Terrain-Induced Turbulence Intensity during Tropical Cyclone Passage as Determined from Airborne, Ground-Based, and Remote Sensing Sources

Abstract

ow-level turbulence [rapid headwind fluctuations below 1600 ft (500 m)] poses potential safety hazards to landing/departing aircraft and is capable of disrupting air traffic. Timely, accurate alerts of low-level turbulence require reliable determination of its intensity, quantified by an internationally adopted aircraft-independent metric [cube root of the eddy dissipation rate (EDR1/3)], which cannot be directly measured but only inferred from observational data. In this paper, a large-scale survey of terrain-induced low-level turbulence intensity around the Hong Kong International Airport (HKIA) during tropical cyclone (TC) passage is presented, utilizing EDR1/3 values determined from multiple remote sensing and in situ sources, including the scanning Doppler lidar, the terminal Doppler weather radar (TDWR), a high-resolution anemometer, and the operational Windshear and Turbulence Warning System (WTWS) at HKIA. Over a 18 720-min study period spanning five TC cases between 2010 and 2012, ground-based EDR1/3 was computed using a variety of first-principle and empirical methods and was shown to demonstrate a strong linear correlation with airborne values determined from quick access recorder (QAR) data of over 350 landing flights. Spatiotemporal features as experienced on board aircraft were also reproduced by the lidar- and TDWR-derived profiles. Positive skill could be extracted from threshold-based alerting of low-level turbulence events by considering each ground-based source individually, while a combination of lidar and TDWR alerts demonstrated enhanced performance and hence the potential value of complementary surveillance under clear-air and in rain conditions. This study serves to establish the ability of ground-based instruments in correlating with airborne EDR1/3 and the performance of threshold-based alerting algorithms for turbulence events, contributing toward improvements in turbulence-alerting techniques for the aviation community.