Large-scale Circulation Control of the Occurrence of Low-level Turbulence at Hong Kong International Airport

Abstract

This study identifies the atmospheric circulation features that are favorable for the occurrence of low-level turbulence at Hong Kong International Airport [below 1600 feet (around 500 m)]. By using LIDAR data at the airport, turbulence and nonturbulence cases are selected. It is found that the occurrence of turbulence is significantly related to the strength of the southerly wind at 850 hPa over the South China coast. On the other hand, the east–west wind at this height demonstrates a weak relation to the occurrence. This suggests that turbulence is generated by flow passing Lantau Island from the south. The southerly wind also transports moisture from the South China Sea to Hong Kong, reducing local stability. This is favorable for the development of strong turbulence. It is also noted that the strong southerly wind during the occurrence of low-level turbulence is contributed by an anomalous zonal gradient of geopotential in the lower troposphere over the South China Sea. This gradient is caused by the combination of variations at different timescales. These are the passage of synoptic extratropical cyclones and anticyclones and the intraseasonal variation in the western North Pacific subtropical high. The seasonal variation in geopotential east of the Tibetan Plateau leads to a seasonal change in meridional wind, by which the frequency of low-level turbulence is maximized in spring and minimized in autumn.

摘 要

透过激光雷达(LIDAR)在香港国际机场的观测数据, 我们把不同的日子界定为湍流和非湍流事件. 研究结果指出, 低空湍流出现频率与中国南方沿岸低空经向风的强度 (850 hPa) 有显著的关系. 另一方面, 结果表明低空纬向风的强度对低空湍流出现频率影响并不显著. 因此, 低空湍流是当低空南风经过大屿山所生成. 此外, 南风把南海的水汽传至香港. 因此, 减弱了大气的稳定性, 为低空湍流的发展提供了有利环境. 研究同时指出中国南方沿岸低空经向风的强度是受高度场的纬向梯度所影响. 而纬向梯度的变化可分为不同的时间尺度. 研究结果显示温带气旋和向西延伸的副热带高压, 分别导致纬向梯度在天气和季节内的变化. 最后, 青藏高原以东的高度场在季节变更中, 有明显的变化. 这样也会影响高度场的纬向梯度, 从而影响经向风的强度. 因此较多湍流事件发生于秋季, 较少湍流事件发生于春季.