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AMOFSG/7-IP/12
/ AMOFSG/7-IP/12
2/9/08

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

SEVENTH MEETING

Montréal, 9 to 12 September 2008

Agenda Item / 5: / Aerodrome observations

DISCUSSION ABOUT THE OBSERVATION OF CLOUD LAYERS AND THE LOCATION OF CEILOMETER(s)

(Presented by Michael Leroy)

SUMMARY
This paper deals with the reporting of the cloud layers, the positioning of the ceilometer(s) and the content of ATIS.

1.INTRODUCTION

1.1For precision approach runways, instrumental cloud base measurements should be carried out close to the location, where a middle marker is, or should be situated (Annex 3 — Meteorological Service for International Air Navigation , App. 3, Par. 4.5.1)1, i.e. "at a distance of 900 –1200 m (3000 to4000 ft) from the landing threshold at the approach end of the runway". SN/13 recalls why.

1.2ICAO Annex 3 — Meteorological Service for International Air Navigation, recommends:

4.6.5.2 Recommendation.— Cloud observations for local routine and special reports should be representative of the approach area.

4.6.5.3 Recommendation.— Cloud observations for METAR and SPECI should be representative of the aerodrome and its vicinity.

1.3ICAO Annex 11 — Air Traffic Services, Chapter 4 (flight information service) refers only to clouds to be transmitted to the pilot, with the following words:

cloud below 1 500 m (5 000 ft) or below the highest minimum sector altitude, whichever is greater

cumulonimbus; if the sky is obscured, vertical visibility, when available;

1.4When available, human observation is primarily used for the cloud layers, the ceilometer(s) being used by the human observer to localize the bases of the cloud layers.

2.DISCUSSION

2.1Based on the French experience, it is doubtful that a human observer reports both the cloud layers representative of the aerodrome (and its vicinity) and the cloud layers representative of the approach area. In fact, the cloud observation of a human observer is based on what he or she sees : the central point is the observer location. The observer being located within the aerodrome, it is quite valid to consider that he or she observes cloud layers representative of the aerodrome (and its vicinity), as recommended for the METAR and SPECI. The measurements of one or several ceilometers are used to get the best evaluation of the height of the cloud layers.

2.2With a human observer, it is probably quite frequent that the cloud layers transmitted in a local report are in fact the same that the cloud layers transmitted in a METAR. In this case, the exact location of the ceilometer, at a distance of 900 –1200 m from the landing threshold or at this landing threshold, doesn’t really change the cloud layers reported.

2.3It is different if the cloud layers representative of the approach area are calculated from the measurements of a ceilometer located as it is recommended, because these cloud layers are derived from the measurements at the ceilometer’s vertical.

2.4The recommended ceilometer’s location is the most important when the height of the cloud base is low, close to the decision’s height. In such a case, the added value of a human observer is quite low. If this observer is located 2 km from the approach end of the runway (where it is recommended to locate the ceilometer) and if the height of the cloud base is close to the decision height (300 ft to 700 ft, depending on the runway’s category and the type of plane), the angle of view of the cloud base from the observer location is about 3 to 6°. With a so low angle of view, it is really quite difficult for a human observer to describe the cloud layers specifically for the approach area !

2.5Therefore, the importance of the location of the ceilometer is much more for the height of cloud base of the lowest layer than for the complete description of the cloud layers. The height of cloud base is operationally critic when it is close to the decision height. In such a case, the cloud layer is very generally wide spread and the difference of height of cloud base from one point and another point at a distance of 1200 –1500 m is quite low. So, the exact location is no so critic. It is what Météo-France found in a study inter-comparing the height of cloud bases between the 4 ceilometers of CDG airport (LFPG). But the results of such a study may depend on the site.

2.6On a airport with several runways equipped with a ceilometer, the local reports shall include the cloud layers for each runway (Annex 3, appendix 3, 4.5.4.3.b). But the content of the meteorological information of the flight information service (Annex 11) defines only “cloud”. Thepossibility of having several cloud layers for several approach areas is not identified. Which one to use ? In France, there is no problem of choice for the cloud layers in the ATIS, as the cloud layers transmitted and used are in fact the cloud layers representative of the aerodrome (the same cloud layers transmitted in the METAR and SPECI). And in this case, the exact location of the ceilometer is not so important !

3.CONCLUSION

3.1The recommended location of a ceilometer is really meaningful when the cloud layers transmitted in the local reports are really representative of the approach area. It is the case in automatic mode, when the ceilometer is located as recommended (at a distance of 900 –1200 m (3000 to4000 ft) from the landing threshold at the approach end of the runway). It is not so true when the cloud layers are reported by a human observer.

3.2When there is more than one runway in use, the local reports should contain cloud layers for the multiple approach area. But the ATIS considers only one set of cloud layers. As for multiple visibility measurements, there is an inconstancy between the requirements of Annex 3 and Annex 11.

3.3In this context, when the cost to install a ceilometer “at a distance of 900 –1200 m (3000 to4000 ft) from the landing threshold at the approach end of the runway” is high, when it is an isolate place, it may not be very efficient to follow the Annex 3’ recommendation for the ceilometer’s location, if the advantage of such a location is not fully used.

4.ACTION BY THE GROUP

4.1The AMOFSG is invited to :

a)note the discussion presented in this paper; and

b)take it’s content into account for further discussions about the ceilometer location (SN/13) and the use of cloud layers in ATIS (SN/6).

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