1. © Crown copyright Met Office Forecasting Runway Visual Range Lauren Reid, Met Office, 12 September 2013 ECAM

2. © Crown copyright Met Office Contents This presentation covers the following areas Introduction Definition of RVR Methodology Models (UKV, MONIM, HT-FRTC) Case studies Conclusions Questions and answers

3. © Crown copyright Met Office Introduction Runway Visual Range (RVR) is used at airports to help determine if Low Visibility Procedures (LVP) are necessary. LVPs affect airport operations by requiring a reduction in the number of aircraft landing and taking-off depending on the cloud base height, visibility and/or RVR The purpose of this study was to determine if it may be possible to generate an RVR forecast to aid airport operations mitigating against the worst impacts of LVP

4. © Crown copyright Met Office RVR ≠ Visibility The view from the cockpit along runway 22R at Copenhagen during a fog event. The RVR in this photo is 500m [1] [1] [1] http://picsfromtheoffice.blogspot.co.uk/2011_11_01_archive.html - Mathieu Neuforgehttp://picsfromtheoffice.blogspot.co.uk/2011_11_01_archive.html

5. © Crown copyright Met Office Definitions Runway Visual Range is defined by ICAO Annex 3 as: The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. Visibility - Visibility for aeronautical purposes is the greater of: a)the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; b)the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background. Note — The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR). (ICAO Document 9328 2005):

6. © Crown copyright Met Office Relationships Koschmieder's Law. A relationship between the apparent luminance contrast of an object, seen against the horizon sky by a distant observer, where σ = extinction coefficient (m -1 ) Allard's Law. An equation relating illuminance (E) produced by a point source of light of intensity (I) on a plane normal to the line of sight, at distance (R) from the source, in an atmosphere having a transmissivity (T). E=(Ie -σR )/R 2

7. © Crown copyright Met Office An illustration of the variation of RVR with MOR for a fixed illumination threshold (E t =10 -5 ) derived from the equation on previous page. Results are displayed for a range of runway lighting intensity (I) from 10 cd to 10000 cd (shown as different colours). The equations from Koschmieder and Allard’s Law can be rearranged to form:

8. © Crown copyright Met Office Data

9. © Crown copyright Met Office MONIM (Met Office Might Illumination Model) MONIM is used by defence as a tactical decision aide for use with night vision goggles. Adapted for use in built up areas with additional light sources. Results in the illumination value, E. E t is required for the RVR calculation, so the ratio of the background light vs the whole hemisphere is utilised - 0.137%

10. © Crown copyright Met Office HT-FRTC (Havemann-Taylor Fast Radiative Transfer Code) The Havemann-Taylor Fast Radiative Transfer Code was developed at the Met Office for use in simulating the electromagnetic radiation from a source at a specified observation location some distance away As the electromagnetic radiation travels to the observer, the radiation is modified by the atmosphere, which absorbs, emits and scatters the radiation by varying amounts depending on the current state of the atmosphere

11. © Crown copyright Met Office Met Office UKV model The ability to accurately forecast fog and the subsequent low visibility is incredibly important for airport operations, it is important to recognise that fog is one of the most difficult meteorological phenomena to forecast with the level of detail required. Output fields from the UKV model valid at 06z 16 October 2011. Left: Visibility at 1.5m height (m). Right: Fog fraction (percentage).

12. © Crown copyright Met Office Observations Transmissometer The three transmissometers record and store the details of the runway lighting, including the background luminance (cd/m2) and the runway light intensity (cd). If the observed RVR is above 1500m then the system does not record the RVR METAR (Meteorological Terminal Air Report) Is a routine meteorological observation given at all airports. METARs give an indication to incoming pilots of the current weather conditions at the runway and includes: temperature; pressure; cloud height and coverage (in octa); wind speed and direction; visibility; and RVR (if below 1500m). These two methods used to identify and test case studies at Heathrow and Bournemouth

13. © Crown copyright Met Office Examples of the output transmissometer RVR for Heathrow Airport on 16 October 2011. a) Shows only the first (touch down) value of RVR from the transmissometer. Note how the METAR values for both the northern and southern runway match with the IRVR data. b) There are three reported values for the RVR for the north and south runways which has been averaged to give the blue and red line, from this the MOR for the northern runway was calculated (green). The overlaid points are the METAR RVR (for both runways) and visibility in blue, red and green respectively.

14. © Crown copyright Met Office Case Studies

15. © Crown copyright Met Office Case Studies Dates were chosen for the case studies based on METAR observations of RVR that lasted longer than 3.5 hours and resulted in a significant reduction in visibility at Heathrow or Bournemouth Airports. The dates investigated were: 30th September 2011 at Bournemouth 16th October 2011 at Heathrow 20th September 2011 at both sites

16. © Crown copyright Met Office Bournemouth Airport 30 September 2011

17. © Crown copyright Met Office Bournemouth Airport 30 September 2011 Forecast Time UKV Visibility (m) 36km box Average (m) Percentage Fog (0-1) METAR Visibility (m) METAR RVR (m) 03z (T+0)450860460.1251000 R26/0450 R08/0600 04z (T+1)466854880.125900 R26/P1500 R08/0250 05z (T+2)540859530.00200 R26/0250 R08/0300 06z (T+3)575462930.00100R26/0400 07z (T+4)637666850.00600R26/0175 08z (T+5)778388580.004000- 09z (T+6)11083117310.009000- 10z (T+7)12254134140.00CAVOK- 11z (T+8)12379133650.00CAVOK-

18. © Crown copyright Met Office The minimum and maximum differences plus RMSE in metres of RVR using the forecast NWP input compared to the average transmissometer observations. DifferenceRVR (m) Minimum2338 Maximum4303 RMSE3514

19. © Crown copyright Met Office Bournemouth Airport 30 September 2011 Poor visibility forecast unrepresentative of the conditions experienced The other case study also had a similar situation where the forecast visibility was too large resulting in unrealistic RVR forecast As Bournemouth has a costal location this impacts on fog formation in the region

20. © Crown copyright Met Office Heathrow Airport 16 October 2011

21. © Crown copyright Met Office Heathrow Airport 16 October 2011

22. © Crown copyright Met Office Heathrow Airport 16 October 2011 - sensitivity

23. © Crown copyright Met Office The minimum and maximum differences plus RMSE of the RVR using the forecast NWP input compared to the average transmissometer observations. DifferenceRVR (m) Minimum-141 Maximum2691 RMSE1722

24. © Crown copyright Met Office Heathrow Airport 16 October 2011 Fog/visibility forecast was reflective of the observed conditions The forecast data used in the RVR forecast did result in a good approximation of the RVR though it did tend to be too large compared to the METARs

25. © Crown copyright Met Office Conclusions When using obviations as inputs to the RVR calculation RMS error is in the region of 300- 900m. When using the forecast visibility the RVR can degrade significantly to be over 2000m larger than the observations. It is possible to forecast RVR, but only if the forecast visibility and low level cloud is an accurate representation of the conditions experienced

26. © Crown copyright Met Office Questions & answers

27. © Crown copyright Met Office RVR Definition Runway visual range is defined by ICAO Annex 3 as: The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. Further details are given by ICAO Doc 9328 section 2.2: The definition implies that RVR is not an “observation” or a “measurement” of a meteorological parameter such as surface wind direction and speed, temperature and pressure; it is an assessment, based on calculations that take into account various elements, including atmospheric factors such as extinction coefficient of the atmosphere, physical/biological factors such as visual threshold of illumination, and operational factors such as runway light intensity.

28. © Crown copyright Met Office Visibility Definitions The definitions of visibility and meteorological optical range are (ICAO Document 9328 2005): Visibility - Visibility for aeronautical purposes is the greater of: a)the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; b)the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background. Note — The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR). Meteorological Optical Range (MOR) - The length of the path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp at a colour temperature of 2,700 K to 0.05 of its original value, the luminous flux being evaluated by means of the photometric luminosity function of the International Commission on Illumination.