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

© Crown copyright Met Office

Forecasting Runway Visual RangeLauren Reid, Met Office, 12 September 2013

ECAM


Contents

This presentation covers the following areas

• Introduction

• Definition of RVR

• Methodology

• Models (UKV, MONIM, HT-FRTC)

• Case studies

• Conclusions

• Questions and answers


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


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] http://picsfromtheoffice.blogspot.co.uk/2011_11_01_archive.html - Mathieu Neuforge

http://picsfromtheoffice.blogspot.co.uk/2011_11_01_archive.html


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):


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)/R2

ln(0.05)MOR


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).

ln(0.05)

ln( ) 2 ln( )LL

LL

RVRMOR

I RVREt

• The equations from Koschmieder and Allard’s Law can be rearranged to form:


Data


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%


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


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).


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


Heathrow AP 16-10-2011 METAR and Average IRVR Obsevations

0

200

400

600

800

1000

1200

1400

1600

1800

2000

02:00 04:00 06:00 08:00 10:00Time of day

Dis

tan

ce (

m)

calc MOR

avg I-RVR South

avg I-RVR North

METAR RVRNorth

METAR RVRSouth

METAR VIS

Heathrow AP 16-10-2011METAR and IRVR Obsevations at Touchdown Point

0

200

400

600

800

1000

1200

1400

1600

1800

2000

02:00 04:00 06:00 08:00 10:00Time of day

Dis

tan

ce (

m)

RVR Touchdown South

RVR Touchdown North

METAR RVRNorth

METAR RVRSouth

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.


Case Studies


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


Bournemouth Airport 30 September 2011

Bournemouth AP 30-09-2011 (METAR and IRVR Obsevations)

0

500

1000

1500

2000

2500

3000

3500

4000

02:00 03:00 04:00 05:00 06:00 07:00 08:00

Time of day

Dis

tan

ce (

m)

calc MOR

avg I-RVR

METARVIS

METARRVR R26

METARRVR R08


Bournemouth Airport 30 September 2011

Forecast TimeUKV Visibility

(m)

36km box Average (m)

Percentage Fog (0-1)

METAR Visibility (m)

METAR RVR (m)

03z (T+0) 4508 6046 0.125 1000R26/0450R08/0600

04z (T+1) 4668 5488 0.125 900R26/P1500R08/0250

05z (T+2) 5408 5953 0.00 200R26/0250R08/0300

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 -


HT-FRTC: Obs of visibility and forecast RVRBournemouth 20110930

0

1000

2000

3000

4000

5000

6000

03:00 04:00 05:00 06:00 07:00 08:00 09:00Time

Dis

tan

ce

(m

)

METAR RVRR26

METAR RVRR08

Continental,I=3000

Koschmieder -MOR

The minimum and maximum differences plus RMSE in metres of RVR using the forecast NWP input compared to the average transmissometer observations.

Difference RVR (m)

Minimum 2338

Maximum 4303

RMSE 3514


Bournemouth Airport30 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


Heathrow Airport 16 October 2011

Heathrow AP 16-10-2011 METAR and Average IRVR Obsevations

0

200

400

600

800

1000

1200

1400

1600

1800

2000

02:00 04:00 06:00 08:00 10:00Time of day

Dis

tan

ce (

m)

calc MOR

avg I-RVR South

avg I-RVR North

METAR RVRNorth

METAR RVRSouth

METAR VIS




Heathrow Airport 16 October 2011 - sensitivity

Sensitivity study: Obs of visibility and forecast RVRHeathrow 20111016

0

200

400

600

800

1000

1200

1400

1600

1800

2000

03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00

Time

Dis

tan

ce

(m

)

METARRVR R27L

METARRVR R27R

IRVR data

MONIM Et,I=3000

IRVR avg


HT-FRTC: Obs of visibility and forecast RVRHeathrow 20111016

0

500

1000

1500

2000

2500

3000

3500

03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00

Time

Dis

tan

ce

(m

)

METAR RVRR27L

METAR RVRR27R

Continental,I=3000

Koschmieder -MOR

The minimum and maximum differences plus RMSE of the RVR using the forecast NWP input compared to the average transmissometer observations.

Difference RVR (m)

Minimum -141

Maximum 2691

RMSE 1722



• 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


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


Questions & answers


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.


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 Forecasting Runway Visual Range Lauren Reid, Met Office, 12 September 2013 ECAM.

Documents