AMOFSG/10-IP/4 21/5/13 AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG) TENTH MEETING Montréal, 17 to 19 June 2013 Agenda Item 5: Aerodrome observations AUTOMATED CLOUD INFORMATION GENERATION (Presented by Jarmo Pilli) SUMMARY This information paper gives background information for automated airport cloud information generation and distribution to the pilots and other users. 1. INTRODUCTION 1.1 Pilots need cloud information to define operational conditions or restrictions and to understand the prevailing conditions at the airport and its vicinity. 1.2 There has been plenty of discussion about correct siting of ceilometers and the number of ceilometers needed to give proper automated cloud (sky condition) information to an airport. 1.3 This paper concentrates on two topics that are relevant to automated cloud information generation and distribution to the pilots: a) use of METAR/SPECI, local reports and air traffic controllers (ATC) in the distribution of the required cloud (sky condition) information to the pilots; and b) cloud data processing period in automated cloud information generation. (5 pages) AMOFSG.10.IP.004.5.en.docx
AMOFSG/10-IP/4-2 - 2. DISCUSSION 2.1 Annex 3 Meteorological Service for International Air Navigation Standards and recommendations are built on some presumptions. METAR/SPECI reports are meant for flight planning and, as such, are more generic and provide data processed for a longer period. Air traffic controllers should have displays, updating in real time, that show cloud information relevant to operational use and air traffic controllers provide cloud information to the pilots when required. Local routine and SPECIAL reports provide operationally relevant information, but updates only periodically or when operationally significant changes appear. Local reports are also defined as a data source for ATIS. All these three means should be considered as a whole and none of them can provide a full picture of required cloud information alone. 2.2 METAR and SPECI reports are useful for a need to understand the prevailing conditions. METAR and SPECI report cloud (sky condition) information should be representative to the airport and its vicinity area. Operationally METAR and SPECI cloud information is useful for VFR flights where generic cloud conditions are important. For IFR flights more specific information is needed. 2.3 Air traffic controllers can and should provide cloud information to the pilots using their display at the tower. ICAO Annex 3, Appendix 3, 4.5.1 and 4.5.2 recommend having approach area cloud information available for the air traffic services when precision approach procedures are used. In practice this means that most significant cloud data is the lowest significant cloud layer on the approach area, especially focusing to the lowest 300ft. Unfortunately, it is not defined how and when the cloud information should be updated. 4.5.1 Siting Recommendation. When instrumented systems are used for the measurement of the cloud amount and the height of cloud base, representative observations should be obtained by the use of sensors appropriately sited. For local routine and special reports, in the case of aerodromes with precision approach runways, sensors for cloud amount and height of cloud base should be sited to give the best practicable indications of the height of cloud base and cloud amount at the middle marker site of the instrument landing system or, at aerodromes where a middle marker beacon is not used, at a distance of 900 to 1 200 m (3 000 to 4 000 ft) from the landing threshold at the approach end of the runway. Note. Specifications concerning the middle marker site of an instrument landing system are given in Annex 10,Volume I, Chapter 3 and at Attachment C, Table C-5. 4.5.2 Display Recommendation. When automated equipment is used for the measurement of the height of cloud base, height of cloud base display(s) should be located in the meteorological station with corresponding display(s) in the appropriate air traffic services units. The displays in the meteorological station and in the air traffic services units should relate to the same sensor, and where separate sensors are required as specified in 4.5.1, the displays should clearly identify the area monitored by each sensor. 2.4 Local routine reports (MET REPORTs) are issued typically in 30-minute periods. Local special report (SPECIAL) should be issued when an operationally significant change is observed. However, according to Annex 3, Appendix 3, 3.2.2, SPECIAL reports may be excluded when ATC has the relevant information on their displays.
- 3 - AMOFSG/10-IP/4 3.2.2 Local special reports shall be transmitted to local air traffic services units as soon as the specified conditions occur. However, by agreement between the meteorological authority and the appropriate ATS authority, they need not be issued in respect of: a) any element for which there is in the local air traffic services unit a display corresponding to the one in the meteorological station, and where arrangements are in force for the use of this display to update information included in local routine and special reports 2.5 It is notable that cloud information Standards and recommendations are still rather much observer oriented. This is understandable as cloud information is one of the most complex of the observed parameters to be automated. Standards or recommendations specify the representative areas and current cloud algorithms try to satisfy that need using calculation periods. The commonly used 30-minute calculation period is too long for operational use in precision approach. 2.6 When an observer is not available, ceilometer(s) provide the base information for clouds. If airports have only single ceilometer (with algorithm), the cloud reporting capability is rather restricted. In typical case the single ceilometer reports cloudiness for the line from centre of the aerodrome to 30-minutes downwind. The report totally lacks cloud data in upwind direction. The area covered with the cloudiness report is highly affected with wind speed. In 30 minutes clouds are moving downwind from the centre of the aerodrome: 2 km, with 1 m/s, 9 km with 5 m/s, 16 km with 9 m/s and 36 km with 20 m/s. The last distance is far out from the aerodrome vicinity area regardless of the aerodrome size. 2.7 The factor of local climatology determines the types of cloud bases and their distribution around the aerodrome. Flat, uniform terrain with homogenous surface characteristics is likely to create a uniform, statistically distributed cloudiness over the area of the airport and its vicinity. Any strong differences in these characteristics (e.g. sea shore, hills, forests, etc.) will lead to typical localized cloudiness features particularly for low and medium cloud. Benefits of an array of ceilometers depends heavily on local weather and terrain characteristics. 2.8 Multi-ceilometer algorithms provide an enhancement for sky condition information. Studies have shown that single and multi-ceilometer algorithms provide similar results even over 90 per cent of cases. One of the reasons is the use of a 30-minute period that smoothens the differences. Also a significant portion of steady conditions make this, at least partially, a statistical fact. This statistical point of view discards the safety and efficiency point of view where rapid change situations have a much greater effect on the airport operations than steady conditions. As METAR reports are not meant for operational decisions, a single ceilometer with sky condition algorithm seems to be a reasonable compromise. 2.9 Studies with single ceilometer and multiple ceilometers less than 5 km apart have shown only little differences when the 30-minute period has been used. The longer the period is, the further the ceilometers must be placed to avoid an overlapping measurement area which does not enhance the results. Five point star form would be an optimum, but unfortunately it is not typically a practical solution for the airports. Triangle may already provide a significant enhancement, especially when ceilometers are 5 km or more from each other.
AMOFSG/10-IP/4-4 - 2.10 Automatic systems with a single ceilometer and 30-minute algorithms typically suffer with slow and late reporting of cloudiness in changing weather conditions. One possible benefit multiple ceilometers wide apart can provide is that the calculation period for the algorithm can be shortened. The same number of measurement points can be collected in less time reducing the wind speed effect to the measurement area. For example, three ceilometers and a 10-minute calculation period provide the same number of measurement data, but can better represent the aerodrome area and adapt faster to changing weather (cloudiness) conditions. 2.11 Providing ceilometers to outer areas of the aerodrome and its vicinity widens the reportable area to upwind direction from the centre of the aerodrome. Even approach direction is normally from downwind, the upwind direction cloudiness may provide a short-term forecast to the approach area. This is also likely the direction where a human observer focuses on changing weather conditions. 2.12 CHMI made a study at Prague airport comparing official METARs amended by observer and AUTOMETARs. A study contained data from September 2011 to June 2012. Total number of compared reports was 6597. AUTOMETAR cloud data was provided by multi-ceilometer algorithms using data from RWY 24, 06 and 12 approach area ceilometers and RWY 30 touchdown ceilometer. The four ceilometers are close to the form of triangle. The cloud comparison focused only to differentiate amounts FEW,SCT vs. BKN,OVC and 10 per cent difference in cloud height reporting. Cloud amount comparison provided a difference of 5.2 per cent and cloud height difference of 3.5 per cent of the reports. Results were better than expected. This may be due to a windy airport and a rather open terrain. CHMI expects that the addition of SPECI reports would have changed to results significantly. 2.13 At Prague airport air traffic controllers displays have cloud information from approach area ceilometer and based on a 5-minute period calculation. According to CHMI, the users have been satisfied. There have been thoughts to test the period of one or two minutes. This has been denied due to fact that ceilometers can send data in 15 or 30 second intervals. The number of samples would be too small for cloud base analysis. The algorithm used is a simplified version of a sky condition algorithm providing just the lowest layer (base) height. This works well enough for the operational purposes. The users are mostly interested in these values when cloud height falls under 300 ft. Typically on those conditions at Prague airport, wind speed is low and the approach area is fully obscured. So sophisticated cloud amount estimation is not necessarily needed. 2.14 Oulu airport in Finland is lying close to sea having two ceilometers located at the approach areas. As an example, air traffic controllers were complaining about not having AUTOSPECI and AUTOMETAR still reporting scattered medium height clouds, while the other runway end had low cloud or a vertical visibility situation. A short study was made using stored data of the most interesting rapidly changed situations. Sky condition algorithms with 30-minute and 10-minute period were compared. A 10-minute algorithm could catch the same cloud layers as a 30-minute algorithm and there were no major differences on steady conditions. From an operational point of view, a 10-minute algorithm provided significant improvement to reaction time in a rapid cloudiness changes. Long tails of passed clouds shrank. At the same time, a multi-ceilometer algorithm was tested with a greater weight factor on reporting vertical visibility. From a safety perspective it seemed to be operationally important. A change to vertical visibility situation or fast and significant change in cloud height or amount could be seen a bit like marked discontinuity situation on winds or RVR.
- 5 - AMOFSG/10-IP/4 2.15 The user s operational needs can be satisfied better by using multiple measurement locations and advanced cloud algorithms. Automatic cloud calculations can be, and should be, developed to react faster to the changes in cloud conditions. This would also improve cloud data validity in reports, especially in automated reports. 2.16 Air traffic controllers have the main role to provide real-time cloud information to the pilots for decision making. In that respect, it is vital that air traffic controller s working positions contain suitable, real-time cloud information for airport operational needs. In precision approaches the main focus in cloud reporting should be the lowest 300ft on approach areas. In rapidly changing weather conditions only air traffic controllers can provide up-to-date cloud information to the pilots. 3. ACTION BY THE AMOFSG 3.1 The AMOFSG is invited to note the information contained in this paper. END