Wind
4
0.5
1.0
Time (s)
FIGURE 5-2. Fluctuations in horizontal and vertical wind recorded with fast
response sensors (after Tatarski, 1961).
components are given the symbols u, v, and w, where u is the velocity
component in the x direction, v in the y direction, and w in the z (vertical) direction. Each velocity component has a mean value (averaged over
a period ranging from 15 minutes to an hour) and a component which
fluctuates about the mean. Since the coordinate system is oriented in the
direction of the mean wind, the mean v and w components are zero. Figure
5.2 shows the u and w components of the wind vector for a period of a
few seconds. These can be compared with the temperature fluctuations
in Fig. 2.4. Wind fluctuation (turbulence) is the underlying cause of all
of these fluctuations, and similar patterns are found in measurements of
all atmospheric scalars (water vapor, COz, etc.)
5.3 Modeling the Variation in Wind Speed
We return to turbulent transport under the topic of transport processes.
We now focus on modeling just the variation in the mean wind speed. Our
goal is the ability to model the wind speed around animals and leaves.
These models are used later to compute the boundary layer conductances
of these objects. We are therefore not so concerned with the fluctuations
or vector components of the wind and are most interested in the wind
speed. A rigorous analysis would distinguish between the wind speed
and the magnitude of the mean wind velocity vector, but in practice the
two are often used interchangeably. We therefore use the symbol u to
represent both the magnitude of the velocity vector and the wind speed.
4
0.5
1.0
Time (s)
FIGURE 5-2. Fluctuations in horizontal and vertical wind recorded with fast
response sensors (after Tatarski, 1961).
components are given the symbols u, v, and w, where u is the velocity
component in the x direction, v in the y direction, and w in the z (vertical) direction. Each velocity component has a mean value (averaged over
a period ranging from 15 minutes to an hour) and a component which
fluctuates about the mean. Since the coordinate system is oriented in the
direction of the mean wind, the mean v and w components are zero. Figure
5.2 shows the u and w components of the wind vector for a period of a
few seconds. These can be compared with the temperature fluctuations
in Fig. 2.4. Wind fluctuation (turbulence) is the underlying cause of all
of these fluctuations, and similar patterns are found in measurements of
all atmospheric scalars (water vapor, COz, etc.)
5.3 Modeling the Variation in Wind Speed
We return to turbulent transport under the topic of transport processes.
We now focus on modeling just the variation in the mean wind speed. Our
goal is the ability to model the wind speed around animals and leaves.
These models are used later to compute the boundary layer conductances
of these objects. We are therefore not so concerned with the fluctuations
or vector components of the wind and are most interested in the wind
speed. A rigorous analysis would distinguish between the wind speed
and the magnitude of the mean wind velocity vector, but in practice the
two are often used interchangeably. We therefore use the symbol u to
represent both the magnitude of the velocity vector and the wind speed.