64
Wind
know the wind speed in the vicinity of the organism. This requires an
understanding of the behavior of average wind in the surface boundary
layer of the earth. The behavior of the wind, in turn, is dictated by rates
of turbulent transfer in the surface boundary layer. Turbulent transfer theory allows us to derive equations for wind, temperature, vapor density,
and COz profiles and fluxes, and are helpful later when we discuss plant
canopies and their environment. In this chapter we first discuss the behavior and characteristics of the wind in natural, outdoor environments. In a
later chapter we present some of the fbdamentals of turbulent transport
theory and derive the profile equations for wind, temperature, and vapor
concentration.
5.1 Characteristics of Atmospheric Turbulence
As was previously mentioned, one of the obvious characteristics of wind
is its variability. We are aware of random temporal variations of the wind
through fluttering of flags and leaves, variations in the force of wind on
us, and other common experiences. Spatial variations are obvious when
one looks at a field of "waving grain" or at "cat's paws" on a lake. We
are also aware that the range of variability is large. We see very small
scale fluctuations in "heat waves" on hot summer days and feel or hear
the effects of very large scale fluctuations as wind gusts which blow dust
or shake the house. All of these characteristics of wind with which we
are intimately acquainted are characteristics of turbulent flow. Except for
a thin layer of air close to surfaces, the atmosphere is essentially always
turbulent, or, in other words, characterized by random fluctuations in wind
speed and direction caused by a swirling or eddy motion of the air. These
swirls or eddies are generated in two ways. As wind moves over natural
surfaces, the friction with the surface generates turbulence. This is called
mechanical turbulence. Turbulence is also generated when air is heated
at a surface and moves upward due to buoyancy. This is called thermal
or convective turbulence. The size of the eddies produced by these two
processes is different, as is shown in Fig. 5.1. The fluctuations from
mechanical turbulence tend to be smaller and more rapid than thermal
fluctuations. A striking demonstration of these types of turbulence can be
seen by watching the plume from a smokestack on a hot day. The plume
is called a looping plume because, in addition to the small scale mechanical turbulence that tears the plume apart and spreads it with distance, the
thermal updrafts and downdrafts cause the entire plume to be transported
upward or downward.
Large eddies, which are produced either mechanically or thermally, are
unstable and decay into smaller and smaller eddies until they are so small
that viscous damping by molecular interactions within the eddies finally
turns their energy into heat. The size of the smallest eddies produced
by mechanical and convective motion (rather than breakdown of larger
eddies) is called the outer scale of turbulence. The eddy size at which
significant molecular interaction (viscous dissipation) begins is called
Wind
know the wind speed in the vicinity of the organism. This requires an
understanding of the behavior of average wind in the surface boundary
layer of the earth. The behavior of the wind, in turn, is dictated by rates
of turbulent transfer in the surface boundary layer. Turbulent transfer theory allows us to derive equations for wind, temperature, vapor density,
and COz profiles and fluxes, and are helpful later when we discuss plant
canopies and their environment. In this chapter we first discuss the behavior and characteristics of the wind in natural, outdoor environments. In a
later chapter we present some of the fbdamentals of turbulent transport
theory and derive the profile equations for wind, temperature, and vapor
concentration.
5.1 Characteristics of Atmospheric Turbulence
As was previously mentioned, one of the obvious characteristics of wind
is its variability. We are aware of random temporal variations of the wind
through fluttering of flags and leaves, variations in the force of wind on
us, and other common experiences. Spatial variations are obvious when
one looks at a field of "waving grain" or at "cat's paws" on a lake. We
are also aware that the range of variability is large. We see very small
scale fluctuations in "heat waves" on hot summer days and feel or hear
the effects of very large scale fluctuations as wind gusts which blow dust
or shake the house. All of these characteristics of wind with which we
are intimately acquainted are characteristics of turbulent flow. Except for
a thin layer of air close to surfaces, the atmosphere is essentially always
turbulent, or, in other words, characterized by random fluctuations in wind
speed and direction caused by a swirling or eddy motion of the air. These
swirls or eddies are generated in two ways. As wind moves over natural
surfaces, the friction with the surface generates turbulence. This is called
mechanical turbulence. Turbulence is also generated when air is heated
at a surface and moves upward due to buoyancy. This is called thermal
or convective turbulence. The size of the eddies produced by these two
processes is different, as is shown in Fig. 5.1. The fluctuations from
mechanical turbulence tend to be smaller and more rapid than thermal
fluctuations. A striking demonstration of these types of turbulence can be
seen by watching the plume from a smokestack on a hot day. The plume
is called a looping plume because, in addition to the small scale mechanical turbulence that tears the plume apart and spreads it with distance, the
thermal updrafts and downdrafts cause the entire plume to be transported
upward or downward.
Large eddies, which are produced either mechanically or thermally, are
unstable and decay into smaller and smaller eddies until they are so small
that viscous damping by molecular interactions within the eddies finally
turns their energy into heat. The size of the smallest eddies produced
by mechanical and convective motion (rather than breakdown of larger
eddies) is called the outer scale of turbulence. The eddy size at which
significant molecular interaction (viscous dissipation) begins is called