Introduction

This Oxford handbook on non-synoptic wind systems is an outlook of the state of knowledge of various aspects of these wind systems and their impacts on our natural and build environment. During the last two decades, it has become clear that these types of winds dominate in terms of damage in some geographical areas; at the same time, they are different from the large-scale synoptic winds for which the knowledge matured. As opposed to the synoptic winds, the non-synoptic ones are localized in both space and time, three dimensional in nature while having similar intensities. The handbook explores the particularities of this type of wind in terms of climatology, surface layer, and aerodynamic and structural impacts on buildings, structures, and natural habitat. It also addresses the implications on risk analysis, engineering guidelines and codes, socioeconomic aspects, and insurance policies. The handbook comes at the moment when the state of knowledge in this area has evolved but is not yet mature. Therefore, it provides the opportunity to inform and trigger debate.

Concluding Summary

This chapter summarizes the book's study on non-synoptic wind storms (NSWSs). The book covers aspects related the general vulnerability to NSWSs in terms of (1) incidence, including the flow field and intensity and the frequency and occurrence of these storms; and (2) exposure, including preparedness for NSWSs. In doing so, it presents the state of the art regarding full-scale data acquisition and analysis, mesoscale and microscale numerical modeling, physical simulations, structural analysis, risk modeling, building codes implementation, and insurance analysis. For each of these aspects, the presentation aims at being informative, reviewing a large palette of approaches and presenting their advantages and limitations. It also stresses the need for future research.

Analytical and Semi-empirical Models of Tornadoes and Downbursts

Analytical and semi-empirical models are inexpensive to run and can complement experimental and numerical simulations for risk analysis-related applications. Some models are developed by employing simplifying assumptions in the Navier-Stokes equations and searching for exact, but many times inviscid solutions occasionally complemented by boundary layer equations to take surface effects into account. Other use simple superposition of generic, canonical flows for which the individual solutions are known. These solutions are then ensembled together by empirical or semi-empirical fitting procedures. Few models address turbulent or fluctuating flow fields, and all models have a series of constants that are fitted against experiments or numerical simulations. This chapter presents the main models used to provide primarily mean flow solutions for tornadoes and downbursts. The models are organized based on the adopted solution techniques, with an emphasis on their assumptions and validity.

Forecasting of Tornadoes and Downbursts

Tornadoes and downbursts cause extreme wind speeds that often present a threat to human safety, structures, and the environment. While the accuracy of weather forecasts has increased manifold over the past several decades, the current numerical weather prediction models are still not capable of explicitly resolving tornadoes and small-scale downbursts in their operational applications. This chapter describes some of the physical (e.g., tornadogenesis and downburst formation), mathematical (e.g., chaos theory), and computational (e.g., grid resolution) challenges that meteorologists currently face in tornado and downburst forecasting.

Novel Physical Simulators for Non-Synoptic Wind Systems

The study of wind effects on buildings and structures is primarily based on physical simulations of wind events. Synoptic, atmospheric boundary layer (ABL) winds have been simulated in boundary layer wind tunnels. Non-synoptic wind events such as tornadoes and downbursts are three-dimensional, dynamic, and non-stationary, and, as a result, a new generation of physical simulators have emerged in the past decades. Some of these simulators, their performances as well as their limitations, are reviewed in this chapter.

Structural Response to Non-Stationary Thunderstorm Outflows

The mechanics associated with thunderstorm outflows differ significantly from traditional turbulence in boundary layer winds both in its kinematics and dynamics. The key distinguishing attributes are the contrasting velocity profile with height, a rapid increase in speed, and the statistical features of the energetic gusts in the wind field, exhibiting a strong non-stationarity. This raises serious questions regarding the applicability of conventional stationary process-based theories, thus calling for a paradigm shift. This chapter reviews popular approaches concerning the structural analysis of non-stationary thunderstorm outflows, such as evolutionary power spectrum-based analysis, wavelet-based analysis, thunderstorm response spectrum technique involving the equivalent wind spectrum, and hybrid simulation-based analysis in the time domain. Finally, some preliminary comparisons between the results obtained using these different methods are presented.

Engineering Perspective of Tornadoes

This chapter is an introduction to tornado storms from an engineering perspective. The material included here relates to warnings and subsequent response by people, the chance of tornado hazard at a location, tornado–structure interaction, and building design for tornadoes for life safety. Other chapters in this handbook, referenced here, give details on interrelated subjects, in this chapter, reader will gain an overview of the available knowledge on tornadoes from an engineering perspective. Other chapters of this handbook and the references at the end of this chapter can provide in-depth understanding of engineering other aspects of tornado.

Other Models: Data-Driven Approaches for Non-Stationary Non-Synoptic Winds

In comparison with atmospheric boundary layer winds, which are generally regarded as stationary, windstorms such as hurricanes, typhoons, and cyclones; thunderstorms and downbursts; and tornadoes generally exhibit non-stationary features characterized by changes in wind speed and direction. Due to these characteristics, it is usually challenging to model them in a simplistic format. To overcome this difficulty, a data-driven approach may be an alternative, one that has gained significant popularity in many fields mainly due to the rapid advance in measurement and monitoring systems that allows the collection of long-term massive datasets. This chapter reviews data-driven approaches employed in the fields of non-stationary non-synoptic winds from their characterization, modeling, and simulation perspectives.

Toward the Codification of Thunderstorm/Downburst Winds

Non-synoptic winds often exhibit rapid changes during a short period, which may be accompanied by changes in direction. This introduces non-stationarity both in the mean and the standard deviation of wind fluctuations. Thus, design loads in non-synoptic non-stationary winds obtained from conventional analysis frameworks included in codes and standards, such as the gust loading factor approach, may not be appropriate, thus calling for a careful examination of traditional design procedures. This chapter reviews a proposed design procedure for non-synoptic non-stationary winds. In particular, a codification of gust front winds originating from thunderstorms and downbursts is discussed because the event occurs frequently and is well-known to exhibit significant non-stationary characteristics. Two major frameworks reported in the past literature, such as the gust front factor and the thunderstorm response spectrum technique, are examined as a step toward the codification of gust front winds. In addition, a comparison is made between the two frameworks to assess their performance. Finally, a living codification concept through learning and updating invoking the emerging “design thinking” approach is discussed.

Simplified Hazard Modeling and Structural Reliability Analysis Considering Non-Synoptic Wind Systems (NSWS) in Canada

High-intensity wind events such as tornadoes and downbursts can be very destructive to structures and infrastructure systems. In the present chapter, an overview of the assessment of the wind hazard due to tornadoes and downbursts for Canadian sites is provided. Available tornado occurrence information available in Canada that can be used as the basis to develop a tornado occurrence model is discussed. The chapter presents an overall framework to develop tornado wind-velocity hazard maps for Canada. It also explores the use of simple equivalent along height wind profile that could be used to evaluate tornadic wind loading for line-like structures and of a practical procedure to evaluate the failure probability of structures subjected to high-intensity wind events. It is indicated that for a certain class of prismatic structures, the use of nonlinear static pushover analysis can be adequate to evaluate the capacity curve of the structure subjected to downburst wind loading. A probabilistic model of the capacity curve obtained in such a manner can then be used to evaluate the structural reliability by incorporating the assessed wind-velocity hazard map and equivalent wind profile.