AMOFSG/9-SN No. 31 22/8/11 AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG) NINTH MEETING Montréal, 26 to 30 September 2011 Agenda Item 5: Observing and forecasting at the aerodrome and in the terminal area 5.1: Observations REVALIDATION OF AERONAUTICAL REQUIREMENTS FOR VERTICAL VISIBILTY, ICE CRYSTALS AND SHOWERS (Presented by Bill Maynard) SUMMARY This study notes asks the study group to validate whether, or not, there is an ongoing requirement for the observation and reporting and, by extension, the forecasting of vertical visibility, showers and ice crystals. 1. INTRODUCTION 1.1 The ongoing aeronautical requirement for the observation, reporting and forecasting of vertical visibility, showers and ice crystals is re-evaluated. It is suggested that given the limitations of the ability to measure these, combined with arguably limited aeronautical requirements for each, that consideration be given to removing them from Annex 3 Meteorological Service for International Air Navigation. 2. DISCUSSION 2.1 Vertical visibility 2.1.1 Vertical visibility is used 30 times in Annex 3. In one case it is used in the record of previous amendments, 10 times in recommendations, 11 times in Tables and 8 times in Standards. Due to multiple use of the term within the same provisions only 4 separate standards and 6 separate (5 pages) AMOFSG.9.SN.031.5.en.doc
AMOFSG/9-SN No. 31-2 - recommendations are involved. There is also an indirect reference to vertical visibility in footnotes to tables which state that it is to be included whenever applicable. Also, the associated code form VV is used 13 times, all but one case being in Tables to the appendices. 2.1.2 The instances of vertical visibility within 4 standards involved are (emphasis added): (4.5.1 refers, in part) Local routine and special reports and METAR and SPECI shall contain the following elements in the order indicated: i) cloud amount, cloud type (only for cumulonimbus and towering cumulus clouds) and height of cloud base or, where measured, vertical visibility; (4.6.5.1 refers) Cloud amount, cloud type and height of cloud base shall be observed and reported as necessary to describe the clouds of operational significance. When the sky is obscured, vertical visibility shall be observed and reported, where measured, in lieu of cloud amount, cloud type and height of cloud base. The height of cloud base and vertical visibility shall be reported in metres (or feet). (Appendix 3, 4.5.4.4 refers in part) In local routine and special reports: a) the units of measurement used for the height of cloud base and vertical visibility shall be indicated; (Appendix 5, 2.2.6 refers) Vertical Visibility - When the sky is expected to remain or become obscured and vertical visibility observations are available at the aerodrome, and the vertical visibility is forecast to improve and change to or pass through one or more of the following values, or when the vertical visibility is forecast to deteriorate and pass through one or more of the following values: 30, 60, 150 or 300 m (100, 200, 500 or 1 000 ft), the trend forecast shall indicate the change. 2.1.3 Of particular note is that the standards related to vertical visibility are all qualified as being applicable when applicable or where measured such that they are enabling and do not mandate its inclusion in meteorological reports. 2.1.4 The qualifiers relate to 2 different situations. First, the reporting of vertical visibility is conditional upon the existence of phenomena that fully obscure the sky. Second, it is not available from all observation programs and, for example, is typically reliant upon human observers to make an assessment. 2.1.5 Based upon the last consolidated supplement to Annex 3, at least 7 States have filed State Differences indicating that they do not provide reports of vertical visibility. In fact, one State remarks that there is no operational requirement for vertical visibility. In another State vertical visibility is assessed only by inference based upon the present weather and visibility. 2.1.6 Selected excerpts related to vertical visibility from WMO No. 8 Guide to Meteorological Instruments and Methods of Observation - may be relevant: Part I 15.1.1 (in part) Vertical visibility: The maximum distance at which an observer can see and identify an object on the same vertical as him/herself, above or below. 15.1.3 Meteorological requirements
- 3 - AMOFSG/9-SN No. 31 For meteorological purposes, observations are required for cloud amount, cloud type and height of cloud base 15.7.1 Some laser ceilometers provide an estimate of vertical visibility based on the integrated reflected energy within range. Comparisons carried out during the WMO International Ceilometer Intercomparison (WMO, 1988) showed that, on many occasions, values reported were unreliable and that further development of this capability would be necessary before estimates could be used with confidence. Part II 2.6.1 (in part) Because LIDAR based ceilometers determine the local extinction coefficient for fixed intervals VV may be derived relatively easily 2.1.7 Notably, WMO No. 8 does not define vertical visibility as a meteorological requirement therefore we must conclude that it is a strictly aeronautical requirement. 2.1.8 Other than by use of the sensor integration, many automated weather stations are not able to assess vertical visibility. On the other hand, many laser ceilometers are able to determine multiple cloud layers such that they are not actually limited by obstructions that a human observer may report as vertical visibility. Therefore, vertical visibility may be a human limitation not shared by many current and most future laser ceilometers. 2.1.9 It may be recalled that vertical visibility can be a result of fog, blowing snow, blowing sand, blowing dust, sandstorm, duststorm, heavy precipitation (especially snow and drizzle), smoke or volcanic ash. The reporting of these phenomena in combination with a low visibility may be sufficient information for operators. 2.1.10 The only information added by the reporting of vertical visibility is that the base of any overlying cloud is not visible, by humans, from the surface. However, given the typical low prevailing visibility associated with obscured skies this would normally be redundant information. Moreover, the systematic suppression of information related to overlying low cloud that is available to laser ceilometers removes information stated as an aeronautical requirement elsewhere in Annex 3. 2.1.11 It is proposed that vertical visibility be eliminated and that all cases of fully obscured skies be reported as overcast with the associated base to be reported in place of vertical visibility but at the same height. For example VV005 would be replaced by OVC005, when applicable. For automated sites that are not limited by surface based obscurations, this would not be applicable, and cloud would be reported in the conventional manner. 2.1.12 A template for the proposed text changes is illustrated below and would be accompanied by the deletion of all references to vertical visibility in favour of the use of overcast. 4.5.1 i) cloud amount, cloud type (only for cumulonimbus and towering cumulus clouds) and height of cloud base or, where measured, vertical visibility; when applicable, the maximum height above ground visible into an obscuring layer. 4.6.5 Clouds 4.6.5.1 Cloud amount, cloud type and height of cloud base shall be observed and reported as necessary to describe the clouds of operational significance. When the sky is obscured, vertical
AMOFSG/9-SN No. 31-4 - visibility an overcast sky shall be observed and reported, where measured, in lieu of cloud amount, cloud type and height of cloud base. when applicable, along with the maximum height above ground visible into the obscuring phenomena. The height of cloud base and vertical visibility shall be reported in metres (or feet). 2.1.13 As an editorial note, it could also be suggested that the restriction of cloud type to only for cumulonimbus and towering cumulus should apply equally to both of 4.5.1 and 4.6.5. 2.2 Ice crystals 2.2.1 Ice crystals are mentioned 5 times in the annex, four times in recommendations and once in a standard. Curiously, the one standard that requires it is for trend forecasts. 2.2.2 Annex 3 refers to ice crystals as both precipitation and an obscuration. In Table A3-2 it is placed in the obscurant s column whilst in appendix 3, 4.4.2.3 a) it is specifically referred to as precipitation. 2.2.3 In practice, no intensity is assigned to ice crystals and they are always of a light intensity yet only reported with the associated visibility is 5 km (3SM) or less (notably, there is no corresponding visibility limitation for its inclusion in a TAF). Also, the amount of accumulation is normally negligible even over several hours. 2.2.4 Ice crystals are frozen and occur at very low temperatures (typically -30 C or less) and therefore do not represent an icing hazard. 2.2.5 Ice crystals can result in reduced visibility but would be reported as fog or a mix of fog and ice crystals, should visibility go below 1 km (0.5 NM). 2.2.6 Ice crystals may contribute to the false reporting of low cloud layers by some ceilometers as a result of their associated backscatter discontinuities being similar to those of the base of frozen clouds. Moreover, ice crystals present a significant challenge for automated systems to report and it may not make sense, from a cost-benefit point of view, to focus on developing this capability. 2.2.7 Although they occur in markedly different temperature ranges there is a potential, in some atmospheric conditions, that snow grains and ice crystals could be confused. The former represents a hazard as it is typically formed from supercooled large droplets (often associated with freezing drizzle) while the later is simply a restriction to visibility. Also, there is an associated possibility that some users may be confused regarding which represents a hazard. In such a case, reporting only the hazardous phenomena would eliminate this possibility. 2.2.8 In consideration of the above, it could be proposed that the reporting of ice crystals does not meet any aeronautical requirement and is redundant information where visibility is being reported. Moreover, it would simplify the Annex and eliminate an area of potential confusion with a hazardous phenomena should it be removed. 2.2.9 Note that WMO No. 407 International Cloud Atlas-Volume I defines ice crystals as being precipitation (Part III, 2.1.2 (8) refers).
- 5 - AMOFSG/9-SN No. 31 2.3 Showers 2.3.1 It can be argued that the reporting and forecasting of showers is problematic, prone to error, redundant and lacking in a clear aeronautical requirement. 2.3.2 The only information added by using the qualifier shower is the intermittent nature of the precipitation and, uniquely, the association with convective cloud types as noted in the following excerpt from WMO No. 8 (Part I, 14.2.1.1, in part): Showers are the precipitation events associated with physically separated convective clouds. 2.3.3 If the purpose of reporting showers is primarily to report on the presence of convective cloud then it may be redundant reporting given that this is already specified for the reporting and forecasting of cloud as noted in the following references from Annex 3 and in Table A3-2 and A5-1: Appendix 3 4.5.4.3 Recommendation. In local routine and special reports and in METAR and SPECI: b) cumulonimbus clouds and towering cumulus clouds should be indicated as CB and TCU, respectively; Appendix 5 (1.2.4, in part) Recommendation. Cloud amount should be forecast in the following order: d) cumulonimbus clouds and/or towering cumulus clouds, whenever forecast and not already included under a) to c). 2.3.4 It can also be argued that it is problematic for either human or automated systems to consistently assess the existence of showers without using remote sensing such as weather radar to detect the convective cloud. It is also notable that shower activity could be associated with altocumulus castellanus for which is not reportable as a cloud type. 2.3.5 Based on the above, it may be appropriate to re-validate the aeronautical requirement for the reporting of shower as a present weather descriptor as it is actually intended to convey cloud type information. 3. CONCLUSION 3.1 A clarification regarding the ongoing aeronautical requirements for the observation, reporting and forecasting of vertical visibility, ice crystals and showers may be appropriate. 4. ACTION BY THE AMOFSG 4.1 The AMOFSG is invited to comment on this paper and whether there is a continued aeronautical requirement for the continued reporting and forecasting of vertical visibility, ice crystals and showers. END