Low Level Wind Effect at Airports

What is low level wind effect?
Low Level Wind Effect refers to significant airflow disruption due to buildings and other man-made structures close to the runway, leading to turbulence and possibly windshear under certain wind conditions.

Why is there low level wind effect?
As the air flows over a building, depending on the background wind speed and direction, the stability of the atmosphere and the size of the building, the air may be blocked, flow over or around the building. This is similar to the occurrence of waves and vortices downstream of a stone in a river.

If the building is rather close to the flight path and it is sufficiently large, it may bring about rapid fluctuations of the wind that are perceived to be wind changes (headwind/crosswind changes) and/or turbulence by the pilots.

How will low level wind effect affect takeoff and landing?
When the low level wind effect causes the crosswind component to fluctuate, pilots may experience control difficulties during take off and landing. Under such conditions, directional control would be more challenging on a wet runway.

Where are the possible locations of low level wind effect at the Hong Kong International Airport (HKIA)?
The hot spots of low-level wind effect are downwind of major buildings. As there is the possibility of encountering building-induced turbulence and windshear, pilots should be aware when landing on:

Similarly, there is the possibility of encountering building-induced turbulence and windshear when landing on:

Pilots should be also aware when taking off from HKIA RWY 07C in northwesterly/northerly/northeasterly winds with a background speed of about 15 knots or more of the possibility of building-induced turbulence and windshear over the runway.

How are pilots warned of the low level wind effect at HKIA?
Possibility of significant low level wind effects at HKIA have been promulgated in the Hong Kong Aeronautical Information Publication (AIP). In addition, windshear warnings may be issued to pilots by the forecasters of the Hong Kong Observatory.

How to detect low level wind effect?
The low level wind effects occur over a very small area and have high temporal and spatial variations. The Observatory developed alerting technology based on the short-range LIDAR (SRL), and installed a unit of SRL close to the Centre Runway near to AsiaWorld-Expo. With its finer and more frequent scans, the SRL can help capture turbulence features, which are of smaller spatial and temporal scales, induced by buildings nearby. The SRL has commenced operational use since the middle of 2019 as an enhancement of the Observatory’s Windshear and Turbulence Warning System. In addition, the Observatory installed a number of additional SRL units on the airport and its vicinity to further the study of building induced turbulence as well as aircraft wake turbulence.

Short-range LIDAR & its wind measurement imagery (inset)

How to study the low level wind effect at HKIA?
For sustainable development of the airport, new buildings and infrastructures will be constructed inside and around HKIA. To evaluate whether the new buildings and infrastructures would lead to low level wind effect, their dimensions and distances from the runway would be studied. If they are found to be large and/or close to the runway, further wind study would need to be conducted by the building owner at the planning stage using numerical simulations before the structures are built. The numerical model so employed has to be calibrated, i.e. its results are found to be proven by comparison with wind tunnel measurements, pilot reports and the available meteorological observations. It is used to simulate the airflow disturbances over the runway when the winds blow over and around the buildings and infrastructures under consideration. If necessary, the existing buildings around the new building would need to be included in the simulation because of possible interaction of the airflow disturbances arising from different buildings.

How to assess and minimize the low level wind effect of new buildings at HKIA?
Based on the experience of overseas airports, it was established that crosswind changes of 7 knots or more in short intervals in a background crosswind of 25 knots could cause difficulties in landing.

Results of the numerical simulations would be compared against this threshold. If the 7-knot criterion is violated, i.e. the crosswind change is more than 7 knots over short distance along the flight path/runway, an extended criteria on the gradient of wind change (2.5 knots per 30 metre) will be examined. Failing the extended criteria, mitigation measures would need to be considered to minimize the effect of the new building on the wind, such as changing the building’s dimensions, appearance, and provision of warnings. The results and possible ways to mitigate the effect of the new structure will be discussed at the Windshear and High Impact Weather Panel which coordinates the work for more detailed understanding of the low level wind conditions at HKIA and to enhance air traffic safety based on the findings of the low level wind conditions. If pilots encounter low level wind effect, please report the event to ATC or fax / email a pilot report to the Observatory.

How was the criterion established?
The criterion, namely crosswind change of 7 knots or more in short intervals in a background crosswind of 25 knots (commonly called the “7-knot criterion”), was established based on the pioneering work of Schiphol airport in Amsterdam, the Netherlands. The matter of low level wind effect was first brought up back in 1995, when in a short period, a number of pilots reported turbulent gusts during approach and rough landings on a particular runway at Schiphol including three missed approaches on one day. Investigations were then conducted on these cases and it was suggested that an engine test-run facility, which was constructed in 1992 near the runway, was the cause of the issue. Subsequent wind tunnel study by National Aerospace Laboratory (NLR) discovered trailing vortices and wake downwind of the test-run facility. The measurements were used in simulated approaches and identified that the wake played a major role in these cases. In 2006, a larger-scale study was conducted by NLR which involved pilots in the assessment. The study re-affirmed the validity of the 7-knots criterion. Based on the experience of Amsterdam Schiphol Airport, the Aerodrome Meteorological Observation and Forecast Study Group (AMOFSG) of International Civil Aviation Organization (ICAO) has included the “7-knot criterion” in the Doc 8896 (Manual of Aeronautical Meteorological Practice) to assist States in the planning of buildings at aerodromes.

Low Level Wind Study by Organisations
Organisations embarking on studying the low level wind effect of proposed new buildings or infrastructures at the airport shall contact the Hong Kong Observatory beforehand.

Further information
Aeronautical Information Publication (AIP):
https://www.ais.gov.hk/eaip_20241128/2024-11-28-000000/html/index-en-US.html

Hong Kong Observatory Web Site:
http://www.weather.gov.hk/en/aviat/amt/amsmain.htm