Lidar determination of the mixed layer height and entrainment zone thickness over Hong Kong

Abstract

In this thesis we present the results of a study aimed at better understanding of the advantages and limitations of lidar determination of the ML height and the EZ thickness in the PBL over Hong Kong. To this end first we have reviewed the background theory underlying the lidar study of the ML height and the EZ thickness, namely, the lidar equation and the entrainment equation. Secondly, we have carried out extensive experimental measurements, involving several case studies under various conditions, in order to gain the better understanding sought. In this phase of the project, we have collected aerosol data in the PBL in a series of observations carried out using the City University of Hong Kong lidar system. Eight cases were selected for the evening and early morning transition study of the ML height and the EZ thickness in different weather regime. The data collected have been analyzed using the visual inspection method and the Steyn et al. (1999) technique. Various factors bearing on the observational procedures as well as noise and induced error have been taken into account. The results of this study indicate that under high pressure system dominating the southeast China, the EZ thickness increases with time for the evening transition, but decreases with time for the midnight to early morning transition. The Steyn et al. (1999) technique and the computer program has been found applicable for the cases of three-layer structure and produced very good results. Alternative approaches and possible modifications have to be considered for multi-layer systems. Additional lidar studies concerning diurnal variation of the ML height and correlation with the site temperature during a cold front passage are also presented in this thesis.