Low frequency transients

In the preceding chapter, Fig. 7.14 compared the eddy correlation tensor for the higher frequency eddies, whose periods are less than around ten days, with the lower frequency transients. The high frequency eddy statistics have a well-defined structure in the midlatitudes, with maxima in the storm track regions. The high pass filter used in Chapter 7 served to isolate a specific family of dynamical processes, namely, those associated with baroclinic instability and the subsequent evolution of baroclinically unstable waves. The low frequency eddy kinetic energy is much less clearly structured. Figure 7.14 shows some evidence of maxima downstream of the high frequency maxima, as well as some correlation between the jet centres and maxima of low frequency variability. But none of these patterns is especially marked.

One reason for this is that the low frequency band covers a very wide range of frequencies. There are disturbances whose periods are very little longer than those of the baroclinic disturbances; indeed, the maxima downstream of the storm track centres are at least partly due to the occlusion and decay of midlatitude cyclones, which become slower moving as they fill. But there are also transients of significant amplitude with very much longer periods. Indeed, a spectral analysis of any long sequence of atmospheric data reveals that variability is observed on as long a period as one cares to specify.

Observations of steady eddies

Despite the eddy–zonal flow partitioning which we have employed in preceding chapters, the seasonal mean flow is very far from being zonally symmetric. Such departures from symmetry are important in accounting for regional variations of climate. They also modify the global patterns of heat and momentum transport, especially in the northern hemisphere winter. In this chapter, we will discuss some observations of the steady wave pattern, and show how rather simple theories based on linear wave propagation can account for some of the gross features of these observations.

The steady waves are most pronounced in the northern hemisphere winter, and have their largest amplitudes in the upper troposphere. In some circumstances, they also become very important at high levels in the winter stratosphere, a point that we will return to in Chapter 9. Figure 6.1 shows the winter mean geopotential height field at 25 kPa in both hemispheres. The characteristic features of the northern hemisphere picture are the pronounced troughs over Canada and Japan, with ridges over the eastern side of the two ocean basins. One's subjective impression is of a predominantly zonal wavenumber 2 pattern. This general pattern is very persistent and can be seen in individual seasons with only relatively small variations. The corresponding picture for the southern hemisphere looks, at first sight, much more axisymmetric.

Major influences on planetary circulations

Until recently, the study of global circulations has been confined to the circulation of a single system, namely that of the Earth. Throughout the earlier part of this book, we, too, have concentrated upon the Earth, showing how the poleward and upward transports of heat generate the kinetic energy associated with observed atmospheric circulations. We have described some of the forms which these heat fluxes can take, including the essentially axisymmetric circulations of the Hadley cells in low latitudes and the wavelike baroclinically unstable waves of the midlatitudes. These principles need not be restricted to the Earth's system alone. In this chapter, we will enquire how general are the particular heat transporting circulations observed in the Earth's atmosphere, and how they might be modified in different circumstances.

Such a discussion has become much more informed in the last 20 years or so, as the study of planetary atmospheres has advanced considerably. Spacecraft have now paid at least fleeting visits to every planet with a substantial atmosphere in the solar system, with the exception of Pluto, which may possess an atmosphere. In the case of Venus and Mars, direct in situ measurements of meteorological parameters have been made in addition to the more usual remotely sensed data. In the coming years, plans are under way for entry probes and direct measurements of other atmospheres, including those of Jupiter and Titan.

Observational basis

The large scale structure of the atmospheric flow varies most rapidly in the vertical direction, and least rapidly in the zonal direction. Zonal averaging therefore makes the important vertical and meridional variations plain, and has been employed for many years as a compact way of studying the global circulation. Indeed, for many writers, the global circulation is simply the pattern of flow projected on to the meridional plane. In this book, we will take a broader view by attempting to summarize our current understanding of the full, evolving three-dimensional pattern of winds and temperature in the atmosphere. But the traditional zonal mean view is a useful starting point which we will explore in this chapter.

The zonal mean wind and vectors of the mean meridional wind are illustrated in Fig. 4.1, based on ECMWF analyses. Rising motion is seen in the tropics, with the maximum vertical velocity in the summer hemisphere. Strongest descent is at latitudes of around 25 – 30° in the winter hemisphere, with flow towards the equator near the surface and away from the tropics in the upper troposphere, as is required by continuity. Such an axisymmetric circulation is the most obvious response of the atmospheric flow to the net heating excess in the tropics and the deficit at high latitudes discussed in the preceding chapter.

The aim of this chapter is to introduce the basic physical laws which govern the circulation of the atmosphere and to express them in convenient mathematical forms. No attempt is made at either completeness or rigour beyond the requirements of the later chapters. Those who wish for a more detailed discussion are referred to one of the many excellent texts on dynamical meteorology which are now available. Those by Holton (1992) and by Gill (1982) are particularly recommended.

The first law of thermodynamics

The first law may be stated simply in its qualitative form: heat is a form of energy. The transformation of heat energy into various forms of mechanical energy is the process which drives the global circulation of the atmosphere and which is responsible for the formation of the weather systems whose cumulative effects define the climate of a particular region. These transformations will be discussed in more detail in Chapter 3. In this section, the first law will be expressed in mathematical terms. But, first, it will be necessary to consider the thermodynamic properties of the air which makes up the atmosphere.

The ‘thermodynamic state’ of a parcel of air is defined by specifying its composition, pressure, density, temperature, and so on. In fact, these properties are not independent of one another, but are related through the ‘equation of state’ of the air.

The seasonal cycle of the stratospheric circulation

Up until this point, we have concentrated almost exclusively upon the troposphere, which is characterized by a relatively weak stratification, with a temperature lapse rate of around 6–7 K km–1. At the tropopause, the lapse rate becomes close to zero; the lower stratosphere is nearly isothermal. The corresponding change in stratification, as measured by the Brunt-Väisälä frequency, is by a factor of around two, from values of 10–2 s–1 in the troposphere to values of 2 × 10–2 s–1 in the lower stratosphere. In the upper stratosphere, from heights of 30 km to around 50 km, the temperature actually increases with height. The transition to stably stratified conditions is called the tropopause, which is extremely sharp in the tropics and midlatitudes. It is rather more gradual in polar latitudes, especially in winter when there is no incoming sunlight. The abrupt increase of stratification at the tropopause means that the stratosphere is dynamically very different from the underlying troposphere. Baroclinic instability is virtually suppressed and disturbances are mainly forced from below. The stratification acts as a filter, removing the smaller scale disturbances and allowing only the longest waves to propagate out of the troposphere to great heights in the stratosphere. Shorter wavelength disturbances are thereby trapped in the troposphere, which behaves as a waveguide, the upper boundary of which is the tropopause.

Zonal variations in the tropics

Up to this point, we have followed a traditional exposition of the global circulation by concentrating upon the zonal mean circulation and upon the zonal mean fields of eddy quantities. But the global circulation is far from zonally symmetric. Tropical heating has distinct maxima at particular longitudes. In the midlatitudes, the transient eddies are not distributed uniformly around the latitude circles, but are concentrated into isolated ‘storm tracks’, especially in the northern hemisphere. This chapter will be devoted to a description of such zonal asymmetries and their consequences.

The various diagnostics of the steady and transient eddy activity which we have considered in earlier chapters become small in the tropics. Eddy kinetic energy is much smaller in the tropics than in the midlatitudes. Similarly, eddy temperature and momentum fluxes, both steady and transient, are much smaller in the tropics. Thus a picture emerges in which heat and momentum are transported, essentially by axisymmetric motions in the tropics, with eddies taking over in the subtropics and midlatitudes.

There is some truth in this picture. But it can also be misleading. First, consider the heating fields shown in Fig. 3.8. The forcing of the circulation is certainly not axisymmetric, especially in the tropics. Rather, there are a small number of centres of intense heating.

Timescales of atmospheric motion

The circulation of the atmosphere is intrinsically unsteady; fluctuations on all timescales are observed. In the last chapter, it was shown that the fluxes of temperature, momentum, and so on, carried by such transients play an important part in determining the time mean circulation of the atmosphere. Our task in this chapter will be to describe the transients on various timescales, and to discuss the mechanisms which can give rise to transient behaviour. Just as the atmosphere contains a wide range of spatial scales, from the molecular to the global, so the atmospheric circulation exhibits a wide range of timescales, ranging from timescales of just a few seconds associated with the overturning of small turbulent eddies, to geological timescales for major climate changes.

Some of the frequencies observed are directly related to the frequencies of periodic forcings. For example, diurnal and semi-diurnal variations of temperature and wind are associated with the diurnal variation of solar heat input. These ‘thermal tides’ are important at high levels in the atmosphere and can be detected in the lower atmosphere. More importantly for our purposes, the annual cycle of radiative forcing has a profound effect on large scale atmospheric circulation. This seasonal cycle of meteorological quantities affects nearly all parts of the globe.

But in addition to these externally imposed periods, the atmospheric flow itself generates all kinds of timescales internally.

Great increases in the cost of fuel and the advent of very large tankers and bulk carriers have focused the attention, during the last decades, on means to enhance the efficiency of ship propulsion. An obvious way of obtaining an efficiency increase is to use propellers of large diameter driven by engines at low revolutions, as can be deduced from the developments in Chapter 9. Such a solution is, however, in many cases not practically possible. This has then given impetus to the study and adoption of unconventional propulsion arrangements, consisting, in general, of static or moving surfaces in the vicinity of propellers.

A distinct indication of the serious and extensive activity in the development and use of unconventional propulsors may be seen in the report of the Propulsor Committee of the 19th ITTC (1990b) which lists seven devices including large diameter, slower turning propellers. Here we summarize the hydrodynamic characteristics of six of these devices omitting larger-diameter propellers. The six devices are: Coaxial contrarotating propellers, propeller with vane wheel, with pre-swirl stators, with postswirl stators, ducted propellers and propellers operating behind flowsmoothing devices.

Emphasis is given in the following to a variational procedure which, in a unified fashion enables nearly optimum design of several of these configurations.

Propulsive Efficiency

Propulsive efficiency is conventionally thought of as the product of the open-water efficiency of the propulsor, the hull efficiency and a factor termed the relative-rotative efficiency.

The flows about wings of finite span are sufficiently analogous to those about propeller blades to warrant a brief examination before embarking on the construction of a mathematical model of propellers. A more detailed account of wing theory is given for example by von K ármán & Burgers (1935).

A basic feature of the flow about a straight upward-lifting wing of finite span and starboard-port symmetry in a uniform axial stream is that the velocity vectors on both the lower and upper surfaces are not parallel to the longitudinal plane of symmetry. On the lower side the vectors are inclined outboard and on the upper side they are inclined inwardly in any vertical plane or section parallel to the vertical centerplane. This is a consequence of the pressure relief at the wing tips and the largest increase in pressure being in the centerplane on the lower side and the greatest decrease in pressure being in the centerplane on the upper or suction side. Thus there are positive spanwise pressure gradients on the lower side and negative spanwise gradients on the upper side which give rise to spanwise flow components which are obviously not present in two-dimensional flows.

In many treatments of wing theory one finds figures purporting to show the flow about a wing of finite aspect ratio in which there is a continuous line of stagnation points near the leading edge, extending from one wing tip to the other.