AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

Transcription

1 23/12/09 AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG) EIGHTH MEETING Melbourne, Australia, 15 to 18 February 2010 Agenda Item 5: Observing and forecasting at the aerodrome and in the terminal area 5.2: Aerodrome forecasts CALCULATION OF THE AMOUNT OF SKY COVER (Presented by Bill Maynard) SUMMARY This paper presents a summary of different methods for the determination of the amount of sky coverage. It seeks clarification regarding the underlying requirements. 1. INTRODUCTION 1.1 The Aerodrome Meteorological Observing and Forecasting (AMOF) Study Group was established by the International Civil Aviation Organization (ICAO) Air Navigation Commission (ANC) to assist the meteorology Secretary of the Air Navigation Bureau in reviewing the observation and forecasting standards and recommended practices in Annex 3 Meteorological Service for International Air Navigation. 1.2 This study note seeks to confirm the current interpretation of the provisions of the Annex related to the observation and reported of the amount of sky cover. It also seeks to confirm or refute whether there is an aeronautical requirement for reporting of summation cloud amounts and / or the inclusion of phenomena that partially obscure the sky into the amount of the lowest cloud layer. (5 pages) AMOFSG.8.IP en.doc

2 DISCUSSION 2.1 Annex 3 currently contains standards and recommended practices for the observation and reporting of cloud layers and amounts as shown in Appendix A. 2.2 Although not defined in Annex 3, there is also an ICAO definition of ceiling in Annexes 2 and 6 which should be noted. The height above the ground or water of the base of the lowest layer of cloud below m ( ft) covering more than half the sky. 2.3 Operationally, and regardless of the reporting methodology in use, a ceiling is assumed to be operationally interpreted as the lowest reported occurrence of BKN or OVC or vertical visibility in a meteorological report. 2.4 It is assumed that the accepted international interpretation of the sky cover reporting provisions is such that: a) partially obscuring phenomena are not considered in the determination of the sky condition; and b) each cloud layer is evaluated and reported, independently, subject to the provision that each successive layer that is reported, to a maximum of three, must be of the next higher reportable category except that an additional layer can be included if it consists of significant convective cloud. 2.5 Based upon the previous assumptions the following examples apply in the event of: a) fog which almost, but does not completely, cover the sky (such that it is not appropriate to report a vertical visibility) and through which scattered low base stratus is visible, the sky condition would be reported as SCT0XX; and b) a sky condition that is half covered (4 ocktas) by stratus over which there was a scattered (3 ocktas) stratocumulus deck the reportable sky condition would be SCT. The next higher layers would all be ignored unless one of them, on its own, meets the criteria for a BKN layer. 2.6 A summation amount reporting practice is one where the amount of cloud coverage reported for each successive layer is the sum of all cloud up to and including that cloud layer. For example, if there was a scattered (3 oktas) layer of stratus over which there was a scattered (4 oktas) layer of stratocumulus then the lower layer would be reported as SCT and the second layer would be reported as BKN given that the summation (total) amount of cloud covering the sky up to that level would be more than half. 2.7 The summation amount concept can also be modified to include any other obscuring phenomena in the total, including those which may be only partially obscuring the sky and / or be based at the surface. This may include blowing dust, blowing snow, precipitation and fog.

3 - 3 - AMOFSG/8-IP No It is assumed that, operationally, all cloud layers are considered to be opaque. However, for the sake of completeness, it should be noted that sky condition reporting can also make a distinction between layers which can be seen through (commonly called thin layers) and those which are opaque. 2.9 In summary of the above there are three methods that can be used to report sky condition as viewed from the surface, starting with the presumed current leading interpretation of Annex 3, each layer is reported: a) independently of any other cloud layer beginning with the lowest layer and including up to two additional layers only if they are of a greater reportable amount than the previously reported layer and other obscuring phenomena are disregarded. ( separate layers ); b) with an amount that is inclusive of the total amount of sky covered by cloud at and below that level. Other obscuring phenomena are disregarded. ( summation ); and c) with an amount that is inclusive of the total amount of sky covered by cloud and obscuring phenomena at and below that level, inclusive of surface based obscurations. ( Summation of cloud and partial obscurations ) It should be noted that none of the automated means for assessing sky condition know to the author is currently able to provide reports in response to either definition 2 or 3 above. Although laser ceilometers use a temporal weighting to assess the amount of each layer, versus a spatial weighting done by a human, it is assumed that it is accepted that these are merely different means of assessing and reporting the same phenomena It is also assumed that there is consensus that an additional layer can always be included to report significant convective cloud Appendix B shows some examples of how the meteorological reports may vary using the different methodologies shown in It is further assumed that there is consensus that the TAF must reflect the same methodology as the METAR / SPECI with regard to the description of sky condition. 3. CONCLUSION 3.1 The current standard in paragraph of Annex 3 includes the words as necessary which are open to interpretation. It may be prudent to clarify the factors to be considered in establishing necessity in this context, in order to foster international commonality.

4 Appendix A APPENDIX A ANNEX 3 PROVISIONS RELATED TO CLOUD AMOUNT Clouds 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; Recommendation. In local routine and special reports and in METAR and SPECI: when several layers or masses of cloud of operational significance are observed, their amount and height of cloud base should be reported in increasing order of the height of cloud base, and in accordance with the following criteria: 1) the lowest layer or mass, regardless of amount to be reported as FEW, SCT, BKN or OVC as appropriate; 2) the next layer or mass, covering more than 2/8 to be reported as SCT, BKN or OVC as appropriate; 3) the next higher layer or mass, covering more than 4/8 to be reported as BKN or OVC as appropriate;

5 Appendix B APPENDIX B EXAMPLES OF ALTERNATIVE MEANS OF REPORTING SKY CONDITION Case 1: Morning radiation fog which obscures most (6 oktas) of the sky with layers of stratus and stratocumulus above. The amount for the layer of stratus, by itself, is 2 oktas while the stratocumulus layer covers 3 oktas. Assume that the stratus is a thin layer that can be seen through. i. Separate layers METAR YUDO Z 27003KT 1/4SM FZFG R14/1300FT/N FEW004 SCT020 M03/M03 A2995 Note that if the stratus layer was scattered (SCT) then the stratocumulus would not be reported unless it was broken or overcast. The amount is all that matters. The component of sky obscured by the fog is not counted in determining ceiling. ii. Summation METAR YUDO Z 27003KT 1/4SM FZFG R14/1300FT/N FEW004 BKN020 M03/M03 A2995 iii. Summation including surface based obscurations METAR YUDO Z 27003KT 1/4SM FZFG R14/1300FT/N BKN004 BKN020 M03/M03 A2995 Note: This example includes a second broken layer for illustration purposes. Case 2. As a result of light snow, 1 oktas of the sky is obscured. There are also a two layers of stratus (2 okta and 4 oktas, respectively) with overcast nimbostratus above. i. separate layers METAR YUDO Z 23006KT 1SM -SN FEW002 SCT008 OVC020 M02/M04 A3006 ii. summation METAR YUDO Z 23006KT 1SM -SN FEW002 BKN008 OVC020 M02/M04 A3006 iii. summation including obscurations METAR YUDO Z 23006KT 1SM -SN SCT002 BKN008 OVC020 M02/M04 A3006 END