AMOFSG/10-SN No. 7 23/4/13 AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG) TENTH MEETING Montréal, 17 to 19 June 2013 Agenda Item 5: Aerodrome observations REPORT FROM THE AD-HOC WORKING GROUP 2: ESTABLISH THE USER REQUIREMENTS FOR THE REPORTING OF INTERMITTENT PRECIPITATION AND SHOWERS (Presented by Colin Hord) SUMMARY This Study Note presents the final report of an ad-hoc working group formed at AMOFSG/9. Its purpose was to review the user requirements for the reporting of intermittent precipitation and showers in local routine and special report and in METAR and SPECI. 1. INTRODUCTION 1.1 At the ninth meeting of Aerodrome Meteorological Observation and Forecast Study Group (AMOFSG/9), an ad-hoc team (WG/2) was set up to consider the user requirements for the reporting of intermittent precipitation. This resulted from discussions during the meeting that in automated reports that contained showers, these were in effect a report of intermittent precipitation and not the classic interpretation of precipitation falling from convective cloud. 1.2 The Summary of Discussions from AMOFSG/9 concluded that the group had mixed views on t he operational requirements for reporting showers in automated reports. It was agreed that showers were difficult to observe using automatic systems and that, in many cases, such systems actually reported intermittent precipitation as showers. It was also acknowledged that the convective nature of the atmosphere was reported by means of the cloud type, namely TCU and CB. (7 pages) AMOFSG.10.SN.007.5.en.docx AMOFSG/10-SN No. 7 - 2 - 1.3 This led the group to consider what the aeronautical requirement was for the reporting of showers other than to report intermittent precipitation in meteorological reports in general. 1.4 Consequently the following action was agreed: Action Agreed 9/26 — User requirements for the reporting of intermittent precipitation That an ad hoc group (WG/2) consisting of Colin (rapporteur), Carole, [IATA], Keith, Steve, Michael and Bill establish the user requirements for the reporting of intermittent precipitation and showers and provide a report for the next meeting of the group. 1.5 This paper recognizes and appreciates the reponses and guidance provided by colleagues comprizing the WG: Carole, Keith, Steve, Michael and Bill. 2. DISCUSSION 2.1 The purpose of this paper is to address this action by summarizing the results of discussions and proposing consequential changes to ICAO Annex 3 — Meteorological Service for International Air Navigation and associated documents. 2.2 The modus operandi for this working group was to provide a view on a range of potential options, following stakeholder consultation, and then for the rapporteur to propose document changes based on feedback received. 2.3 Specifically, this paper: a) sets out the options for reporting showers; b) summarizes the feedback received from the group; and c) offers a number of proposed changes to ICAO documentation as a result. 2.4 The options for reporting showers 2.4.1 Four possible outcomes were presented: Continue to report SH in manual & automated reports (based on classical interpretation) 2.4.2 This solution would arise from an Industry view that the notification of SH in present weather reports should be based upon the classical interpretation of showers (that is to say precipitation falling from a convective source). 2.4.3 It would be up States then to devise the technology that would allow automated systems to report SH through the detection of specific cloud types. - 3 - AMOFSG/10-SN No. 7 Continue to report SH in manual & automated reports (but with the latter based a new definition of “Intermittent”) 2.4.4 This solution would arise from an Industry view that the reporting of SH in both manual and automated reports should continue. 2.4.5 However, this solution recognizes that some automated systems cannot readily detect specific cloud types and that the algorithms used by automated systems characterize the precipitation according to whether the precipitation is continuous or intermittent (the latter promoting the use of SH in AUTOs). 2.4.6 There would be an understanding, therefore, that the definitions of SH are very different according to whether the information is manually or automatically derived. The latter would need to be defined with additional guidance drafted for Annex 3 and the ICAO Manual on Automatic Meteorological Observing Systems at Aerodromes (Doc 9837). Remove the requirement to report SH in automated reports 2.4.7 This solution would arise from an Industry view that, whilst the characteristic of precipitation remains valuable to know in terms of the convective nature of the sky, there is a recognition that some automated systems cannot readily detect specific cloud types and that the algorithms used by automated systems characterize the precipitation according to whether the precipitation is continuous or intermittent (the latter promoting the use of SH). 2.4.8 On that basis, the definition of SH as intermittent precipitation in automated reports (rather than the classical definition) does not provide value and so should be removed from automated reports entirely. Remove the requirement to report SH entirely 2.4.9 This solution would arise from an Industry view that the characteristic of precipitation is unimportant for any reports (irrespective of whether they are manually or automatically derived), and that the reporting of SH is a means to an end to identify the presence of significant convective cloud (convective cloud being adequately accounted for by the use of TCU and CB). 2.4.10 The result would be the removal of SH from Annex 3 entirely. 3. DISCUSSION 3.1 Feedback received from the group 3.1.1 There was little support for the removal of SH entirely. SH can occur without the reported presence of TCU or CB, and feedback from stakeholders confirmed the value of seeing precipitation differentiated between convective and dynamic type, when augmented by human observation. 3.1.2 Maintaining a classical interpretation of SH in automated reports was not well supported. Reporting showers in automated reports based on algorithms that take account of the convective nature of the sky is not widely adopted. Though the use of remote sensing (radar reflectivity, satellite imagery, etc.) can assist in the determination of convective precipitation, most automated systems rely on the AMOFSG/10-SN No. 7 - 4 - intermittent nature of the precipitation. To change this definition to account for the presence of convective clouds would require widespread and costly changes to the algorithms used and may not generate significant improvement in detection of showers. It should be an inspirational target for States to achieve this, and that States with the existing remote sensing capability should be encouraged to utilize this to report showers from a classical interpretation. 3.1.3 To construct a new definition for “intermittent” accepts that the classical definition of showers is not applicable for automated reports and this received some support. “Intermittent” would require a definition from ICAO and widespread amendments to ICAO, World Meteorological Organization (WMO) and State documentation. There was some support for moving towards this solution in the longer term through an approach that defined showers in automated reports strictly in terms of rapid changes in intensity and the removal of an explicit connection with convection, though it was considered it would take time to achieve consensus on how this should be achieved. 3.1.4 The removal of the requirement to report SH in automated reports received widespread support. 3.1.5 Views expressed from stakeholders in New Zealand noted the limited value of knowing whether the precipitation in automated reports is showery or not, given the information available from cloud reports and that merely knowing the nature of the precipitation (rain, snow, hail, etc.) is the most important consideration. 3.1.6 Further, the United States has recognized the limited values of reporting showers in automated reports for the past 20 years. 3.1.7 Consultation of Canadian stakeholders also favoured an approach that removed SH from automated reports. 3.1.8 The United Kingdom supports the removal of SH from automated reports on the basis that the definition of showers cannot be realistically applied and that the reporting of intermittent precipitation offers little additional operational benefits. 3.1.9 There was, therefore, widespread support for the removal of SH from automated reports, and the group recommend changes to documentation to reflect this. That noted, it should be appreciated that for States in a position to do s o, showers should be reported in automated reports based on t he classical interpretation of showers. 3.1.10 The feedback received is available at Appendix A. 3.2 Resultant proposed changes to ICAO documentation 3.2.1 ICAO Annex 3: New para: Recommendation.— In automated local routine and special reports and METAR and SPECI when showers (SH) cannot be reported based upon a definition taking account of the presence of convective cloud, the precipitation should not be characterised by SH. 3.2.2 ICAO Manual of Aeronautical Meteorological Practice (Doc 8896): - 5 - AMOFSG/10-SN No. 7 Table 2-6. Characteristics of present weather phenomena Shower SH Used to report showers of rain “SHRA”, snow “SHSN”, ice pellets “SHPL”, hail “SHGR”, small hail and/or snow pellets “SHGS”, or combinations thereof, for example, “SHRASN”. In METAR, showers observed in the vicinity of the aerodrome should be reported as “VCSH” without qualification regarding type or intensity of precipitation. SH in automated reports should not be reported unless based upon an algorithm taking account of the presence of convective cloud. 4. CONCLUSION 4.1 The group was unanimous in its support for Option 3 (the removal of the requirement to report SH in automated reports).