AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

Transcription

1 AMOFSG/10-SN No. 5 19/4/13 AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG) TENTH MEETING Montréal, 17 to 19 June 2013 Agenda Item 5: Aerodrome observations REPORTING OF RUNWAY VISUAL RANGE (Presented by HU Jia-mei) SUMMARY This paper discusses the reporting of runway visual range (RVR) for situations with rapidly varying visibility. In order to provide sufficient information to the users about the visibility changes, the mean as well as the minimum RVR may need to be reported. 1. INTRODUCTION 1.1 According to ICAO Annex 3 Meteorological Service for International Air Navigation, the 10-minute average runway visual range (RVR) is normally reported in aerodrome routine meteorological report (in meteorological code form)/aerodrome special meteorological report (in meteorological code form) (METAR/SPECI). However, if there is significant variation of RVR, the minimum and maximum values of 1-minute mean RVR in the previous 10-minute period (or shorter in case of marked discontinuity of RVR) would be reported. The details could be found in the following extract from the latest version of Annex Recommendation. In METAR and SPECI when instrumented systems are used for the assessment of runway visual range, the variations in runway visual range during the 10-minute period immediately preceding the observation should be included as follows: a) if the runway visual range values during the 10-minute period have shown a distinct tendency, such that the mean during the first 5 minutes varies by 100 m or more from the mean during the second 5 minutes of the period, this should be indicated. When the variation of the runway visual range values shows an upward or downward tendency, this should be indicated by the abbreviation U or D, respectively. In circumstances when actual fluctuations during the 10-minute period show no distinct

2 AMOFSG/10-SN No tendency, this should be indicated using the abbreviation N. When indications of tendency are not available, no abbreviations should be included; and b) if the 1-minute runway visual range values during the 10-minute period vary from the mean value by more than 50 m or more than 20 per cent of the mean value, whichever is greater, the 1-minute mean minimum and the 1-minute mean maximum values should be reported instead of the 10-minute mean value. If the 10-minute period immediately preceding the observation includes a marked discontinuity in runway visual range values, only those values occurring after the discontinuity should be used to obtain variations. Note. A marked discontinuity occurs when there is an abrupt and sustained change in runway visual range, lasting at least 2 minutes, which reaches or passes through criteria for the issuance of SPECI given in c). 1.2 In the 9th meeting of ICAO Aerodrome Meteorological Observation and Forecast Study Group (AMOFSG) in September 2011, there were discussions about the reporting of variations in RVR and, in particular, when SPECI was reported. It was considered that, since any significant variations would already be captured by the use of a tendency, the additional reporting of the variations appeared to be redundant. It was also noted that the provision relating to the reporting of these variations was complex and likely to be confusing to the user because sometimes the 10-minute mean value was reported, and sometimes the 1-minute mean values were reported. If variations of RVR were reported, it would be difficult for the users to track why they needed to be reported in the present METAR/SPECI message but not afterwards, because the users were not provided with all the 10-minute averages in all the METAR/SPECI messages. As such, there was a suggestion to report the 10-minute mean values with the tendencies only. The suggestion was later adopted in ICAO and included in Amendment 76 to Annex The present paper discusses the reporting of RVR variation in METAR/SPECI based on the weather situations at Hong Kong International Airport (HKIA). 2. WEATHER CONDITIONS IN HONG KONG 2.1 The reporting of RVR variation in METAR has been studied since the opening of HKIA and the results are given in Table 1. It could be seen that the reporting of RVR variation is about 1 per cent or less of all the METAR messages. For foggy/hazy situations in which the maximum 1-minute mean RVR value is less than 1000 m, the frequency of such RVR variation is about 0.1 per cent or less of all the METAR messages. As such, the reporting of RVR variation, especially for RVR less than 1000 m, is rather rare in HKIA.

3 -3- AMOFSG/10-SN No. 5 Note: 2012 covers January and February only Table 1 Percentages of the reporting of RVR variation at HKIA 2.2 Though the frequency of occurrence is low, as the tendency does not provide any information on how large the variation is, the reporting of 10-minute mean RVR together with the tendency may not reflect the potentially limiting RVR, even with the implementation of marked discontinuity. A case in point is the fog case on 19 February At about 04:28 HKT, 19 February 2013, fog patches started to affect HKIA. The 1 minute mean RVR dropped rapidly to below 800 m (Figure 1). However, as the fog that was advected to HKIA at that point was rather patchy, it failed to sustain a change lasting 2 minutes initially and did not meet the marked discontinuity criterion at 04:30 HKT. Based on the latest proposed amendment, 1200D would be reported based on the 10-minute RVR and the tendency for the RVR to drop. This, however, provided no hint on how low the RVR was, which later dropped further to below 500m mean1minrvr mean10minrvr RVR (m) :10 4:12 4:14 4:16 4:18 4:20 4:22 4:24 4:26 4:28 4:30 Hong Kong time Figure 1 A case of falling RVR in fog at R2E, 19 February A similar situation would arise for the fog case of Figure 2. The 1-minute mean RVR dropped below the threshold of CAT I runway, namely, 550 m at about 03:25:50 HKT, 15 March 2011, at which the 10-minute mean RVR was still 2000 m. From 03:27:30 HKT onwards, the 1-minute mean

4 AMOFSG/10-SN No RVR rose above 550 m, but the 10-minute mean RVR dropped below 550 m due to marked discontinuity of RVR. It appears that the 1-minute mean RVR and 10-minute RVR are out of phase. Moreover, based on the latest proposed amendment, the reported RVR at 03:30 HKT would be 0550N, which could not capture the lower 1-minute mean RVR of about 400 m in the 10-minute period before 03:30 HKT. Figure 2 A case of falling RVR in fog at R2E, 15 March For the case of Figure 2, if METAR is to be issued at 03:26 HKT, the report would be 0450VP2000D in the existing scheme, and 1800D according to the latest proposed amendment. The lower RVR of about 450 m could not be reflected in future. 2.6 For the fog case of Figure 3, the 1-minute mean RVR was fluctuating about the RVR threshold of CAT II runway, namely, 300 m. On the other hand, the 10-minute mean was slow in responding to such cases, and remained below 300 m in the whole period. If the proposed scheme of AMOFSG is adopted, the reported RVR at 09:00 HKT would be 0250N, which could not capture the lower 1-minute mean RVR of about 150 m in the 10-minute period before 09:00 HKT. Figure 3 A case of rapidly varying RVR in fog at R2E, 28 February 2011.

5 -5- AMOFSG/10-SN No Apart from fog, rain/shower could also cause rapid changes of RVR. A case of RVR variation in shower is given in Figure 4. The 1-minute mean RVR dropped below 1000 m (initiation of low-visibility procedure) starting from 08:41 HKT, 5 April The 10-minute mean RVR, however, stayed above 2000m at that point. Based on the latest amendment, if the whole event happened 11 minutes earlier, the report would be P2000D, which again could not capture the lower 1-minute mean RVR of about 800m mean1minrvr mean10minrvr :29 8:33 8:37 8:41 8:45 8:49 8:53 8:57 9:01 9:05 9:09 9:13 9:17 Figure 4 A case of changing RVR in shower at R2C, 5 April USER S COMMENTS 3.1 The matter of reporting RVR variation was discussed with the users in Hong Kong in January The pilots indicated that they were concerned about the minimum RVR and the reporting of average value might not be sufficient to the pilots as very different minimum RVR could have the same averaged value. It was suggested that the minimum 1-minute mean RVR in addition to the average value be reported should the variation be significant. 4. ACTION BY THE GROUP 4.1 The group is requested to: a) note the content of this paper; and b) consider that the minimum RVR value be reported in addition to the 10-minute average RVR and the tendency, in case of significant variation of RVR. END