AMOSSG/6-SN No. 15
3/10/06
AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP
(AMOSSG)
SIXTH MEETING
Exeter, United Kingdom, 17 to 20 October 2006
Agenda Item 5: Aerodrome observation requirements
5.1: Reporting requirements and criteria
REMOVAL OF ARTIFICIAL GUSTS IN
SURFACE WIND MEASUREMENTS AT AERODROMES
(Presented by C.M. Cheng)
SUMMARY
This paper presents the issue of artificial gusts in surface wind measurement at
aerodromes and invites discussion on the need of more guidance on the
provision of representative surface wind measurements at aerodromes.
1.
INTRODUCTION
1.1
At aerodromes where heavy jet aircraft frequently operate, jet efflux and wake vortices
are known to occasionally cause artificial gusts in surface wind measurements. While algorithms to
remove these artificial gusts are being used at some international aerodromes, there is no established
practice in such procedure. This paper presents the issue of artificial gusts in surface wind measurements
and invites discussion on the need of more guidance on the provision of representative surface wind
measurements at aerodromes.
2.
THE ISSUE OF ARTIFICIAL GUSTS
2.1
The occurrence of artificial gusts is closely related to the siting of the wind sensor. When
the wind sensor is closer to the runway, it would be more exposed to artificial gusts from jet efflux and
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wake vortices. If the wind sensor is located farther away from the runway, then its measurement might be
less representative of the wind conditions on the runway.
2.2
Annex 3 — Meteorological Service for International Air Navigation, Appendix 3, 4.1.1.2
recommends that “Representative surface wind observations should be obtained by the use of sensor
appropriately sited”. This annex does not contain explicit provision on how to site a wind sensor relative
to the runway to achieve representative wind measurements. Annex 10 — Aeronautical
Telecommunications does contain specifications on siting of equipment on operational areas but mainly
from the perspective of reducing the hazard to aircraft.
2.3
The Manual of Aeronautical Meteorological Practice (Doc 8896), Table A5-3 advises
that, in respect of surface wind speed and direction measurements, “Siting will be governed by obstacle
limitation surfaces and local prevailing surface wind regime. …. depending on local conditions, it may be
necessary to locate a frangible and lighted mast within the runway strip. …. The site must not be affected
by buildings, etc., or by aircraft operations (e.g. jet efflux during taxiing)”. Part II Chapter 2 of the WMO
Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8) also advises that “If
wind sensors installed on aerodromes are to be representative of the conditions at take-off or landing
areas, then any disturbance or turbulence due to the proximity and passage of the aircraft themselves must
be avoided (false gust indications due to landings and take-offs)”.
2.4
In reality, from the experience in Hong Kong, China and elsewhere, it is indeed very
difficult to site the aerodrome anemometers so that all the above guidance could be followed at the same
time. It is not possible to eliminate the effect of artificial gusts especially at aerodromes in which heavy
jet aircraft frequently operate.
2.5
At the Hong Kong International Airport (HKIA), there are two parallel runways.
To provide representative surface wind measurements along the runway and the touchdown zones, three
anemometers are installed for each runway within the zone “B” in Figure A5-2 of Doc 8896. Two of them
are located near the two touchdown zones and one of them is sited near the mid-point of the runway.
Yet, under suitable wind conditions, artificial gusts were recorded by these anemometers (see example in
Figure 1, below).
Figure 1 Time series of 3-second mean winds, i.e. gusts, recorded by one anemometer at HKIA on
29 August 2006. A few instances of artificial gusts are clearly discernible within a period of 30 minutes.
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2.6
The Hong Kong Observatory implemented an algorithm to automatically identify and
remove artificial gusts. It is based on a Dutch study presented at the WMO Technical Conference on
Meteorological and Environmental Instruments and Methods of Observation (TECO-98) and published
by WMO in Instruments and Observing Methods Reports No. 70 (WMO/TD - No. 877). It was
understood from the Royal Netherlands Meteorological Institute (KNMI) that the Dutch algorithm is used
at the Amsterdam Schiphol Airport to automatically remove artificial gusts from measurements of a
number of anemometers for the ATC wind displays, local routine and special reports and METAR/SPECI
in response to user’s request.
2.7
At HKIA, artificial gusts are identified by the algorithm. These events of artificial gusts
are flagged for the attention of the weather observer who determines, based on flight movement
information and meteorological assessment, whether the identified events are indeed non-meteorological
and should be removed from the reports. On average, 97 per cent of the identified events were removed
before the issuance of the local routine and special reports and METAR/SPECI. There were about 6
observations from the 6 runway anemometers being affected each day. This frequency is expected to
increase with the operation of ultra-large aircraft such as A380. Relocation of the anemometers does not
appear to be a viable option considering the very stringent constraints imposed by the obstacle clearance
requirements, the existing and planned developments in the airport, and the need to provide surface wind
measurements representative of the conditions along the runways and the touchdown zones.
2.8
There are justifications to remove artificial gusts:
a) artificial gusts are non-meteorological in nature. It does not represent the underlying
wind conditions over the runway.
b) jet efflux from an aircraft is short-lived. Depending on the background wind
condition, wake vortices can sustain for a longer period. However, it is intended that
ATC aircraft separation procedures would avoid one aircraft running into the wake
vortex of another. Artificial gust, even if it is caused by wake vortices, should not be
reflected in the weather reports for ATC and pilots.
c) artificial gusts from jet efflux and wake vortex affect only a part of the runway. They
can be considered to be only a local phenomenon. It appears not necessary to include
artificial gusts in METAR/SPECI, which is intended to represent the conditions over
the whole runway or the whole runway complex.
d) once artificial gusts are removed from the 10-minute winds for METAR/SPECI,
there may be a need to also remove artificial gusts from the 2-minute winds for ATC
display and local routine and special reports. If not, there would be inconsistency in
data in that the 2-minute gusts could be greater than the 10-minute gusts.
2.9
As Annex 3 does not yet contain provisions on how to handle artificial gusts, the group is
invited to consider the following issues:
a) should artificial gusts be removed from affected anemometer readings on ATC
displays, in local routine and special reports and METAR/SPECI considering the
different averaging periods and usage?
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b) should automatic removal of artificial gusts be used operationally, given that there
might be a few percent of identified events being incorrectly removed, i.e. they are
genuine gusts? and
c) should such artificial gust removal algorithm be standardized so that instruments
vendors and meteorological authorities could follow?
3.
3.1
ACTION BY THE GROUP
The group is invited to:
a) note the information in this paper; and
b) consider providing more guidance on the provision of representative wind
measurements at aerodromes, in particular the removal of artificial gusts.
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