AMOFSG/9-SN No. 32 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 5.2: Forecasts THE FUTURE OF THE TAF (Presented by Bill Maynard) SUMMARY This study notes seeks comments regarding the long term future of the TAF, identifies emerging issues, and proposes some options on how it could be potentially changed to better meet some user requirements. 1. INTRODUCTION 1.1 Notwithstanding that the Meteorological Services in the Terminal Area (MSTA) continues to progress it is probable that the aerodrome forecast (in meteorological code form) (TAF) will continue to be used as a primary flight planning tool for the foreseeable future and certainly for the next 3 or more amendment cycles to Annex 3 Meteorological Service for International Air Navigation. It is therefore important to consider whether the current standards and recommendations for the aerodrome forecast (in meteorological code form) (TAF), within Annex 3, are and will remain suitable. 2. DISCUSSION 2.1 It is notable that the MSTA initiative is anticipated to provide an alternative, and superior, flight planning tool for most applications. However, there remains uncertainty regarding whether the MSTA will supersede all current uses of the TAF and, in particular, it s regulatory applications. (6 pages) AMOFSG.9.SN.032.5.en.doc
AMOFSG/9-SN No. 32-2 - 2.2 The TAF is currently well integrated into regulatory frameworks that exist in many States and resultant operational procedures by air carriers that, in part, ensure the carriage of sufficient fuel to proceed to an alternate aerodrome should a diversion become necessary. This requirement is also reflected in Annex 6 Operation of Aircraft, Part I International Commercial Air Transport Aeroplanes section 4.3.4. 2.3 The combination of the aforementioned factors leads to a high probability that the TAF will continue to be necessary and be widely used for at least the next decade. In fact, there is no known specific plan to replace it other than a general expectation that the MSTA will eventually prove to be a better and more widely used alternative. 2.4 If the assumption that the TAF will continue to be used into the long term is valid, it is appropriate to have an ongoing review of the applicable Annex 3 Standards and Recommended Practices. If there is an opportunity to improve the TAF then there is, over this period of time, a reasonable expectation of net benefits. 2.5 TAF Recommendation in Chapter 6 2.5.1 The standards and recommended practices for the TAF are found in Chapter 6 and Appendix 5 of Annex 3. Within Chapter 6, there remains only a single recommendation among the standards for the TAF which is in section 6.2 of in Part I of Annex 3, as follows: 6.2.6 Recommendation. The period of validity of a routine TAF should be not less than 6 hours nor more than 30 hours; the period of validity should be determined by regional air navigation agreement. Routine TAF valid for less than 12 hours should be issued every 3 hours and those valid for 12 to 30 hours should be issued every 6 hours. 2.5.2 Upon closer inspection, it may be noted that this recommendation could be divided into 2 or more subcomponents which might, individually, be appropriate for to upgrading to standards. One option would be as follows: 6.2.x The period of validity of a routine TAF shall, except where limited by aerodrome or observation program hours of operation, be not less than 6 hours nor more than 30 hours. 6.2.x Routine TAF for any aerodrome shall be issued at least every six hours. 6.2.x Recommendation - The period of validity and frequency of issue for routine TAF should be determined by regional air navigation agreement. 2.5.3 There may be a few cases where, due to aerodrome restrictions, the final routine issue of a TAF for the day may not allow for a period of validity of at least 6 hours. The addition of the words, except where limited by aerodrome of observation program hours of operation, are proposed to address this.
- 3 - AMOFSG/9-SN No. 32 2.5.4 The discussion in Study Note 12, to this meeting, should also be noted in the review of the above proposal which would set a standard requiring routine TAF to be issued at least every six hours but would, by that wording, also enable more frequent issues. 2.6 Alignment of SPECI and TAF amendment criteria 2.6.1 It should be recalled that the SPECI criteria in the Annex are now divided into standards and recommended practices. Key among the SPECI standards are requirements to issue SPECI in response to hazardous and / or operationally significant present weather conditions. Specifically, (Appendix 5, paragraph 2.3.2 refers, in part): d) when the onset, cessation or change in intensity of any of the following weather phenomena occurs: freezing precipitation moderate or heavy precipitation (including showers thereof) thunderstorm (with precipitation); e) when the onset or cessation of any of the following weather phenomena occurs: freezing fog thunderstorm (without precipitation); f) when the amount of a cloud layer below 450 m (1 500 ft) changes: 1) from SCT or less to BKN or OVC; or 2) from BKN or OVC to SCT or less. 2.6.2 The comparable standards for TAF amendment for these phenomena are as follows (Appendix 5 refers): 1.3.1 The criteria used for the inclusion of change groups in TAF or for the amendment of TAF shall be based on any of the following weather phenomena or combinations thereof being forecast to begin or end or change in intensity: freezing precipitation moderate or heavy precipitation (including showers thereof) thunderstorm (with precipitation) duststorm sandstorm. 2.6.3 It is notable that there are some standards for SPECI criteria for which there are no corresponding TAF amendment standards. A comparison of these criteria reveals that: a) although it is a standard to amend the TAF in response to a duststorm or sandstorm, there is no corresponding SPECI standards for these phenomena, b) although there is a SPECI standard related to thunderstorms (without precipitation), there is no corresponding TAF amendment standard for this hazardous phenomena, and c) although there is a SPECI standard for freezing fog, there is no corresponding TAF amendment standard. 2.6.4 It is possible to have SPECI criteria that provide information beyond that required for a TAF amendment. However, it is essential that any TAF amendment criteria be a subset of the SPECI criteria. Also, it would seem reasonable that all potentially hazardous phenomena requiring a SPECI should also warrant a TAF amendment.
AMOFSG/9-SN No. 32-4 - 2.6.5 Based upon the above, the following amendment to Appendix 5 of Annex 3 is suggested. 1.3.1 The criteria used for the inclusion of change groups in TAF or for the amendment of TAF shall be based on any of the following weather phenomena or combinations thereof being forecast to begin or end or, as applicable, change in intensity: freezing precipitation freezing fog moderate or heavy precipitation (including showers thereof) thunderstorm (with or without precipitation) duststorm sandstorm. 2.6.6 Consideration could also be given to introducing a TAF amendment standard in response to one or more SPECI criteria related to defined amounts of cloud based at specified levels. One possibility may to use current recommended values such as for below 450 m (1 500 ft) while another option may be the values corresponding, in some States, to the limits for no alternate IFR use of an aerodrome. 2.6.7 A separate Study Note will propose corresponding revisions to the SPECI standards. 2.7 Use of temporal and probability qualifiers in TAF 2.7.1 The differences in some State practices regarding the use of temporal qualifiers in the TAF may be such that notification through State Difference to operators is justified and should be required through the development of related standards. 2.7.2 In some States, forecasters are encouraged to make full use of all temporal and probability groups necessary to provide a description of the range of weather conditions that can be expected. Alternatively, other State practices restrict or even prohibit the use of one or more of TEMPO, BECMG or PROB especially during the first 6 to 9 hours of a TAF. 2.7.3 As a result of this difference in practice the TAF may provide significantly different information, to operators, regarding hazards such as thunderstorms. In one State the forecaster may indicate a 30 or 40 per cent probability of thunderstorms while the exact same weather situation may not include any mention of thunderstorms in the TAF for a neighbouring State. 2.7.4 It is also notable that the provisions related to change groups for TREND forecasts are all in the form of standards already. The case for having similar standards in place for the TAF is at least as strong, and likely stronger, given the near global implementation and regulatory applications of the TAF. 2.7.5 In order to address the above, and ensure that all such major differences in practice are effectively communicated to operators, it is proposed that paragraphs 1.3.3 to 1.3.6 be upgraded to standards on their current basis. 2.7.6 Consideration could also be given to upgrading section 1.4 of Appendix 5 to Annex 3 to a standard although it is notable that some significant differences of opinion and practice exist regarding the combined use of PROB and TEMPO that should be resolved beforehand.
- 5 - AMOFSG/9-SN No. 32 2.8 Possible Enhanced TAF 2.8.1 As a bridge until the MSTA products are widely available, and for any long term ongoing use, consideration could be given to allowing additional flexibility in the use of the TAF code form. In particular, the use of PROB could be expanded to provide additional information regarding the probability of forecasted events. Among those possibilities could be: a) to allow PROB to be used for any probability that is a multiple of ten (other than zero or one hundred), or b) to use PROB in tandem with all TEMPO, other than certain events, to quantify the probability of occurrence. 2.8.2 Consideration could also be given to specifying events that will be of a brief or intermittent nature in the TAF code. The general re-introduction of the obsolete INTER code as a modifier may be one means to achieve this goal. 2.9 Automated TAF 2.9.1 It is anticipated that automated TAF production will be proposed in some States in the foreseeable future. In some cases, computer generated TAF are already being used as the starting point or first guess for human forecasters to reduce workload and allow forecasters to focus upon rapidly changing weather and / or high profile locations. 2.9.2 It could be argued that a TAF generated and directly transmitted without human intervention is only a means of production and does not require any specific text in Annex 3. 2.9.3 On the other hand the introduction of direct to user from computer TAF may raise sufficient concerns that some specific enabling text that also addresses potential concerns should be considered for the Annex. That is, the TAF could be considered to be more than a simple product but rather an overall service with associated aspects that are impracticable to deliver without human intervention. 2.9.4 Some of the concerns that may be raised include the following: backup and emergency capability, adequacy of weather watch, ability to integrate multiple sources of real-time information (i.e. remote sensing, public and pilot reports etc.) and demonstrability of equivalent (or better) performance to humans over the full range of weather conditions and adequate accounting for all relevant local effects. 2.9.5 There may also be a systematic difference between automated and human produced TAF. The human TAF may typically add value relative to the probability of weather being within defined decision making criteria (i.e. above or below landing limits) whereas the automated TAF may simply forecast the most probable weather condition regardless of its impact on users. 2.9.6 It could be proposed that the following text be added to Annex 3 to address automated TAF; x.x For States in a position to do so, in consultation with users, the TAF production and dissemination may be fully automated for some or all aerodromes. Such TAF shall be equivalent to, or better than, human produced TAF, as demonstrated by suitable statistical analysis for a full range of expected weather conditions at each of the aerodromes of interest and by an expert
AMOFSG/9-SN No. 32-6 - review of the algorithms. They shall include the ability to maintain a continuous weather watch and the prompt issuance of all types of amendments and shall include real-time monitoring of output, be responsive to real-time concerns regarding their content and have suitable backup and emergency capabilities. 2.9.7 The word prompt, which is also used in an existing standard in paragraph 6.2.4, may require further definition. It is proposed that prompt should correspond to an achieved performance of 90 per cent of required amendment being received within 20 minutes of the time of the corresponding change in weather conditions. This performance target has been demonstrated to be achievable in Canada. Alternatively, these criteria could be applied only for deteriorating weather conditions. 2.9.8 It may also be prudent to consider a phased in approach to any automated TAF. At first, they could be used during non-operational hours and during high confidence weather situations and for lower profile aerodromes for which real time human intervention / review remains possible. Furthermore, these TAF could be labelled as AUTO TAF until they are widely implemented and used by operators.. 2.9.9 The World Meteorological Organization (WMO) could also be invited to review related guidance material and revise it to address the probable development of automated TAF. 3. CONCLUSION 3.1.1 The long term future of the TAF should be considered and options such as those suggested in this paper should be evaluated for possible implementation. In particular, routine review to ensure consistency with other parts of Annex 3 should be ongoing. 4. ACTION BY THE AMOFSG 4.1.1 Comment on this paper and, in particular, the assessment of the long term requirement for the TAF and each of the amendment proposals to Annex 3 suggested in paragraphs 2.5.2, 2.6.5, 2.7.5, 2.8 and 2.9.5. Further consideration could also be given to inviting the WMO to provide further guidance material specific to any, or all, of these issues. END