Investigation of Atmospheric Boundary Layer Wind Structure and Wind Resource Assessment Under Climate Change

大氣邊界層風結構研究及氣候變化下風能資源評估

Student thesis: Doctoral Thesis

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Award date3 Oct 2022

Abstract

Atmospheric boundary layer (ABL) is the lowest part of the atmosphere where most human activities take place. Scientific knowledge of atmospheric motions (i.e., winds) within the ABL is of great significance for many applications, such as design of structures and infrastructures, hazard mitigation of extreme windstorms, and generation of wind power.

However, due to the complicated fluid dynamics and thermodynamics of the ABL, the understanding of winds within the ABL remains limited. In view of this, this dissertation aims to comprehensively investigate the wind structure of the ABL. Two major parts constitute this thesis: the first part investigates the wind and turbulence structures of hazardous windstorms, mainly from a micrometeorological viewpoint (in terms of seconds, minutes, or hours), while the second part analyzes the wind characteristics in relation to wind energy resource, primarily in a climatological manner (in terms of months or years).

In the first part, an investigation of the ABL wind and turbulence structures of tropical cyclones (TCs) over land is firstly conducted. Based on the observations during two record-breaking TCs (Super Typhoons Hato and Mangkhut) from the 356-m-high Shenzhen Meteorological Gradient Tower (SZMGT), mean wind structure, including the wind profile and power law coefficient, as well as turbulence characteristics, consisting of the turbulence intensity, turbulence integral length scale, gust factor, turbulent kinetic energy and budget, momentum flux, drag coefficient, power spectrum, and coherence, are thoroughly analyzed. Vertical eddy diffusivity for momentum and mixing length are also evaluated, which can facilitate the numerical simulation of TCs. Next, mean wind and turbulence structures of four thunderstorm events are investigated with special attention paid to the nonstationarity of the thunderstorm wind speeds. A comparative analysis of TC, monsoon, and thunderstorm wind characteristics is subsequently carried out, and commonalities and differences among the vertical profiles of mean wind speed, turbulence intensity, and turbulence integral length scale for these storm events are explored. Whilst these studies based on SZMGT provide important information for high-rise structure design over land, they cannot serve as references for design of structures over ocean (e.g., offshore wind turbines and oil platforms). To address this, Global Positioning System (GPS) dropsonde observationsare utilized to study the wind profiles of TCs over open ocean. Inflow and outflow properties and their connections with the strengthening and weakening of TCs are examined. Moreover, the variation of gust factor with wind speed during the eyewall passage of Hato over an offshore meteorological station is analyzed, and the phenomenon of decreasing gust factor with increasing wind speed at very high wind speeds over 50 m/s is discovered for the first time.

While the first part focuses on short-term or medium-term windstorm events with timescales of minutes to days, the second part pays more attention to the long-term wind climatology over months or years, so as to facilitate wind energy harvesting and help combat climate change. Based on the joint use of a Doppler wind sodar (sonic detection and ranging) system and a microwave radiometer, wind and thermal characteristics and wind energy resources in a coastal region of Hong Kong are investigated. The probability distribution and variation of atmospheric stability and air density are examined, and their effects on vertical wind shear and wind potential at turbine hub heights are revealed. To assess wind resources over extensive areas, a numerical weather prediction (NWP) model is utilized to simulate the offshore wind fields over the entire territory of Hong Kong. Wind energy related parameters, such as wind power density, capacity factor, annual energy production, and levelized cost of electricity, are estimated, and a high-resolution offshore wind atlas of Hong Kong is presented and discussed. Furthermore, due to the evolving global temperature patterns under climate variability, substantial change may occur in spatial and temporal distributions of wind fields and wind resources. To quantify the climate change impacts, future wind energy resources in Hong Kong are evaluated based on the combined use of 28 global climate models (GCMs) from Coupled Model Intercomparison Project Phase 6 and observations from 6 local meteorological stations. A multi-model and multi-method ensemble approach is proposed to minimize the prediction uncertainties, and a significant increase in summer wind potential and a remarkable decline in autumn wind resource at the end of this century is projected. The outcomes are expected to guide the wind farm siting and turbine selection in Hong Kong, and contribute to mitigating climate change and achieving carbon neutrality.