Forecasting Runway Visual Range ECAM Lauren Reid, Met Office, 12 September 2013 © 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 © 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 © 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] © Crown copyright Met Office [1] http://picsfromtheoffice.blogspot.co.uk/2011_11_01_archive.html - Mathieu Neuforge 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): © 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) MOR ln(0.05) • 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)/R2 © Crown copyright Met Office • The equations from Koschmieder and Allard’s Law can be rearranged to form: MOR ln(0.05) RVRLL ln( I ) 2 ln( RVRLL ) Et © Crown copyright Met Office An illustration of the variation of RVR with MOR for a fixed illumination threshold (Et=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). Data © 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. • Et is required for the RVR calculation, so the ratio of the background light vs the whole hemisphere is utilised - 0.137% © 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 © Crown copyright Met Office Met Office UKV model Output fields from the UKV model valid at 06z 16 October 2011. Left: Visibility at 1.5m height (m). Right: Fog fraction (percentage). © Crown copyright Met Office • 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. 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 © Crown copyright Met Office Heathrow AP 16-10-2011 METAR and IRVR Obsevations at Touchdown Point 2000 1800 RVR Touch down South 1600 Distance (m) 1400 RVR Touch down North 1200 1000 METAR RVR North 800 600 400 METAR RVR South 200 0 02:00 04:00 06:00 08:00 10:00 Time of day 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. Heathrow AP 16-10-2011 METAR and Average IRVR Obsevations 2000 calc MOR 1800 1600 avg I-RVR South Distance (m) 1400 avg I-RVR North 1200 1000 METAR RVR North 800 600 METAR RVR South 400 METAR VIS 200 0 02:00 04:00 © Crown copyright Met Office 06:00 Time of day 08:00 10:00 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. Case Studies © 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 © Crown copyright Met Office Bournemouth Airport 30 September 2011 4000 Bournemouth AP 30-09-2011 (METAR and IRVR Obsevations) 3500 calc MOR 3000 Distance (m) avg I-RVR 2500 METAR VIS 2000 1500 METAR RVR R26 1000 METAR RVR R08 500 0 02:00 03:00 04:00 05:00 06:00 Tim e of day © Crown copyright Met Office 07:00 08:00 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) 4508 6046 0.125 1000 R26/0450 R08/0600 04z (T+1) 4668 5488 0.125 900 R26/P1500 R08/0250 05z (T+2) 5408 5953 0.00 200 R26/0250 R08/0300 06z (T+3) 5754 6293 0.00 100 R26/0400 07z (T+4) 6376 6685 0.00 600 R26/0175 08z (T+5) 7783 8858 0.00 4000 - 09z (T+6) 11083 11731 0.00 9000 - 10z (T+7) 12254 13414 0.00 CAVOK - 11z (T+8) 12379 13365 0.00 CAVOK - © Crown copyright Met Office HT-FRTC: Obs of visibility and forecast RVR Bournemouth 20110930 6000 METAR RVR R26 Distance (m) 5000 4000 METAR RVR R08 3000 Continental, I=3000 2000 Koschmieder MOR 1000 0 03:00 04:00 05:00 06:00 07:00 08:00 09:00 Time The minimum and maximum differences plus RMSE in metres of RVR using the forecast NWP input compared to the average transmissometer observations. © Crown copyright Met Office Difference RVR (m) Minimum 2338 Maximum 4303 RMSE 3514 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 © Crown copyright Met Office Heathrow Airport 16 October 2011 Heathrow AP 16-10-2011 METAR and Average IRVR Obsevations 2000 calc MOR 1800 1600 avg I-RVR South Distance (m) 1400 avg I-RVR North 1200 1000 METAR RVR North 800 600 METAR RVR South 400 METAR VIS 200 0 02:00 04:00 06:00 Time of day © Crown copyright Met Office 08:00 10:00 Heathrow Airport 16 October 2011 © Crown copyright Met Office Heathrow Airport 16 October 2011 - sensitivity Sensitivity study: Obs of visibility and forecast RVR Heathrow 20111016 2000 METAR RVR R27L 1800 Distance (m) 1600 METAR RVR R27R 1400 1200 IRVR data 1000 800 MONIM Et, I=3000 600 400 IRVR avg 200 0 03:00 04:00 05:00 06:00 07:00 Time © Crown copyright Met Office 08:00 09:00 10:00 HT-FRTC: Obs of visibility and forecast RVR Heathrow 20111016 3500 METAR RVR R27L Distance (m) 3000 2500 METAR RVR R27R 2000 1500 Continental, I=3000 1000 500 Koschmieder MOR 0 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 Time The minimum and maximum differences plus RMSE of the RVR using the forecast NWP input compared to the average transmissometer observations. © Crown copyright Met Office Difference RVR (m) Minimum -141 Maximum 2691 RMSE 1722 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 © Crown copyright Met Office Conclusions • When using obviations as inputs to the RVR calculation RMS error is in the region of 300900m. • 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 © Crown copyright Met Office Questions & answers © 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. © 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. © Crown copyright Met Office