Lagrangian Coherent Structures and Turbulence Detection near the Hong Kong International Airport based on LIDAR Measurements

Terrain-disrupted turbulent airflow brings windshear to aircraft at the Hong

Kong International Airport (HKIA). Better knowledge of the structures of

the turbulent airflow would be very useful to detection of the windshear and

understanding of the effects of the shear on aircraft. A methodology is

developed to identify Lagrangian coherent structures (LCS) from the

turbulent airflow sampled by Doppler velocity measurements of LIght

Detection And Ranging (LIDAR) systems at HKIA. As a demonstration of the

methodology, strong southerly flow associated with a tropical cyclone in

April 2008 is analyzed. In this methodology, a variational method is first

applied to the conical scans of the radial velocity from the LIDAR to derive

the 2D wind field. Lagrangian airflow analysis is then used to identify the

LCS in the airflow as revealed in the 2D wind data. The Lagrangian flow

field is integrated backward and forward in times to determine the updraft

and downdraft of the flow respectively, and the results are compared with

the vertical scans of the LIDAR which have not been used in the Lagrangian

flow analysis. It is found that the updraft and downdraft identified from

the Lagrangian flow analysis of the LIDAR conical scans are generally

consistent with the airflow convergence and divergence as analyzed from the

LIDAR vertical scans. The Lagrangian flow analysis presented in the study

provides a way to infer the vertical air motion from the conical scan data

of the LIDAR. This could bring a significant progress in LIDAR based

windshear detection as both the vertical and horizontal components of the

air motion account for the windshear effects to aircraft, but so far, the

vertical component could not be measured directly by the LIDAR or other

ground-based weather sensors.