Introduction

In Chapter 2 we derived the equations that govern the evolution of the atmosphere, and in Chapter 3 we discussed the numerical discretizations that allow the numerical integration of those equations on a computer. The discretization of the continuous governing equation is limited by the model resolution, i.e., by the size of the smallest resolvable scale. We have seen that in a finite difference scheme, the smallest scales of motion that can be (poorly) resolved are those which have a wavelength of two grid sizes. In spectral models, the motion of the smallest wave present in the solution is more accurately computed, but for these and for any type of numerical discretization there is always a minimum resolvable scale. Current climate models typically have a horizontal resolution of the order of several hundred kilometers, global weather forecast models have resolutions of 50–100 km, and regional mesoscale models of 10–50 km. Storm-scale models have even higher resolution, with grid sizes of the order of 1–10 km. In the vertical direction, model resolution and vertical extent have also been increased substantially, with current models having typically between 10 and 50 vertical levels, and extending from the surface to the stratosphere or even the mesosphere. As computer power continues to increase, so does the resolution of atmospheric models.

Despite the continued increase of horizontal and vertical resolution, it is obvious that there are many important processes and scales of motion in the atmosphere that cannot be explicitly resolved with present or future models.

Introduction to atmospheric predictability

In his 1951 paper on NWP, Charney indicated that he expected that even as models improved there would still be a limited range to skillful atmospheric predictions, but he attributed this to inevitable model deficiencies and finite errors in the initial conditions. Lorenz (1963a, b) discovered the fact that the atmosphere, like any dynamical system with instabilities, has a finite limit of predictability (which he estimated to be about two weeks) even if the model is perfect, and even if the initial conditions are known almost perfectly. He did so by performing what is now denoted an “identical twin” experiment: he compared two runs made with the same model but with initial conditions that differed only very slightly. Just from round-off errors, he found that after a few weeks the two solutions were as different from each other as two random trajectories of the model.

Lorenz (1993) described how this fundamental discovery took place: His original goal had been to show that statistical prediction could not match the accuracy attainable with a nonlinear dynamical model, and therefore that NWP had a potential for predictive skill beyond that attainable purely through statistical methods. He had acquired a Royal-McBee LGP-30 computer, with a memory of 4K words and a speed of 60 multiplications per second, which for the late 1950s was very powerful.

Introduction

In previous chapters we saw that NWP is an initial/boundary value problem: given an estimate of the present state of the atmosphere (initial conditions), and appropriate surface and lateral boundary conditions, the model simulates (forecasts) the atmospheric evolution. Obviously, the more accurate the estimate of the initial conditions, the better the quality of the forecasts. Currently, operational NWP centers produce initial conditions through a statistical combination of observations and short-range forecasts. This approach has become known as “data assimilation”, whose purpose is defined by Talagrand (1997) as “using all the available information, to determine as accurately as possible the state of the atmospheric (or oceanic) flow.”

There are several excellent reviews of this subject, which has become an important science in itself. The book Atmospheric data analysis by Daley (1991) is a comprehensive description of methods for atmospheric data analysis and assimilation. Ghil and Malanotte-Rizzoli (1991) have written a rigorous discussion of present data assimilation methods with special emphasis on sequential methods. Talagrand (1997) gives an elegant introductory overview of current methods of data assimilation, and Zupanski and Kalnay (1999) also provide a short introduction to the subject. The book Data assimilation in meteorology and oceanography: Theory and practice (Ghil et al., editors, 1997) contains a wealth of important papers on current methods for data assimilation. An earlier but still useful book is Dynamic meteorology: Data assimilation methods (Bengtsson et al., editors, 1981). Thiebaux and Pedder (1987) provided a description of spatial interpolation methods applied to meteorology.

During the 50 years of numerical weather prediction the number of textbooks dealing with the subject has been very small, the latest being the 1980 book by Haltiner and Williams. As you will soon realize, the intervening years have seen impressive development and success. Eugenia Kalnay has contributed significantly to this expansion, and the meteorological community is fortunate that she has applied her knowledge and insight to writing this book.

Eugenia was born in Argentina, where she had exceptionally good teachers. She had planned to study physics, but was introduced to meteorology by a stroke of fate; her mother simply entered her in a competition for a scholarship from the Argentine National Weather Service! But a military coup took place in Argentina in 1966 when Eugenia was a student, and the College of Sciences was invaded by military forces. Rolando Garcia, then Dean of the College of Sciences, was able to obtain for her an assistantship with Jule Charney at the Massachusetts Institute of Technology. She was the first female doctoral candidate in the Department and an outstanding student. In 1971, under Charney's supervision, she finished an excellent thesis on the circulation of Venus. She recalls that an important lesson she learned from Charney at that time was that if her numerical results did not agree with accepted theory it might be because the theory was wrong.

Previous chapters have described coastal landforms and discussed morphodynamic frameworks for interpreting the pattern of adjustments for different coastal types. The range of adjustments is complex and individual coasts change at varying rates and in varying directions. The level of uncertainty about what will happen in the future increases as the time scale increases. Although coastal landforms and the natural processes of erosion and deposition that shape them are the focus of this book, this natural pattern of adjustment is increasingly influenced directly and indirectly by human activities. Many coasts have been substantially modified by local structural and ecological changes brought about intentionally or unintentionally by humans. The impact of human activities can be felt beyond the local scale. Climate change as a result of the enhanced greenhouse effect and the associated threat of accelerated sea-level rise imply human impact on a global scale at an unprecedented rate. These impacts are added to natural pattens of change.

No coast is now likely to be beyond the influence of humans who have become a force ‘as powerful as many natural forces of change, stronger than some and sometimes as mindless as any’ (Meyer, 1996, p. 2).

This chapter is concerned with coral reefs and associated carbonate environments on tropical and subtropical coasts. Reefs are dynamic geomorphological systems demonstrating a complex interplay between physical and biological processes. They form solid limestone, simultaneously producing, breaking down and redistributing sediments of different sizes to construct a range of landforms. As a result of their ability to build rigid, wave-resistant structures, corals modify the environment in which they live, as expressed by Darwin in the quotation above. Reefs contain a variety of interacting subsystems operating over a broader range of time scales than generally seen on rocky coasts, comprising construction, destruction and various responses to extreme perturbations, such as storms.

Deltas and estuaries are dynamic systems associated with the mouths of rivers. Deltas are accumulations of river-derived sediment whereas estuaries are the tide-influenced lower parts of rivers and their valleys. The distinction between them is sometimes difficult to discern, and it is useful to consider a continuum of deltaic–estuarine landforms. Deltaic–estuarine morphology is influenced by geological setting and topography, and landforms are shaped by hydrodynamic processes. Riverine and coastal sediments are affected by both alluvial and marine influences, together with minor local processes, such as direct input of colluvium from hillslopes, cliff retreat, wind redistribution of sediment, and chemical and biological action.

River discharge and the rate of delivery of sediment to the ocean or embayment vary in relation to catchment size, lithology and climate (Milliman, 2001). The rivers that drain from the continental area of southern and eastern Asia, with highly tectonic hinterlands and prominent monsoon climates, for instance, deliver large volumes of sediment to the oceans (Milliman and Meade, 1983). However, steep, tectonically active island catchments, such as those throughout the Indonesian island arc, also contribute disproportionately large sediment volumes to the ocean (Milliman and Syvitski, 1992).

This book outlines the way that coasts operate. It is written for students of coastal geomorphology, coastal environments, and coastal geology, and for all those with an interest in coastal landforms or who seek insights into the way the coast behaves. It brings together studies of process operation and studies of coastal evolution concerned with longer-term landform development, into morphodynamic models. Coastal morphodynamics involves the mutual co-adjustment of process and form. It provides a framework from which to generalise across space and time scales. The book introduces these concepts and outlines geological setting, materials and coastal processes. Although there are physical principles which govern the response of sediment to forcing factors such as wave energy, the complexity of non-linear interactions means that it is generally difficult to scale up to explain behaviour over time scales that are relevant to human societies.

The book is based heavily on my own research experiences in Australia, Britain, the United States, New Zealand and on many islands in the West Indies, Pacific and Indian Oceans. It also draws extensively on the scientific literature and pays particular tribute in terms of historical perspective to those coastal scientists who have built the foundations of what we know. I hope that it instils something of the sense of wonder that I feel about the coast, and offers new perspectives on how the coast is shaped.

Coasts are often highly scenic and contain abundant natural resources. The majority of the world's population lives close to the sea. As many as 3 billion people (50% of the global total) live within 60 km of the shoreline. The development of urbanised societies was associated with deltaic plains in semiarid areas, and the first cities appeared shortly after the geomorphological evolution of these plains (Stanley and Warne, 1993a). The coast plays an important role in global transportation, and is the destination of many of the world's tourists.

The shoreline is where the land meets the sea, and it is continually changing. Coastal scientists, and the casual ‘wanderer by the shore’, have attempted to understand the shoreline in relation to the processes that shape it, and interrelationships with the adjacent shallow marine and terrestrial hinterland environments. Explaining the geomorphological changes that are occurring on the coast is becoming increasingly important in order to manage coastal resources in a sustainable way.

This book examines the coast as a dynamic geomorphological system. Geomorphology is the study of landforms, and coastal geomorphology is concerned primarily with explaining the many different types of coastal landforms, and understanding the factors that shape them.

Beaches represent some of the most dynamic coasts; they are attractive, not only from aesthetic and recreational points of view, but also as field areas for geomorphological research. The term beach describes wave-deposited sediment. As sediments continue to accrete on beaches they build a larger feature much of which is no longer actively reworked by waves, termed a barrier.

Coastal landforms adjust towards, but rarely find, delicate and dynamic balances with the processes that operate. This is expressed very clearly in the above quotation from G.K. Gilbert, and has been a recurrent theme in the preceding chapters.

Rocky coasts occur where rugged or relatively resistant terrestrial lithology abuts the ocean, forming a distinct or abrupt transition between land and sea. They are typically high-energy coasts, primarily of sea cliffs and other steeply inclined shorelines, on which the influence of underlying rock type is plainly apparent. Many of these coasts evolve at slow rates; in places, resistant pre-Tertiary rocks appear to have changed little over millions of years. However, it is clear that tectonic activity and fluctuations of sea level, particularly during the Quaternary, have caused shoreline position to adjust over time.