Environmental Hazards

The following sections are included:

• Contents

• Foreword

• Preface

The emphasis in this short introductory chapter is on those fluid dynamical phenomena that are best understood in terms of convection and diffusion of vorticity, the curl of the velocity field. Vorticity is generated at fluid boundaries and diffuses into the fluid where it is subject to convection, stretching, and associated intensification. Far from boundaries, viscous effects may be negligible, and then vortex lines are transported with the fluid. Vortex rings, which propagate under their own self-induced velocity, are a widely observed phenomenon, and a fundamental ingredient of fluid flow. Stretching and intensification is best illustrated by the “Burgers vortex” (the simplest model for a hurricane) in which these processes are in equilibrium with viscous diffusion. Instabilities of Kelvin–Helmholtz type are all-pervasive in highly sheared flow, and inexorably lead to transition to turbulence. In turbulent flow, the vorticity is random, but these fundamental processes still dictate many features of the flow. Fully three-dimensional turbulence is characterized by a cascade of energy through a broad spectrum from large scales to very small scales at which kinetic energy is dissipated by viscosity, a scenario that leads to the famous (−5/3) Kolmogorov spectrum. These topics are reviewed and discussed with a view to geophysical applications. The phenomena of intermittency and concentrated vortices as revealed by direct numerical simulation are also briefly discussed.

In this chapter, the basic mechanics of stratified, rotating fluids as the background to geophysical and environmental flows are discussed. The following topics are included: Stable stratification and internal waves; gravity currents; plumes and convective flows; similarity theory of the atmospheric boundary layer; geostrophic motion and inertial waves; geostrophic adjustment.

Fluid dynamics is at the heart of our climate system, governing the motion of the atmosphere, oceans, and ice sheets. Indeed, as will be described here, the equations of motion for a fluid on a rotating sphere form the core of the numerical climate models used to predict future change. The spherical shape of the Earth leads to different approximations being appropriate within and outside the tropical regions. Aspects of weather phenomena such as mid-latitude winter storms and the tropical monsoons can be understood in this way. The Pacific and Atlantic Oceans are bounded in longitude by the continents, whereas the Southern Ocean is not. This leads to the dominance of different physical processes and hence different circulation patterns. Some processes, such as those associated with the El Niño Southern Oscillation, can only be understood in terms of the coupled dynamics of the atmosphere and ocean. In both the atmosphere and the oceans, global-scale overturning circulations transport heat as well as chemical species around the Earth. The physical mechanisms responsible for these aspects of the climate system are reviewed, and their potential for change with increased atmospheric greenhouse gas concentrations is discussed.

Tropical oceans play a major role in natural variability of the world climate. Anomalous coupled ocean-atmosphere phenomena generated in the tropical oceans produce changes in global atmospheric and oceanic circulation that influence regional climate conditions even in remote regions. On the inter-annual timescale, the El Niño Southern Oscillation (ENSO) of the tropical Pacific Ocean is known as a typical example of such phenomena and has received worldwide attention because of its enormous societal impact. Recently a new mode of variability has been identified with a distinct central Pacific warming pattern. This “El Niño Modoki” mode involves ocean-atmosphere coupled processes, indicating the existence of a unique atmospheric component during the evolution, analogous to the Southern Oscillation in the case of El Niño. The Modoki's impact on world climate is very different from that of ENSO. Interestingly, the Modoki's influences over regions such as the Far East including Japan and the western coast of the USA are almost opposite to those of the conventional ENSO. Modoki events have been more frequent and persistent during recent decades. Inter-annual variability originating in the tropical Indian Ocean includes an ocean-atmosphere coupled phenomenon known as the Indian Ocean Dipole (IOD). The IOD has a unique teleconnection pattern that implies regional climate variability and thus societal impacts in various parts of the globe. These phenomena are described and discussed in detail in this chapter.

Tropical cyclone activity has long been understood to respond to changing properties of the large-scale atmospheric and oceanic environment. In this essay, I review evidence for changing tropical cyclone activity, and the controversy surrounding the quality of the data itself and the attribution of these environmental changes to various natural and anthropogenic causes. At the same time, there is growing evidence that global tropical cyclone activity may itself affect climate in such a way as to mitigate tropical climate change but amplify climate change at higher latitudes. I will review this evidence and suggest possible routes forward in exploring these effects.

It is now realized that air quality is determined not only by local emissions and local meteorology, but also by long-range atmospheric transport of chemical species from emission regions that may be thousands of kilometers from the region of interest.

Predicting and understanding air quality requires consideration of many different processes, including emissions, boundary layer physics, chemical reactions, and interactions with clouds and particles. These lecture notes focus on the role of atmospheric transport and mixing, emphasizing the fundamental ideas and describing relevant mathematical models. In many parts of the atmosphere large-scale quasi-horizontal flow appears to play the dominant role in transport and in the stirring process that leads ultimately to true (molecular) mixing at very small scales. This means that calculations based on large-scale meteorological datasets can give valuable quantitative information on transport. To make local predictions requires more detailed information on transport, e.g., from regional-scale models.

Extreme precipitation (rain) and subsequent flooding is a major environmental hazard. Methods have been developed, mainly on a statistical basis, to estimate maximum probable or even maximum possible precipitation. Higher resolution weather forecast models, with complex representation of the physical processes involved, can be used for calculation of maximum precipitation. This requires input data with known probabilities, but also allows for estimation of the sensitivities of relevant parameters. Orographic (mountain) structures are a major influence in the formation of very high precipitation. The model applied here is used to study the sensitivity of orographic influence. The long-term climatic development of heavy precipitation and floods in Europe during recent centuries will be used to illustrate these numerical studies.

An overview is presented of the physical and biological factors that cause disasters and of their relationships in quantitative terms to the outcomes of these disasters. The chapter begins with an introduction to the atmosphere, which is the starting point of all hydro-meteorological disasters, including the different processes and links that lead to precipitation. The relationship between precipitation and runoff, or floods, including their forecasting techniques is described. The chapter also covers the types and causes of water-related environmental disasters. A quantitative description of mixing processes by Fickian diffusion and by convective dispersion is given. The governing equations and simplifications for conservative and non-conservative types of pollutants, point and non-point sources of pollution, reaction kinetics for non-conservative pollutants, and modeling approaches, are presented. As the health of a waterbody is measured by the dissolved oxygen concentration, an introduction to the oxygen sag curve in rivers is also given.

Nonlinear waves are observed in all branches of science and engineering, and are present in different aspects of daily life. The great Sumatra-Andaman tsunami (December 2004) in the Indian Ocean provides an example of a series of dramatic events dominated by nonlinear wave dynamics. This chapter will review tsunami behavior at all stages, starting from a source in the open ocean, through trans-oceanic propagation, and up to breaking on shore. Major observed features of tsunami can be analyzed using mathematical wave models of various complexities, such as soliton theory and the Boussinesq approximation.

Several numerical models have been specifically developed for tsunami research and operational forecasts worldwide, and application of general hydrodynamic solvers is becoming common due to the availability of ever-increasing computational power and resources. Operational forecasters need instant solutions, which can as yet be achieved only using data-driven models, such as those based on Artificial Neural Networks or Empirical Orthogonal Functions. Operational procedures for tsunami forecasting and warning at major centers are reviewed in the light of lessons learned from the 2004 tsunami.

Rogue waves are fascinating: once part of the folklore, they now make the news each time an observation is made. At the time of printing, the last example is that of the Louis Majesty cruise ship that was hit by an abnormal wave in March 2010, off the coast of Catalonia in the Mediterranean. The wave impact was immortalized by several videos taken by tourists on board the ship. Fortunately for travelers but unfortunately for scientists, rogue waves do not occur very often and their origin remains a mystery, even if the state-of-the-art in the understanding of rogue waves has witnessed some unprecedented progress in the last five years. Recently similar phenomena were observed in different fields of physics, in particular in optics. Is there hope to learn more on rogue waves from other fields or are these extreme events disconnected phenomena? This chapter provides a review on rogue waves, with an emphasis on the modulational instability and the absolute or convective character of this instability.

The following sections are included:

• Index