WAFSOPSG/4-WP/18
International Civil Aviation Organization
14/12/07
WORKING PAPER
WORLD AREA FORECAST SYSTEM OPERATIONS GROUP (WAFSOPSG)
FOURTH MEETING
Cairo, Egypt, 26 to 28 February 2008
Agenda Item 5: Operation of the WAFS
5.4: Other issues related to the operation and implementation of the WAFS
HEIGHT OF THE TROPOPAUSE AND MAXIMUM WIND IN WAFS GRIDDED FORECASTS
(Presented by WAFC Provider States)
SUMMARY
During 2007, questions were raised by a number of WAFS users regarding the
height co-ordinates used by the WAFCs in gridded binary (GRIB 1) forecasts
of tropopause and maximum wind. This paper presents the Annex 3
provisions, examines the current WAFC practices, and makes
recommendations to correct the identified differences that currently exist in
the provision of these data fields.
1.
INTRODUCTION
1.1
Chapter 3 to Annex 3 — Meteorological Service for International Air Navigation relates
to the World Area Forecast System (WAFS) and Meteorological Offices, with Appendix 2 providing
appropriate technical specifications. Amendment 74 to Annex 3 (effective November 2007) introduced
editorial revisions to the specification of WAFS upper air data, which consequently led to queries from a
number of WAFS users.
1.2
(7 pages)
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Definitions related to height can be found in Chapter 1 of Annex 3. Of relevance here are:
Altitude
The vertical distance of a level, a point or an object considered as a
point, measured from mean sea level (MSL)
Flight level
A surface of constant atmospheric pressure which is related to a specific
pressure datum, 1 013.2 hectopascals (hPa), and is separated from other
such surfaces by specific pressure intervals.
WAFSOPSG/4-WP/18
-2Note 1.— A pressure type altimeter calibrated in accordance with the
Standard Atmosphere:
a) when set to a QNH altimeter setting, will indicate altitude;
b) when set to a QFE altimeter setting, will indicate height above
the QFE reference datum;
c) when set to a pressure of 1 013.2 hPa, may be used to indicate
flight levels.
Note 2.— The terms “height” and “altitude”, used in Note 1, indicate
altimetric rather than geometric heights and altitudes.
Height
The vertical distance of a level, a point or an object considered as a
point, measured from a specified datum.
Level
A generic term relating to the vertical position of an aircraft in flight and
meaning variously height, altitude or flight level.
1.3
Appendix A to this working paper presents excerpts from Chapter 3 and Appendix 2 of
the final draft of Amendment 74 to Annex 3. The excerpts illustrate that the term “height” has, in many
instances, been replaced by the term “flight level” under the amendment. The final text of Chapter 3,
notably paragraph 3.2.1, is provided below for information:
“3.2.1 A Contracting State, having accepted the responsibility for providing a WAFC within the
framework of the world area forecast system, shall arrange for that centre:
a) to prepare for grid points for all required levels global forecasts of:
1) upper wind;
2) upper-air temperature and humidity;
3) geopotential altitude of flight levels;
4) flight level and temperature of tropopause; and
5) direction, speed and flight level of maximum wind;”
1.4
During 2007, the WAFSOPSG member from IATA, and a number of WAFS workstation
suppliers, raised questions regarding the height co-ordinates used by the WAFCs in fields of tropopause
height and maximum wind level. This paper examines the current situation and makes recommendations
to correct the identified differences that currently exist in the provision of these data fields.
2.
DISCUSSION
2.1
The requirement for the provision of grid point data (gridded binary) for the height of the
tropopause and maximum wind level is expressed in flight levels as per paragraph 3.2.1 of Annex 3
outlined above.
2.2
Detailed investigations by the WAFCs have shown that WAFC London is producing
GRIB 1 forecasts of tropopause and maximum wind in flight levels, but WAFC Washington is producing
GRIB 1 forecasts of tropopause and maximum wind as an altitude above mean sea level. Further
confusion is introduced by the fact that WAFC London is using an incorrect parameter number for these
gridded forecasts of 7 (height AMSL) when it should be using a parameter number of 5 (“ICAO Height”).
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2.3
In addition, there appears to be an artificial limit of FL530 assigned to tropopause heights
that cannot be exceeded within the gridded data issued by WAFC London. This is probably so that
automated flight planning systems do not try to interpolate data above the top level of standard WAFS
winds and temperatures – which is currently at FL530 (100 hPa). Meteorologically, the limit at FL530 is
unsound, since in reality the tropical tropopause may be as high as FL580 in some circumstances.
2.4
The above shortcomings may produce some inaccuracies in temperature forecasts
interpolated from the tropopause temperature data and to wind forecasts interpolated using the maximum
wind data, if the wrong height co-ordinates have been followed. The difference between the altitude and
the flight level of the tropopause or maximum wind level can be as much as 3000 feet, in air which is
significantly warmer or colder than ISA conditions. For example, average 300 hPa heights range from
8300 m over the Arctic in January to 9750 m over southern Asia in July (from 27200 to 32000 feet).
Appendix B to the working paper provides an illustration of the variation of the 300 hPa height across the
Northern Hemisphere on a selected date/time in October 2007. Temperature data at the height of the
Tropopause and wind data at the Maximum wind level reported in metres above mean sea level will
therefore be inconsistent with the winds and temperatures reported at the standard pressure (Flight) levels.
2.5
The approved WAFS/SADIS workstation vendors were consulted for their opinions on
the impact of making a change to the upper limit of the tropopause height within the WAFS gridded
binary data. Whilst all respondents agreed that the risk of significant impact on their workstations (and
thus users) was considered low, some commented there may be complications for more specialised flight
planning systems which combine multiple areas and layers. If the tropopause height field was not given a
definitive upper bound (such as FL600), such systems may not adequately interpret the model data.
3.
RECOMMENDATIONS
3.1
In order to effect a change to the height co-ordinates used by WAFC Washington, correct
the parameter number used by WAFC London, and to change the upper limit of the tropopause height,
changes will be necessary to the GRIB encoders employed by both WAFCs. More importantly, end user
systems will probably require updated software to cope with the changes. The WAFCs therefore advise
strongly against making any changes to the current GRIB 1 formatted data, but to incorporate these
changes during the transition to GRIB 2 formatted data.
3.2
In this regard, the group may wish to formulate the following conclusion:
Conclusion 4/..― Height coordinate of the tropopause and
maximum wind in WAFS gridded forecasts
That, the WAFC Provider States be invited to:
a)
use flight levels as the height co-ordinates for the
tropopause and maximum wind;
b)
increase the maximum permitted flight level of the
tropopause to flight level 600; and
c)
implement the changes referred to in a) and b) above
simultaneously with the introduction of WAFS
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WAFSOPSG/4-WP/18
forecasts in the GRIB 2 code form.
Note. — Units used are geopotential metres referenced to the
ICAO Standard Atmosphere rounded to the nearest flight level.
4.
4.1
ACTION BY THE WAFSOPSG
The WAFSOPSG is invited to:
a) note the information in this paper; and
b) decide on the draft conclusion. proposed for the group’s consideration.
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Appendix A
APPENDIX A
Excerpts from the final draft of Amendment 74 to ICAO Annex 3 – Meteorological Service for
International Air Navigation.
WAFSOPSG/4-WP/18
Appendix A
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Appendix B
APPENDIX B
In the 300 hPa northern hemisphere chart below, the 300 hPa height can be seen to vary from 8500 m
over the North Pole to 9720 m over the tropics. This equates to a range of 27,886 Feet to 31,889 Feet.
The ICAO Standard height for 300 hPa is 30,065 Feet.
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