INTERNATIONAL AIRWAYS VOLCANO WATCH OPERATIONS GROUP (IAVWOPSG)
Paris, France, 15 to 19 September 2008
Agenda Item 5: Operation of the IAVWOPSG
5.2: Improvement of the format and content of all volcanic ash related messages
HOW TO DEFINE MOVEMENT OF VOLCANIC ASH IN A GRAPHICAL SIGMET
(Presented by United States)
This paper recommends that the movement of volcanic ash should be included in a
Graphical SIGMET product. The movement should be annotated in a text box on the
graphical product itself. Use of standard ICAO contractions should be used to define the
1.1 In accordance with ICAO Annex 3, Meteorological Watch Offices (MWO) if they have
the capability are to provide a graphical SIGMET for volcanic ash. The graphical SIGMET has to provide
the same information that is included in the text VA SIGMET (WV). Under current practices a graphical
WV is to be provided in the WMO BUFR coded format.
1.2 Amendment 74 to Annex 3 introduced an example of a graphical WV but the example
did not provide any guidance on the movement of the ash. At the International Airways Volcano Watch
Operations Group/3 meeting, Conclusion 3/16 the Secretariat was invited to evaluate the feasibility of
how to depict movement on a graphical WV. In the discussion at the meeting it was the opinion of the
group that adding an arrow would cause clutter. However, as noted above, the text WV and the graphical
WV are to contain the same information. The purpose of this paper is to address the issues related to
providing a graphical WV and a recommendation on how to accomplish this.
2.1 MWO have the responsibility to provide WV for their Flight Information Region (FIR).
In addition to the WV, those MWO that have the capability are to provide a graphical WV. The provision
of a graphical WV has to be consistent with the text WV. Amendment 74 to Annex 3 provides an
example of a graphical WV but there is no reference to ash movement. In the text WV a MWO is
required to include the direction of the ash movement.
2.2 The METLINK Study Group was faced with a similar issue when tasked to develop a
prototype example of a graphical Tropical Cyclone SIGMET (WC). However, the depiction of ash
movement for a graphical WV is more complicated than what was agreed for a graphical WC for a 6 hour
smear of the ash hazard.
2.3 An ash cloud can easily penetrate multiple flight levels. Major eruptions are known to
reach flight levels greater than 45000 ft. Ash when penetrating multiple flight levels is effected by wind
shear where the ash is transported in different directions sometimes as much as 180 degrees. Further
complicating this is, that under current practices it is understood that the ash cloud goes from surface to
the top of the flight level where it is observed. In reality the ash is transported and dispersed in multiple
directions as the ash begins to fall out of the cloud. Transport and dispersion models such as the
U.S.Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model do a fairly good job in
projecting the transport of the ash cloud. The same applies to the PUFF model which is used by the
Anchorage Volcanic Ash Advisory Center in addition to the HYSPLT. While these models and other
models developed by other VAACs, provide the meteorologist with trajectory of the ash cloud, the
meteorologist is still required to take this information and describe the ash hazard in a SIGMET for a 6-
hour time smear not necessarily discrete snap shots which can shown through an interactive Web based
interface of the projected movement of the ash cloud.
2.4 The SIGMET requires the meteorologist to assimilate all of these variables and then
describe complex phenomena in a general manner. For the most part it works quite well in that a trained
aviation meteorologist can take the meteorological information available to him along with the output of
numerical models to provide a time smear of the ash cloud. As part of that function the meteorologist also
provides a statement on the general movement of the ash cloud. For a simple polygon or a situation
where the ash is generally confined to a limited area and is not complicated, this process works well.
2.5 However, when viewing the example as provided in Appendix 1 to Annex 3 Figure SVA
one can see that the ash cloud shown is encountering wind shear and is being transported in different
directions for different flight levels. Putting a simple arrow on anyone of these boxes is no easy task in
that it will add clutter to the graphic and requires some type of symbol to associate the arrow with the
correct box. This same situation becomes even more complicated when there are overlaying boxes for
different flight levels. This underlying principal was the reason that the U.S. advocated the need for a
four panel chart for the VAG so the user can easily discern the location of the ash cloud at discrete time
intervals while also obtaining a sense of movement.
2.6 It can be agreed that volcanic ash once dispersed into the atmosphere will take on many
different configurations depending on wind shear and the winds at different flight levels. However, there
is still a need to show the general direction of the ash movement in the graphical SIGMET.
2.7 Therefore it is proposed that the most practical means to do this is to include a text box
for each polygon that states the general movement of the ash. Note that each polygon as shown in the
example in Appendix 1 of Annex 3 uses a different line type to identify the specific area of where the ash
is situated for a specific flight level. Use of the same line type in a legend with a text message that states
the general ash movement would be easily understood by the user without adding clutter to the graphical
WV. The text box can also be included in the BUFR format so users who elect to ingest BUFR into their
flight planning system or display would have this same information.
2.8 To ensure there is no confusion on the meaning of movement it is also advised that the
language used in the text box be plain and simple. In the past there has been confusion over the
methodology used in defining ash movement. Ash movement in a SIGMET is understood to be used as
“moving to” or “moving towards” whereas the winds will always be described as “from” a particular
direction. (e.g. NW WIND 10-15 KTS. ASH PLUME MVG SE 10-15 KTS.) In other words, this states
that the winds are from the northwest at 10 to 15 knots and the ash plume is moving towards the southeast
at 10 to 15 knots.
3.1 It is recommended that the IAVWOPSG adopt the proposed conclusion as formulated
Conclusion 4/xx — Depiction of Ash Movement in a
Graphical VA SIGMET
That the Secretariat invite WMO to consider the proposal to
show movement by:
a) movement in a graphical SIGMET is understood to be
moving to or towards a direction;
b) movement in a graphical VA SIGMET be stated in plain
English in a text box for each polygon that is shown in the
SIGMET with associated line type; and
c) the example provided in Appendix A be used as a model for
WMO to consider for inclusion in Appendix 1 of Annex 3.
4. ACTION BY THE IAVWOPSG
4.1 The IAVWOPSG is invited to:
a) note the information in this paper; and
b) decide on the draft conclusion proposed for the group’s consideration.
THIS IS AN EXAMPLE OF A GRAPHICAL SIGMET WHICH INCLUDES MOVEMENT OF THE
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