex 3 — Meteorological Service for
International Air Navigation allows reporting the cloud type as missing in the automated METARs.
2.2
When clouds have been detected then cloud amount and cloud height are reported, and
cloud type should be reported in automated METAR and SPECI as missing (///), eg. SCT030///. If TS or
VCTS is reported it means that the system has indirectly detected the presence of the CB clouds and so
the cloud group could be coded as SCT030CB.
2.3
Currently in automated METAR and SPECI code NCD can be used when automatic
system has not detected any clouds. Anyway due to limited detection area it is possible that clouds do
exist in the vicinity and even a thunderstorm cell is approaching the airport.
2.4
In order to fulfil ICAO standards the system should be equipped or interfaced with
thunderstorm (lightning) detecting sensor or system. Then it is possible that present weather group would
have VCTS when cloud group would have NCD. According to ICAO Annex 3 App. 3 paragraph 4.5.4.5
b) METAR could look like following: METAR ABCD 250830Z AUTO 12013G24KT 9999 VCTS NCD
21/16 Q1014. In this case there's a clear contradiction between present weather and cloud groups.
Anyway for the pilot it is clear that if VCTS is reported then there must be CB clouds in the vicinity even
though no clouds have been detected. According to ICAO guidance in Doc 9837 CB clouds should be
reported at least in the vicinity which is in line with TS reporting.
2.5
According to ICAO Annex 3 App. 3 paragraph 4.5.4.5 a) it could be interpreted that if
VCTS is reported it means that the system has indirectly detected the presence of the CB clouds and so
the cloud group could be coded as //////CB instead of NCD.
2.6
Anyway it may distract the pilots if the same automated system provides CB code only
when it has detected lightning and does not have true capability of detecting CB and TCU clouds. It may
be clearer that only TS or VCTS is reported and cloud section is always without cloud type detection. It is
obvious that CBs are present if there are lightning, but lack of TS does not mean that there would not be
CBs or TCUs present.
2.7
Providing VCTS NCD is actually easier to interpret for the pilot than VCTS //////CB.
Pilots have also emphasized that use of solidi (/) is not recommendable. Also the automated systems are
simpler to make when there's no dependency when providing present weather and cloud group
information for the reports based on sensor information.
3.
CONCLUSION
3.1
Current ICAO documentation leaves space for interpretation how to handle the reporting
in situation where automated system has not detected any clouds but lightning has been detected.
Reporting thunderstorm in the vicinity and no clouds detected at the same time seems to be the most
practical way of reporting instead of trying to combine information that is insufficient anway.
3.2
There's no necessity to append ICAO Annex 3 but the study group could consider if note
to Annex 3 or other guidance material for example in Doc 9837 would be provided about this case.
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