WAFSOPSG/4-WP/14
International Civil Aviation Organization
12/12/07
WORKING PAPER
WORLD AREA FORECAST SYSTEM OPERATIONS GROUP (WAFSOPSG)
FOURTH MEETING
Cairo, Egypt, 26 to 28 February 2008
Agenda Item 6: Development of the WAFS
6.3: Improved WAFS forecasts for icing, turbulence and convective clouds in GRIB2
code form
GENERAL GUIDELINES FOR THE OPERATIONAL USE OF WAFS FORECASTS OF ICING,
TURBULENCE AND CONVECTIVE CLOUDS IN GRIB2 FORM
(Presented by WAFC Provider States)
SUMMARY
This paper provides general guidelines on the usage of gridded forecasts of
icing, turbulence and convective clouds.
1.
INTRODUCTION
1.1
The group will recall that it formulated Conclusion 3/13 regarding the implementation of
gridded forecasts for icing (ICE), turbulence (TURB) and convective clouds (CB). The WAFS Provider
States were also tasked with the development of initial guidelines or models for the visualization of these
grids as well as general guidelines on their usage in lieu of WAFS Significant Weather (SIGWX)
forecasts.
1.2
Gridded ICE/TURB/CB forecasts produced by WAFC London have been available for
operational trials on the SADIS FTP service since October 2006. Similar grids produced by WAFC
Washington are expected to be available on the SADIS FTP service by the WAFSOPSG/4 Meeting.
Pictorial representations of the WAFC Washington and WAFC London gridded data are available on the
Internet at
http://www.emc.ncep.noaa.gov/mmb/gtrojan/WAFAVN/gifs/ or
http://aviationweather.gov/testbed/globalgrids
1.3
The group will note that the WAFS 5-year plan (WP/6 refers) indicates that gridded
ICE/TURB/CB forecasts are to become operational in November 2010, to coincide with Amendment 75
to Annex 3 – Meteorological Service for International Air Navigation. It has been proposed that these
(8 pages)
116093855
WAFSOPSG/4-WP/14
-2-
forecasts co-exist with meteorologist-produced SIGWX forecasts for a 3-year transition period, from
November 2010 to November 2013, to coincide with Amendment 76 to Annex 3, as well as offer users
time to accommodate the new products in GRIB 2 format.
1.4
products.
The purpose of this paper is to provide general guidelines on the use of these automated
2.
DISCUSSION
2.1
Increased Temporal Resolution of SIGWX Forecasts
2.1.1
The group will agree that one of the greatest advantage of gridded ICE/TURB/CB
forecasts will be the increase in temporal resolution of SIGWX forecasts. This increase provides users
with current forecasts which are valid for operations at all timescales (T+6 through T+36); something
which is not currently provided by today’s WAFS SIGWX forecasts.
2.1.2
Gridded ICE/TURB/CB forecasts will be valid at three hour time intervals, from T+6, 9,
12, 15, 18, 21, 24, 27, 30, 33 and 36 hours from model initialization time (DT). Meteorologist-produced
SIGWX forecasts are a fixed time prognostic chart, T+24 hours, and have a recommended use for flights
from three hours before SIGWX forecast validity time until three hours after SIGWX forecast validity
time. Users needing SIGWX forecasts for short-haul flights occurring prior to the use time of most
recently issued SIGWX forecast, often use SIGWX forecast issued six hours or more earlier and derived
from previous computer model runs. Long-haul flights with flight times exceeding 6 and 12 hours desire
SIGWX forecasts for two and three validity times, which require the use of old and new SIGWX
forecasts. No valid SIGWX forecasts are available for the later portions of very long-haul flights (for
example an 18-hour flight from New York to Singapore). In addition, since the meteorologist-produced
SIGWX forecasts are not amendable, the use of “old” SIGWX forecasts does not provide the user with
latest forecast information offered by the newer model run.
2.1.3
WAFS upper wind and air temperature forecasts are available in the same time steps as
gridded ICE/TURB/CB forecasts and updated every 6 hours. Each new update of upper wind and air
temperature cancels previous issuances. The Gridded ICE/TURB/CB forecasts will provide users with
corresponding upper wind and air temperature forecasts, whereas using older SIGWX in conjunction with
new wind and temperature may not harmonize.
2.2
Differences Between Forecasts
2.2.1
Differences between gridded ICE/TURB/CB forecasts and forecaster-produced SIGWX
forecasts will only occur at one time step, T+24 hrs. Forecaster-produced WAFS SIGWX forecasts are
not produced for valid times T+6, 9, 12, 15, 18, 21, 27, 30, 33, and 36 hours, thus there are no differences
with using gridded ICE/TURB/CB forecasts at these time steps. Differences at T+24 hours will often
occur, largely due to differences in product content, which are described in the following paragraphs.
2.2.2
Subtle differences exist today between WAFC-London forecaster produced SIGWX
forecasts and WAFC-Washington forecaster produced SIGWX forecasts. These differences exist
primarily due to different models and the subjective forecaster evaluation of the model’s data, in addition
to differences within each WAFC with forecaster experience. Objective gridded ICE/TURB/CB forecasts
by the two WAFCs based on common forecast algorithms greatly minimizes the differences between
WAFS SIGWX forecasts.
-3-
WAFSOPSG/4-WP/14
2.2.3
Gridded CB forecasts contain all convective clouds producing precipitation, whereas
meteorologist-produced SIGWX contain only areas of isolated or occasional embedded cumulonimbus or
frequent cumulonimbus cloud coverage.
2.2.4
Gridded TURB forecasts contain all turbulence, including clear air turbulence (CAT), incloud turbulence, and convective turbulence for the entire globe. Forecaster-produced SIGWX contain
only areas of jet-stream CAT over the globe, as well as in cloud turbulence for portions of the North
Atlantic, Europe, Middle East and South Asia.
2.2.5
Gridded ICE forecasts contain all icing potential, including convective icing, for the
entire globe whereas forecaster-produced SIGWX contain only non-convective icing for portions of the
North Atlantic, Europe, Middle East and South Asia.
2.2.6
Gridded ICE/TURB/CB forecasts will be provided on the WAFS grid scale. National
Meteorological Services may choose to utilize gridded ICE/TURB/CB forecasts for their production of
non-WAFS products, but they may find that meso-scale and regional models may provide finer resolution
for their non-WAFS aviation forecasts. Therefore, it is recognized that differences may exist between
WAFS forecasts and non-WAFS forecasts.
2.2.7
Gridded ICE/TURB/CB forecasts will be included in the routine post-processed output
from the WAFC global models; and therefore be available to users approximately 3 hours prior to the
meteorologist-produced T+24 hour SIGWX forecasts.
2.2.8
More information regarding the content of gridded ICE/TURB/CB forecasts is included
in Appendix B, which was taken in part from the Preliminary assessment of Gridded fields of Icing,
Turbulence and Cb clouds presented the WAFCs at the October 2007 IATA Task Force Meeting.
3.
RECOMMENDATION
3.1
The group may wish to note that the WAFC Provider States have produced draft
guidelines related to the use of the gridded forecasts of turbulence, icing and cumulonimbus clouds
(Appendix A refers). The group is invited to review the guidelines and formulate the following decision
and conclusion:
Decision 4/.. — Guidelines of the use of gridded forecasts of
turbulence, icing and cumulonimbus clouds
That, the generic guidelines for using gridded forecasts of
turbulence, icing and cumulonimbus clouds contained in
Appendix1….. to this report be endorsed by the group.
1
Guidelines are at Appendix A to this working paper.
-4-
WAFSOPSG/4-WP/14
Conclusion 4/... — Development of detailed guidelines for the
use of gridded WAFS forecasts of turbulence,
icing and cumulonimbus clouds
That, the WAFC Provider States be invited to produce a general
guidance document on the intended use of the gridded WAFS
forecasts of turbulence, icing and cumulonimbus clouds, in time of
the WAFSOPSG/5 Meeting.
Note. — The guidance to be developed will be
placed on the WAFSOPSG website.
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 decision and conclusion proposed for the group’s consideration.
————————
WAFSOPSG/4-WP/14
Appendix A
APPENDIX A
GUIDELINES FOR THE USE OF GRIDDED FORECASTS OF TURBULENCE, ICING AND CB
CLOUDS
1)
Gridded ICE/TURB/CB forecasts will be stand-alone products, similar to today’s WAFS
upper wind and air temperature forecasts;
2)
All gridded ICE/TURB/CB forecasts should be considered as flight planning information
for flight documentation;
3)
Gridded ICE/TURB/CB forecasts valid at T+6 hours are not advisories or warnings, such
as SIGMET or AIRMET, and are not to be considered as a replacement or amendment to
any such advisories or warnings;
4)
Gridded ICE/TURB/CB forecasts will be available in 3-hour time steps, from T+6 to
T+36. The recommended period of use will be 1 hour 30 minutes before and after each
time step. E.g., a 0000 UTC T+15-hour gridded ICE/TURB/CB forecast valid at 1500
UTC would be appropriately used for flight information from 1330 UTC to 1630 UTC.
Flight documentation for 1700 UTC would be provided in the 0000 UTC T+18-hour
gridded ICE/TURB/CB forecast; and
5)
With respect to flight documentation, newly issued gridded ICE/TURB/CB forecast
automatically updates and cancels the corresponding forecast issued six hours earlier.
Note. — Users may continue to use forecaster-produced SIGWX for flight
documentation, but should be advised that only the most recently issued T+24 hour SIGWX was produced
using the latest computer model run, and that SIGWX forecasts issued six hours earlier were produced
from an earlier computer model run.
————————
WAFSOPSG/4-WP/14
Appendix B
APPENDIX B
ADDITIONAL INFORMATION ON GRIDDED WAFS FORECASTS
(Extracted, in part, from the Preliminary assessment of Gridded fields of Icing, Turbulence and Cb clouds
presented the WAFCs at the October 2007 IATA Task Force Meeting, with minor additions and
revisions).
1.
CONVECTIVE CLOUDS
1.1
The WAFCs are providing 5 different fields describing the presence of convective
clouds: namely the horizontal extent of Cb cloud within each grid box; the height of the base and top of
the convective cloud; and the height of the base and top of any layered cloud that the convective cloud is
embedded within.
1.2
The WAFCs have made the following observations when analysing the convective
gridded products:
a) The algorithm that has been used by WAFC London to identify convective cloud has
been found to perform poorly at high latitudes, and occasionally poorly at midlatitudes. WAFC London will adopt the enhanced algorithm in use by WAFC
Washington – which is based on convective precipitation rate and appears to offer a
much more realistic convective cloud forecast at all latitudes.
b) Tropical convection is still poorly diagnosed by the models and there are occasions
when significant areas of convection are poorly represented by the gridded products –
even around tropical cyclones. These deficiencies can be considered to include:
missing significant convection areas altogether; incorrect placement of significant
convection areas; and over- or under-estimating the horizontal and vertical extent of
significant convection.
c) Significantly more detail is provided by the gridded products in the convective cloud
structure, than compared to the existing human generated SIGWX products. This
makes the gridded products potentially more useful for providing regional detail.
However, it is not known how accurate these forecasts are in practice, especially at
longer time frames.
d) The models can not discriminate clearly between embedded Cb and non-embedded
Cb. It is also fair to say that there are some occasions when forecasters would be
unable to make this judgment too. Embedded Cb within mid-latitude frontal systems
and tropical depressions seems to be reasonably well represented by the gridded
output, but in other convective systems, we believe that reliance on the ‘embd Cb’
parameter could be misleading and provide pilots with false information.
Recommendation: that all Cb, whether embedded or not, be used in the generation of
products based on the gridded Cb fields.
WAFSOPSG/4-WP/14
B-2
Appendix B
2.
TURBULENCE
2.1
The WAFCs are providing a number of turbulence gridded fields, namely:- CAT
potential (wind shear and mountain wave induced) fields at 5 levels (400, 300, 250, 200 and 150hPa). In
cloud turbulence centered on 5 levels (700, 600, 500, 400, 300 hPa).
2.2
The WAFCs have made the following observations when analysing the turbulence
gridded products:
a) Far greater detail in the CAT potential fields is realised compared to the forecaster
derived products. As with Cb, it is not known how accurate these small scale features
are. The WAFCs can say that if too low a threshold is set then the fields become very
cluttered.
b) An in-cloud turbulence field is also provided. Although this is designed to replicate
information currently provided on SWM charts, it is not clear how it might be used in
practice. The fields remain difficult to evaluate. (conclusion from the IATA TF
meeting is that a combined turbulence product containing CAT and in-cloud
turbulence will be produced)
3.
ICING
3.1
Icing potential is centered on 6 levels at 850 through 300hpa. These evaluate the Icing
potential of the layer and not the probability of icing within the layer. Results are good, and show strong
correlation with weather systems at their respective latitude/altitude. As with Cb and CAT, small scale
details appear in the gridded data. However, the accuracy of these small scale features is unknown.
(Conclusion from the IATA TF meeting was to add icing potential for the layer centred at 850 hPa).
— END —