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.
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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):
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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). There were a number of reasons for this, primarily:
a) a recognition that the classical definition of showers is not widely achievable for use in automated reports; and
b) a perceived lack of value to report SH based on a definition of ‘intermittent precipitation’.
4.2 Reporting of showers defined as intermittent precipitation offer little values to users. The reporting of showers based upon an algorithm taking account of the presence of convective cloud should be an inspirational target for States to achieve. For States to achieve this significant investment is required, and it is for Industry to consider the value of this. For this reason the reporting of SH in automated reports should be permitted for those States in a position to do so.
4.3 This proposal requires a number of changes to ICAO documentation based on these conclusions, which have been proposed in this paper.
5. ACTION BY THE GROUP
5.1 The group is invited to:
a) note the contents of this paper;
b) consider the removal of the requirement to report showers in automated reports; and
c) consider the consequential amendments to ICAO documentation.
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AMOFSG/10-SN No. 7 Appendix
APPENDIX
SELECTED COMMENTS RECEIVED FROM GROUP PARTICIPANTS
1. United States, from Steve
The US supports option #3. A review of our national practices shows that we require the reporting of showers for only those airports with manual reports, which are very few. The U.S. has successfully used automated stations to provide weather observations for nearly 20 years. Those automated stations serving commercial domestic and international operations can be augmented with human oversight if it is deemed that the level of service is required. In review of our existing service standards, human oversight does not include the reporting of showers and therefore, we would support the removal of reporting of showers (SH) from Annex 3 for automated reports.
2. Canada, from Bill
The reporting of showers in Canada is largely covered by the following excerpt from our domestic standard (The Manual of Surface Weather Observations, as referenced from Canadian Aviation Regulations 804.01 c), in part):
1. 3.11.2 Showers. Showery precipitation falls from cumuliform cloud and can be further identified by one or more of the following features:
2. a) Showers often (but not always) begin and end abruptly.
3. b) Showers usually occur in periods of short duration, perhaps 15 minutes or so, but they may last much longer.
4. c) Usually there are rapid fluctuations in the intensity of the precipitation.
5. d) There is a noticeable brightening of the sky between showers.
3.11.2.1 Certain types of precipitation, such as Snow Pellets and Hail, always occur as showery precipitation. Rain, Snow and Ice Pellets, can occur either with showery or non-showery characteristics; to indicate a showery character, the terms “Rain showers”, “Snow showers” and “Ice pellet showers” are used.
(Note that Canada has retained the option to report SHPL and no change is anticipated to this practice.)
For ease of reference; in capsule form Option 1 – status quo Option 2 – status quo with new SH definition Option 3 – remove SH requirement from AUTO Option 4 – remove SH entirely
The results of my internal consultation favoured option #3. If this is not possible then option #1 was the second choice.
AMOFSG/10-SN No. 7
Appendix A-2
Option #2 was perceived to be “another algorithm fix that would over report SH and render its use meaningless”. Option #4 did not receive any support from my stakeholders.
My own personal thoughts were also initially to favour option 3. However, I think that the longer term solution may be in the form of option 2, perhaps if we remove the explicit connection to convective cloud and define it strictly in terms of rapid changes in intensity (which includes starting and stopping). Regrettably, I do not think that we can achieve consensus over how this should be done within the short term so this is not really a practical solution to suggest in the current report.
3. New Zealand, from Keith Regarding the NZ practice for reporting showers in automated reports;
Present weather sensor algorithms used in New Zealand do a reasonable job of identifying shower-type precipitation, but obviously such observations don’t provide definitive evidence of convective cloud. Comparisons between ATC reports (ATIS) and METAR AUTO confirm acceptable agreement on the reporting of showers. However, discussions with users (airlines and GA) indicate that differentiating between showers and intermittent precipitation is not particularly important to them. Simply knowing that there is precipitation (and the type, i.e. rain/drizzle, snow, hail, etc.) is all they really want. Most of them have noted that a combination of a TAF, METAR AUTO and an ATIS report (landing/take-off report) provides the best overall picture rather than simply relying solely on a METAR AUTO. The pilots' MET exams in NZ (PPL, CPL and ATPL) test students on the use of the three components (TAF, METAR AUTO and ATIS) for pre-flight planning.
For manual reports, showers are reported in the normal way, i.e., as observed by the tower controllers who carry out all manual observations in New Zealand. We no longer have dedicated MET observers.
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