AMOFSG/7-IP/6 13/8/08

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

SEVENTH MEETING

Montréal, 9 to 12 September 2008

Agenda Item 6: Aerodrome forecasts

A COMPARISON OF RVR AND VISIBILITY REPORTS

(Presented by Bill Maynard)

SUMMARY This paper summarizes the results of an analysis of RVR and visibility reports included in METAR / SPECI for three aerodromes. It concludes that any future forecasting of RVR, based upon visibility, must be by reference to sensors used for both RVR and visibility.

1. INTRODUCTION

1.1 A comparative analysis was undertaken of the RVR and prevailing visibility as included in METAR and SPECI over a five-year period at three aerodromes. Each of these aerodromes is seasonally exposed to winter weather and one is located north of the Arctic Circle. The assessment of prevailing visibility at each is by human estimation from a separate location from the RVR sensors, which are of a forward scatter design.

1.2 The results of this analysis suggest some sources of uncertainty that should be considered if visibility reports are to be used to infer a most probable RVR value. It concludes that these uncertainties are such that any useful forecast of RVR, based upon visibility reports, must be by reference to the same sensors being used for both RVR and visibility.

(5 pages) 0ce4fe868b67d3800d67db29a61d535f.doc AMOFSG/7-IP/6 2

2. DISCUSSION

2.1 Previous studies have shown that the reported value of RVR is typically higher than the associated visibility value. In fact, empirical rules can be used to relate values of visibility with the most probable value of RVR (see IP3 from the sixth meeting of this study group). These findings are consistent with the fact that the assessment of RVR takes advantage of high intensity runway lights while the visibility assessment is based upon lights of moderate intensity.

2.2 The following definitions from Annex 3 relate;

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).

2.3 Also, Appendix 3, 4.3.5 states (in part) that: In METAR and SPECI, the runway visual range should be based on the maximum light intensity available on the runway.

2.4 A comparative review of the 5,662 METAR / SPECI from three aerodromes over a five year period that contained both prevailing visibility and RVR information revealed that, in equivalent terms;

a) for 2825, or 50 per cent, the RVR report was less than the visibility;

b) for 1085, or 19 per cent, the RVR was less than half the visibility;

c) for 951, or 17 per cent, the RVR was less than one third of the visibility; and

d) for 596, or 11 per cent, the RVR was less than one quarter of the visibility.

2.5 The runway lighting in use for each case could not be determined. Regardless, the significant proportion of comparative RVR and visibility reports with large differences suggest that there are major limitations upon the ability to confidently predict RVR using subject prevailing visibility reports from another location on the aerodrome.

2.6 Analysis of this dataset also suggests that empirical relationships between RVR and visibility should be used with caution during non-uniform conditions, especially if the point of interest in RVR is displaced from the point(s) of assessment for visibility.

2.7 It is also evident from this dataset that there is a high degree of variability of visibility, in space and time, during blowing snow events, with a few examples shown in the Appendix. 3 AMOFSG/7-IP/6

3. CONCLUSION

3.1 Further data should be obtained to validate these findings. Such data should be based entirely upon RVR reports that refer to maximum runway light intensity only. It should also be explored whether these variations in time and space apply to cases with blowing dust and blowing sand.

3.2 Two hypotheses are drawn regarding the relationship between visibility and RVR:

e) the spatial and temporal variability during blowing snow conditions can be large resulting in significant uncertainties when inferring probable RVR from prevailing visibility assessed from a different location on the aerodrome; and

f) meaningful predictions of RVR from prevailing visibility reports can only be accomplished if the assessment of both RVR and visibility is by reference to common sensor(s).

3.3 The need to retain flexibility in the application of recommendation 4.3.1.1 of Appendix 3 to the Annex that calls for RVR to be assessed at a height of approximately 2.5 m should be noted. It may be prudent to adjust this height at locations that are exposed to significant snow and / or blowing snow events.

— — — — — — — —

AMOFSG/7-IP/6 Appendix

APPENDIX

Chapter 1 XXX Examples of cases with significant differences between RVR and visibility (Visibility is in units of Statute Miles, RVR is in units of feet and included as remarks)

Received: 2002-04-17 08:24:35 SACN31 CWAO 170800 RRA METAR CYFB 170800Z 33036G44KT 1/2SM BLSN VV000 M20/M23 A2997 RMK BLSN8 RVR RWY 35 0900 FT SLP155=

Received: 2002-12-13 00:13:21 SACN31 CWAO 130000 RRA METAR CYFB 130000Z 32039G45KT 1/8SM +BLSN VV001 M15/M16 A2921 RMK BLSN8 RVR RWY 35 400 FT SLP894=

Received: 2003-02-05 23:07:00 SACN31 CWAO 052300 METAR CYFB 052300Z 33023G29KT 6SM BLSN SKC M31/M36 A2983 RMK RVR RWY 35 4500 FT SLP106=

Received: 2003-03-04 14:07:04 SACN31 CWAO 041400 METAR CYFB 041400Z 32026KT 1 1/4SM BLSN FEW047 BKN088 M32/M37 A2939 RMK BLSN1SC2AC2 RVR RWY35 1600 FT VSBY N 3/4 SLP959=

Received: 2003-04-16 16:07:00 SACN31 CWAO 161600 METAR CYFB 161600Z 32024G31KT 20SM DRSN FEW091 BKN230 M19/M20 A2963 REBLSN RMK AC2CI2 RVR RWY 35 600 FT VIS N 2 SLP036=

Received: 2003-04-16 16:41:01 SPCN31 CWAO 161638 SPECI CYFB 161638Z 32030G35KT 2SM BLSN SCT091 OVC240 RMK AC3CI2 RVR RWY 35 1600 FT VIS VRB 11/2-21/2=

Received: 2002-06-13 01:07:00 SACN31 CWAO 130100 METAR CYJT 130100Z 23005KT 1/8SM FG VV001 06/06 A2993 RMK FG8 RVR RWY 27 P6000FT SLP137=

— END —