WEATHER ON WINGS
Newsletter for the Aviation Community
Hong Kong Observatory
Issue No. 11 - May 1999
Hong Kong and Mainland Signed a
Memorandum on Cooperation in Aviation Meteorological Services
The Hong Kong Observatory (HKO) and the Air Traffic Management
Bureau (ATMB) of the Civil Aviation Administration of China (CAAC) signed
a memorandum on long term technical cooperation in aviation meteorological
services on 21 April 1999.
The memorandum was signed at the Hong Kong Observatory
by the Director of HKO, Dr Lam Hung-kwan, and the Director of the Meteorological
Division of ATMB, CAAC, Mr. Li Huibin.
The scope of the cooperation covered many areas of aviation
meteorological services including meteorological communication, exchange
of information, meteorological instrumentation and training of personnel.
In the exchange of meteorological information, the two
sides would hold regular discussions to explore new communication links
and to improve communication methods so as to increase the data throughput
and the types of information exchanged. In this way, aviation forecasters
and air traffic controllers of both sides can better grasp the weather
situation, and provide better services to aircraft and airlines.
In respect of meteorological instrumentation, both parties
will exchange technical information on meteorological equipment which will
facilitate improvement of the meteorological facilities of airports in
the Mainland and in Hong Kong.
Under the memorandum, exchange visits will be arranged
more frequently through which staff members of both sides can learn from
each other in technology and operation. This will help raise their professional
standard. Depending on operational needs, the two sides will also undertake
joint technical projects.
The signing of the memorandum has laid a sound foundation
for further cooperation between HKO and ATMB of CAAC in future.
Signing of the memorandum
Eleventh Liaison Group Meeting
The Eleventh Meeting of the Liaison Group on Aviation
Meteorological Services was held at the Conference Hall of the HKO on 15
December 1998. Sixteen members participated. Dr. H.K. Lam said that the
transition of the aviation meteorological service from Kai Tak to Chek
Lap Kok was a great success. HKO could now focus on enhancing the services
to various users. He stressed the importance of the Liaison Group and urged
members to put forward views on the meteorological services and new requirements.
The Eleventh Liaison Group Meeting
Aviation Meteorological Services
TAFs and METARs are provided to airline operators for
pre-flight planning and in-flight re-planning under centralized operational
control and ETOPS flights. TAFs and METARs not routinely received from
AFTN are extracted from the SADIS broadcast and put on the Meteorological
Information Dissemination System (MIDS) for retrieval by airlines. In addition,
the new low level significant weather charts for the Middle East Asia area
are included in flight document for ETOPS flights to the Middle East.
56 Kbps access to the MIDS was launched in March 1999.
To further enhance security, only registered users can access the MIDS
with user caller ID authentication.
Transition to Year 2000
In order to ensure the proper functioning of all the meteorological
systems during the transition to Year 2000, modification to both the hardware
and software of some of the meteorological systems was made in early 1999.
System-wide tests against all published high-risk dates were subsequently
conducted in March and April. We are delighted to announce that all the
systems are now validated to be Year 2000 compliant.
In order to assist airlines to verify that they can download
flight documents from the MIDS at Year 2000 transition, a Year 2000 MIDS
test site with sample data for the Year 2000 transition period has been
set up. MIDS users who have not yet tested their PC in downloading flight
documents from the MIDS during Year 2000 roll-over are encouraged to contact
the Airport Meteorological Office (AMO) to arrange such tests.
A set of manually operated meteorological equipment has
been acquired for the AMO as the ultimate contingent. Should all automatic
observing systems fail, essential meteorological elements will be measured
manually on site and reported to AMO for onward transmission to air traffic
control every half an hour via Trunk Mobile Radio (TMR) or other available
radio communication means such as mobile phones.
To cater for failure of essential communications, e.g.
AFTN on Year 2000 critical dates, a contingency plan is being drawn up
to obtain OPMET data via alternate paths such as WAFS satellite broadcasts,
Internet and facsimile polling. This forms part of the overall Year 2000
contingency plan of the Civil Aviation Department of Hong Kong which in
turn will be part of the ICAO Regional Year 2000 Contingency Plan for the
Asia/Pacific Region.
In case public telephone networks become inoperative on
entering Year 2000, airline operators can come to the AMO or Meteorological
Briefing Area (MBA) inside the Passenger Terminal Building to collect flight
documents. The briefing station in the MBA is connected to our MIDS equipment
at the AMO via private cables which are date-insensitive.
Extension of the Aerodrome Meteorological
Observing System for the Northern Runway
The extension of the Aerodrome Meteorological Observing
System (AMOS) for the northern runway (AMOS-N) was installed in September
last year. It was subsequently put to trial operation. So far the operation
of AMOS-N has been satisfactory. The northern runway was put to emergency
use since 12 April 1999.
Windshear Events on 14 December
1998
A number of pilots reported encountering severe windshear
at around 1,500 ft on departure on 14 December 1998. In particular, the
pilot departing at around 08UTC reported that the airspeed varied by -15
kt to +25 kt around 3/4 and 2 nm from the end of the runway. However, no
reports of severe windshear were received for arrival flights.
The northerly monsoon strengthened on that day. The Strong
Monsoon Signal was hoisted at noon and remained in force throughout the
day. Surface winds at Chek Lap Kok were northerlies around 10 kt throughout
the morning and afternoon. However, winds on the mountain tops of Lantau,
namely Nei Lak Shan and Yi Tung Shan were generally gale (40 - 50 kt) in
the early afternoon. The Sha Lo Wan profiler captured a low level jet at
around 2,000 ft.
Fig. 1 Sha Lo Wan profiler data
The shear experienced by the departing aircraft was due
to vertical shear as the plane passed through the low level jet. Based
on the winds measured on-board a flight departing 8 minutes earlier, there
was a rapid increase in head wind component, from 13 kt at 1,420 ft to
39 kt at 3,100 ft.
Fig. 2 Variation of Head Wind with Height
The on-board wind measurement showed that wind was 10
kt or less near the surface up to around 800 ft. Wind then increased rapidly
and was strongest at around 3,350 ft, reaching 50 kt. This corresponded
very well with the observation from the Sha Lo Wan profiler. The wind recorded
by the aircraft dropped sharply to around 15 kt at 3,800 ft, which was
much sharper than that detected by the Sha Lo Wan profiler.
The presence of the low level jet was also well captured
by the Terminal Doppler Weather Radar (TDWR) at 6 deg scan. At 0740UTC,
around the time the flight departed, the core of the low level jet (with
a radial speed of 40 kt or above and is coloured in pink to red) extended
from around 800 m to 1,200 m, in fairly good agreement with the Sha Lo
Wan profiler.
Fig. 3 TDWR Radial Velocity (6 deg scan)
Departing flights were climbing at very steep angles,
9.6 deg for the one shown above. As a result, the aircraft transit the
low level jet very close to the end of the runway and the change in head
wind was very rapid. Meanwhile for arrival flights, since they were descending
at a lesser angle (3 deg), they experienced the shear further away from
the runway end and more gradually.
Fig. 4 Variation of Head Wind for Arriving
Flights
Based on the Sha Lo Wan profiler data at 0740UTC, the
vertical shear at around 1,100 ft was close to 8 kt per 100 ft. Although
this was lower than the 8 kt per 100 ft which has been used in the Windshear
Warning System at the old Kai Tak Airport, the increase in head wind was
nonetheless significant due to the steep climb angle.
The Windshear and Turbulence Warning System (WTWS) was
not able to warn the windshear associated with the elevated low level jet
since the system used the lowest elevation scan (0.6 deg)
of
the TDWR to calculate the windshear, which did not intercept the jet stream
within the warning area.
Manual Windshear Warnings
The HKO is looking into severe windshear cases reported.
Post-flight reports are very important in identifying the location and
severity of windshear. We have been receiving post-flight windshear and
turbulence reports since airport opening. However, the number of reports
has declined significantly in the past few months. A new and simplified
pilot report form has been developed to reduce the time required to complete
it. Airlines are requested to encourage their pilots to file post-flight
report after each flight.
Apart from convective storms and terrain, low level jets
and land/sea breezes have been found to cause significant windshear at
Chek Lap Kok. These types of windshear are not what the WTWS had been previously
designed to detect. Furthermore, although the TDWR is already exceptionally
sensitive in being able to detect clear air returns a lot of times, when
the weather is fine, e.g. sea breeze cases, the return can be just too
weak for the TDWR to determine the wind field. To cover such scenarios,
forecasting rules have been developed for our aviation forecasters to issue
windshear warnings manually. These rules are also being coded into the
WTWS for automatic warning.
Telephone and Fax Numbers
Telephone:
Officer-in-charge AMO (852) 2910 6300,
(852) 2922 5805
Duty Forecaster AMO (852) 2910 6920,
(852) 2922 5806
Enquiry on flight documents (852) 2910 6921
Fax: (852) 2910 0080
Correspondence
We welcome comments and suggestions on our aviation meteorological
services. Please send them to the following address:
Director of the Hong Kong
Observatory
134A Nathan Road
Kowloon, Hong Kong
(Attn.: Officer-in-charge,
Airport Meteorological Office)
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