An Introduction to the Global Circulation of the Atmosphere

David Randall

• Publisher: Princeton University Press
• Year: 2015
• ISBN:
  9781400866526
• Topic: Ciencias físicas
• Subtopic: Meteorología y climatología

The most comprehensive advanced graduate-level textbook on the subject This is a graduate-level textbook on the global circulation of the Earth's atmosphere—the large-scale system of winds by which energy is transported around the planet, from the tropical latitudes to the poles. Written by David Randall, one of the world's foremost experts on the subject, it is the most comprehensive textbook on the topic. Intended for Earth science students who have completed some graduate-level coursework in atmospheric dynamics, the book will help students build on that foundation, preparing them for research in the field.The book describes the many phenomena of the circulation and explains them in terms of current ideas from fluid dynamics and thermodynamics, with frequent use of isentropic coordinates and using the methods of vector calculus. It emphasizes the key roles of water vapor and clouds, includes detailed coverage of energy flows and transformations, and pays close attention to scale interactions. The book also describes the major historical contributions of key scientists, giving a human dimension to the narrative, and it closes with a discussion of how the global circulation is evolving as the Earth's climate changes.

• The most comprehensive graduate-level textbook on the subject
• Written by one of the world's leading experts
• Connects global circulation and climate phenomena
• Addresses energy, moisture, and angular-momentum balance; the hydrologic cycle; and atmospheric turbulence and convection
• Emphasizes the energy cycle of the atmosphere; the role of moist processes; and circulation as an unpredictable, chaotic process
• Helps prepare students for research
• An online illustration package is available to professors

CHAPTER 1

Perpetual Motion

The atmosphere circulates. The circulation is global in extent (see fig. 1.1). The circulating mass consists of “dry air” and three phases of water. Energy and momentum are carried with the air but evolve in response to various processes along the way. Many of those same processes add or remove moisture.

The circulation is sustained by thermal forcing, which ultimately comes from the Sun. On the average, the Earth absorbs about 240 W m⁻² of incoming or “incident” solar energy, of which roughly 2% is converted to maintain the kinetic energy of the global circulation against frictional dissipation. Additional, “primordial” energy leaks out of the Earth’s interior but at the relatively tiny rate of 0.1 W m⁻² (Sclater et al., 1980; Bukowinski, 1999). The thermal forcing of the global circulation is strongly influenced by the circulation itself—for example, as clouds form and disappear. The interactions between the circulation and the heating are fascinating but complicated.

Averaged over time, the global circulation has to satisfy various balance requirements: for example, the infrared radiation emitted at the top of the atmosphere must balance the solar radiation absorbed, precipitation must balance evaporation, and angular momentum exchanges between the atmosphere and the ocean–solid Earth system must sum to zero. We will discuss the global circulation from this classical perspective. We will also supplement this discussion with descriptions and analyses of the many and varied but interrelated phenomena of the circulation, such as the Hadley and Walker circulations, monsoons, stratospheric sudden warmings, the Southern Oscillation, subtropical highs, and extratropical storm tracks. In addition, we will discuss the diabatic and frictional processes that maintain the circulation, and the ways in which these processes are affected by the circulation itself.

The circulations of energy and water are closely linked. It takes about 2.5 × 10⁶ J of energy to evaporate 1 kg of water from the oceans, and the same amount of energy is released when the water vapor condenses to form a cloud. The energy released through condensation drives thunderstorm updrafts that in one hour or less can penetrate a layer of the atmosphere 10 or even 20 km thick. The cloudy outflows from such storms reflect sunlight to space and block infrared radiation from the warm surface below. Shallower clouds cast shadows over vast expanses of the oceans. One of the aims of this book is to give appropriate emphasis to the role of moisture in the global circulation of the atmosphere.

Figure 1.1. A full-disk image of the Earth on July 27, 2009, looking down on the equator, with North and South America in view. Many elements of the global circulation can be seen in this picture, including the “intertropical” rain band in the eastern North Pacific, swirling midlatitude storms, and the low clouds associated with the high-pressure systems over the eastern subtropical oceans. From http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2009/07/FIRST_IMAGE_G14_V_SSEC.gif

It is conventional and useful, although somewhat arbitrary, to divide the atmosphere into parts. For purposes of this quick sketch, we will divide the atmosphere vertically and meridionally, only briefly mentioning the longitudinal variations. Let’s start at the bottom.

Most of the solar radiation that the Earth absorbs is captured by the surface rather than within the relatively transparent atmosphere. Several processes act to transfer the absorbed energy upward from the ocean and land surface into the lower portion of the atmosphere.

The layer of air that is closely coupled with the Earth’s surface is, by definition, the planetary boundary layer, or PBL. The top of the PBL is often very sharp and well defined (see fig. 1.2). The depth of the PBL varies considerably in space and time, but a ballpark value to remember is 1 km. The air in the PBL is turbulent, and the turbulence is associated with rapid exchanges or “fluxes” of sensible heat (essentially temperature), moisture, and momentum between the atmosphere and the surface. These exchanges are produced by the turbulence, and also promote the turbulence, through mechanisms that will briefly be discussed later. The most important exchanges are of moisture, upward into the atmosphere via evaporation from the surface, and of momentum, via friction. The latent heat associated with the surface moisture flux is a key source of energy for the global circulation, and surface friction strongly influences the ocean currents.

Figure 1.2. This figure shows lidar backscatter from aerosols and clouds. The wavelength of the beam is 532 nm, which is in the green portion of the visible spectrum. The figure was created using data from CALIPSO. The lidar beam cannot penetrate thick clouds, which explains the vertical black stripes in the figure. The PBL is visible because the aerosol concentration decreases sharply with height at the PBL top. The data shown represent observations extending from over the North Atlantic Ocean on the left toward the southeast, over Africa, on the right. Longitudes and latitudes are given along the bottom of the figure. The data also show the Cantabrian Mountains of northern Spain (at about 42° N) and the Atlas Mountains of northern Africa (at about 33° N). The image was kindly provided by Dr. David Winker and the CALIPSO team, of the NASA Langley Research Center.

Above the PBL is the free troposphere. Because the troposphere includes the PBL, we add the adjective “free” to distinguish the part of the troposphere that lies above the PBL. The free troposphere is characterized by positive static stability, which means that buoyancy forces resist vertical motion. The depth of the troposphere varies strongly with latitude and season.

A turbulent process called entrainment gradually incorporates free-tropospheric air into the PBL. Over the oceans, entrainment is, with a few exceptions, relatively slow but steady. Over land, entrainment is promoted by strong daytime heating of the surface, which helps generate turbulence. As a result, the turbulent PBL rapidly deepens during the day. When the Sun goes down, the processes that promote turbulence and entrainment are abruptly weakened, and the PBL reorganizes itself into a much shallower nocturnal configuration, leaving behind a layer of air that was part of the PBL during the afternoon. This diurnal deepening and shallowing of the PBL acts as a kind of “pump” that captures air from the free troposphere and adds it to the PBL starting shortly after sunrise, modifies the properties of that air during the day through strong turbulent exchanges with the surface, and then releases the modified air back into the free troposphere at sunset. This diurnal pumping is one way that the PBL exerts an influence on the free troposphere.

In addition, moisture and energy are carried upward from the PBL into the free troposphere by several mechanisms. Throughout the tropics and the summer-hemisphere middle latitudes the most important of these mechanisms is cumulus convection. Cumulus clouds typically grow upward from the PBL. The updrafts inside the clouds carry PBL air into the free troposphere, where it is left behind when the clouds decay (see fig. 1.3). One of the effects of this process is to remove air from the PBL and add it to the free troposphere.

Figure 1.3. A space shuttle photograph of tropical thunderstorms. The storms are topped by thick anvil clouds. Much shallower convective clouds can be seen in the foreground. From http://eoljscnasa.gov/sseop/EFS/lores.pl?PHOTO=STS41B-41-2347.

Frontal circulations also can carry air from the PBL into the free atmosphere, essentially by “peeling” the PBL from the Earth’s surface, like the rind from an orange, and lofting the detached air toward the tropopause. This process is especially active in the middle latitudes in winter.

Figure 1.4 shows somewhat idealized observed midlatitude vertical distributions of temperature, pressure, density, and ozone mixing ratio, from the surface to the 70 km level. In the lowest 12 km, the troposphere, the temperature decreases (almost) monotonically with height. The troposphere is cooled radiatively, because it emits infrared radiation much more efficiently than it absorbs solar radiation. The net radiative cooling is balanced mainly by the release of the latent heat of water vapor as clouds form and precipitate.

Figure 1.4. Idealized midlatitude temperature, pressure, density, and ozone profiles, for the lowest 70 km of the atmosphere. The temperature, pressure, and density profiles are based on the U.S. Standard Atmosphere (1976). The ozone profile is from Krueger and Minzner (1976).

The upper boundary of the troposphere is called the tropopause. The height of the tropopause varies from 17 km or so in some regions of the tropics to about half that near the poles. Above the tropopause, the temperature becomes uniform with height and then begins to increase with altitude in the region known as the stratosphere. The temperature increase is due to the absorption of solar radiation by ozone, which is created in the stratosphere by photochemical processes. Without ozone there would be no stratosphere. The summer-hemisphere stratosphere is almost devoid of active weather and has warm air over the pole. The winds of the summer-hemisphere stratosphere are predominantly easterly; that is, they blow from east to west. In contrast, the winter-hemisphere stratosphere experiences much more active weather, mainly owing to waves propagating upward from the troposphere below; has very cold air over the pole; and experiences strong westerly winds. During winter, the polar stratosphere is occasionally disturbed by sudden stratospheric warmings, which are dramatic changes in temperature (and wind) that occur sporadically in the Northern Hemisphere and much less frequently in the Southern Hemisphere.

Even though the stratosphere is very dry, its moisture budget is quite interesting. It receives small amounts of moisture from the troposphere and also gains some moisture through the oxidation of methane. The upper boundary of the stratosphere, called the stratopause, occurs near the 1 hPa (~50 km) level. In this book we focus mainly on the tropospheric circulation. We discuss selected aspects of the stratospheric circulation, but we do not discuss the portion of the atmosphere that resides above the stratopause.

For meteorological purposes, the tropics can be defined as the region from about 20° S to 20° N. Although the tropical temperature and surface pressure are remarkably uniform in space and temporally monotonous, the winds and rainfall are quite variable. In many parts of the tropics deep cumulus and cumulonimbus clouds— that is, thunderstorms—produce lots of rain and transport energy, moisture, and momentum vertically, essentially continuing the job begun closer to the surface by the turbulence of the PBL. The convective clouds often produce strong exchanges of air between the PBL and the free troposphere, in both directions: positively buoyant PBL air “breaks off” and drifts upward to form the cumuli, while negatively buoyant downdrafts associated with the evaporation of falling rain can inject free-tropospheric air into the PBL. In the convectively active parts of the tropics, the air is slowly rising in an area-averaged sense.

The mean flow in the tropical PBL is easterly. This is the trade wind regime. The tropical temperature and surface pressure distributions are generally very flat and monotonous, for simple dynamical reasons (discussed in chapter 3) that are connected to the smallness of the Coriolis parameter in the tropics. The tropical moisture and wind fields are more variable than the temperature, however. The tropics is home to a variety of distinctive traveling waves and vortices that organize the convective clouds on scales of hundreds to thousands of kilometers. Finally, the tropics is dominated by powerful and very large-scale monsoon systems that extend into the subtropics and even...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface
• Handy Numbers
• Chapter 1: Perpetual Motion
• Chapter 2: What Makes It Go?
• Chapter 3: First Impressions
• Chapter 4: The Rules of the Game
• Chapter 5: Go with the Flow
• Chapter 6: Up Moist, Down Dry
• Chapter 7: Heat Where It’s Hot, and Cool Where It’s Cold
• Chapter 8: A Taxonomy of Eddies
• Chapter 9: What the Eddies Do
• Chapter 10: A Fluid Dynamical Commotion
• Chapter 11: The Future of the Circulation
• Appendix A: Vectors, Vector Calculus, and Coordinate Systems
• Appendix B: Dimensional Analysis, Scale Analysis, and Similarity Theories
• Appendix C: Why Is the Dissipation Rate Positive?
• Appendix D: Vertical Coordinate Transformations
• Appendix E: The Moist Adiabatic Lapse Rate
• Appendix F: Eddy Kinetic Energy and Zonal Kinetic Energy
• Appendix G: Spherical Harmonics
• Appendix H: Hermite Polynomials
• Bibliography
• Index