DOI: 10.7274/28188356
title: Towards Modeling Tropical Cyclone Boundary Layer: From Meteorological Perspective to Wind Engineering Applications

description: Extreme winds in tropical cyclones (TC) are responsible for the considerable loss of civil engineering structures in TC-prone areas. For analyzing and mitigating TC-induced damage, the estimation of TC-induced extreme winds by the Monte Carlo simulation is a primary step in determining the intensity measure in performance-based wind engineering (PBWE). This requires a tropical cyclone boundary layer (TCBL) wind model, which solves for near-surface wind fields from the primitive equations of fluid motion driven by prescribed gradient wind/pressure profiles. This dissertation is thus aimed at utilizing cutting-edge meteorological advances to enhance the TCBL models in the PBWE. Theoretically, the 3D nonlinear TCBL model is the most rigorous, while a family of simplified linear TCBL models exists with various approximations. This dissertation investigates these models from the following perspectives.
(1) The effects of four key factors (algorithm’s order, thermal effects, vertical diffusivity, and surface drag coefficients) on the 3D nonlinear TCBL model are analyzed by the idealized simulation of an ensemble of exhaustive examples based on TC parameters (VMax, VT, and Holland-B). Patterns of various effects are observed by selected typical examples. At the same time, four indicators (differences between maximum surface and overall wind speeds and MAPE (mean average percentage error) of surface and overall wind speeds) are estimated over the ensemble to quantify their effects. The interaction between these effects and with other factors is also analyzed. It is noted that most of these effects center around RMW (radius to maximum winds) and have asymmetric horizontal distribution with both negative and positive portions concurrently. Mostly, the influence of these factors is less than 22%. 
(2) A benchmark dataset has been established for validating TCBL models, which consists of 692 sorties (a combination of input and output data from measurements). Each sortie combines the input of Holland radial profiles given by flight-level reconnaissance data (Flight+) and at least one type of output measurement (surface stations, SFMR, H*Wind). The dropsonde data is also included as the output of vertical profiles. Among these, the 82 sorties associated with all three types of outputs are finally utilized to demonstrate the established benchmark dataset by comparing the simulation results of the 3D nonlinear and linear TCBL models. The measurements are also analyzed to update and examine the underlying models.
(3) The hierarchy of linear models has been revisited, enhanced, and further developed to enable expeditious computation of the 3D nonlinear model. The unified governing equations are examined in terms of various types of gradient wind models (asymmetric and axisymmetric). The height-dependent vertical diffusivity model, asymmetric gradient model, and vertical and radial thermal profiles are introduced into the 3D linear model by deriving semi-analytical solutions. The height-resolving column model is expressed and enhanced by wind-dependent drag coefficients and height-dependent vertical diffusivities. Additional enhancements are introduced to assess the effects of vertical velocity and nonlinear horizontal advection. Selected enhanced linear models are numerically examined by extensive examples and compared to the 2D slab model and the benchmark 3D nonlinear model. It is shown that the height-resolved column model may be an acceptable approximation, provided some restrictions are satisfied. 
(4) The additional asymmetry resulting from the land-sea roughness contrast for land-falling TCs has been analyzed by the 3D nonlinear model. Even when the coastline is remote from the TC center, this distortion in surface wind speeds is discovered near the TC eyewall. A conceptual model is proposed to represent the contrast-induced asymmetry as the summation of the transitional effect and the global distortion effect, which may interact with the translation-induced asymmetry. Extensive numerical examples show that this asymmetry could influence the overall and surface over-ocean wind speeds by 22% and 14%, respectively. A numerical example also demonstrates its influence on the PBWE. 
This systematic research has exhaustively explored the capability and sensitivity of the 3D nonlinear model and a hierarchy of linear models. Accordingly, the enhanced understanding and modeling of TCBL will promise to advance the reliability of the PBWE-enabled wind-resistant design of civil engineering structures.