advanced

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.