One of the first program tasks was to perform a meteorological review and analysis of historical Hong Kong data. An initial report included a review of scientific theory on airflow around complex terrain, analysis and identification of the conditions which could cause terrain–induced windshear and turbulence near Chek lap Kok, numerical experiments aimed at gaining additional insight into conditions that produce windshear caused by terrain, and an estimate of the timing and location of significant windshear at the new airport.
That review indicated that the primary parameters for determining the nature of the airflow include wind direction and speed, stability, and the presence of critical levels. These parameters were further studied by performing small–scale modeling simulations using the Clark model of a wide range of atmospheric phenomena such as downslope winds, gravity waves and wave amplification, which are often associated with terrain–induced wind flow perturbations. Preliminary results indicated that both mechanical and gravity–wave processes contribute to windshear and turbulence and confirmed that the intensity and location of the turbulence are sensitive, among other things, to wind direction, speed and stability.
A study of test flight results indicated that terrain–induced windshear and turbulence was not related to a single weather phenomenon and should be expected at any time of year. The atmosphere during the turbulence events was variable, although wind speed appeared to have a dominant role in determining intensity. Using 10 years of data, it was found that significant episodes of terrain–induced windshear and turbulence could last from a period of several hours up to a number of days since they were governed by long–term atmospheric motions. Principal phenomena likely to affect aircraft operations at Chek Lap Kok include crosswinds, longitudinal windshear, large wind changes, turbulence, updrafts and downdrafts. It was unclear which condition, if any, would dominate.
Using the knowledge gained from preparing the first meteorological report and from other sources, WITI designed a field experiment to understand fine–scale wind flow in the vicinity of Chek Lap Kok. The basic objectives of the experiment, which was conducted between March 1994 and September 1995, were to quantify the frequency and severity of turbulence and windshear; define more clearly the meteorological conditions under which significant terrain–induced windshear and turbulence occur; validate airflow predictions made by the small–scale model; determine the elements necessary to develop the windshear warning system; and collect verification data.
The major observational platforms used in the field experiment included a King Air research aircraft operated by the U.S. National Center for Atmospheric Research, a scanning Doppler light detection and ranging (lidar) device, integrated sounding system, wind profiler and a network of surface weather stations. Digital flight data from Cathay Pacific B–747–400s operating at Hong Kong also were collected.
Principal findings of the second meteorological report generally supported the conclusions of previous studies. It indicated that moderate to severe terrain–induced turbulence occurs in the vicinity of Chek Lap Kok and that it is more frequent than terrain–induced windshear. Turbulence is found almost exclusively in a well–defined region in the wake of the surrounding terrain. Ambient wind speed determines the magnitude of this turbulence, whereas ambient wind direction governs location. Other factors, such as stability, affect the wind flow response but are secondary.
Both mechanical and gravity–wave processes appear to be important in the dynamics and the net response of both processes appears to be quasi–linear and similar. There is no indication of resonance or unusual responses. Significant terrain–induced windshear and turbulence will occur in episodes that are typically several days in duration and separated by several weeks.
The wealth of scientific information provided by the field study was used in the design of the WTWS system. Coupled with feedback from controllers, pilots and aviation meteorologists, this information was used to develop an operational concept for the new alerting system.
WTWS graphical display
Designers developed the WTWS products to enhance the safety, capacity and efficiency of operations at Chek Lap Kok by automatically providing pilots with concise windshear and turbulence alerts. The system also was designed to provide air traffic managers and supervisors with information to aid effective decision–making and to present high–resolution, real–time meteorological data and forecast guidance to forecasters.
The WTWS integrates data from various sensors and sources, including anemometers, Doppler weather radar, Doppler wind profilers, numerical weather prediction models and an array of global weather observations. Indirectly, it receives data from the ICAO world area forecast system (WAFS) and the World Meteorological Organization (WMO) global telecommunications system.
To enhance flexibility, the system was designed to run on commercially available Unix platforms. Modular system software, written in the C and Fortran programming languages, was designed to conform to international standards, allowing WTWS to run on vendor–neutral platforms. To heighten reliability, a redundant array of independent computers environment was employed. This ensures that all critical system processes are running somewhere on a network of WTWS computers. The capability to refine or tune system algorithms after, as well as before, installation is an important feature because optimization can be achieved only after it is used in an operational environment. The system also includes provisions for accommodation of new types of data, such as real–time aircraft and lidar data, after initial deployment.
An advanced version of the mesoscale model (MM5) developed by Penn State University and NCAR is used to predict atmospheric conditions around the new airport. The MM5 modelling system was adapted to provide real–time, short–range prediction of the mesoscale atmospheric conditions conducive to terrain–induced windshear and turbulence.
Although the MM5 produces weather guidance for the entire Hong Kong region, airport–specific forecasts are generated with a post-processing algorithm. Similar to model output statistics (MOS) techniques commonly used at major forecast centers, this algorithm provides 12–hour forecasts with 30–minute resolution of wind and turbulence at Chek Lap Kok.
The WTWS algorithms, many of which utilize an analysis technique known as 'fuzzy logic', produce turbulence and windshear products based on sensor data. This technique makes use of disparate data types and keeps important information throughout the decision process, maximizing algorithm performance. Sensor inputs include automatic weather stations, TDWR, and an aerodrome meteorological observing system – and each is used by specific windshear and turbulence detection algorithms. The weather product algorithms are used to produce gridded information, which is integrated using decision algorithms. By employing such a scheme, a new turbulence or wind shear algorithm can be added to the system by tailoring its output to a standardized format.