intermittent incursions of large eddies, which cause a positive heat flux (negative
Ri f ) interrupted by longer periods of thermal stability (positive Ri g ).
In hardwood stands in temperate zones, the intermittent low-frequency phenomena, that last about 25 s, can increase pressure by about 2.5 Pa (Shaw et al.
1990). During transitional periods of instability, periodic waves tend to form in a
similar way to the flow over isolated hills (Chap. 5) with a natural frequency of the
order of Brunt–Väisälä, N BV , given by the expression:
N BV ¼
g
h
@h
@z
0:5
ð4:7Þ
At night, trunk spaces are unstable, while the upper levels are thermally stable
with little turbulence, as shown in Fig. 4.1 (Kaimal and Finnigan 1994). This leads
to the formation of dew at the top of the crown due to radiative cooling, while the
environment surrounding the trunks is dry because of turbulent mixing. At higher
levels corresponding to d height (Chap. 2) where fluxes vary according to the
gradient direction, the Ri f parameter can be used to characterize thermal stability.
4.3 Turbulent Transport of Kinetic Energy
Turbulent flow in plant environments is associated with processes governing
momentum, heat, and mass exchanges between the atmosphere and forest canopies
(Raupach and Thom 1981). These exchange processes regulate microclimate in
canopies and have a special impact on carbon dioxide and water vapor fluxes,
2
0
(θ - θ hc ) (K)
w' θ'
0
1
1
Daytime
Night-time
-1
-1
-2
-3
2
a)
b)
3
1
z/h
c
0
Fig. 4.1 Typical profiles within and above the forest canopy in day and night-time conditions for
a potential temperature, and b sensible heat, (h c = canopy height) (after Kaimal and Finnigan
1994)
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
111
Ri f ) interrupted by longer periods of thermal stability (positive Ri g ).
In hardwood stands in temperate zones, the intermittent low-frequency phenomena, that last about 25 s, can increase pressure by about 2.5 Pa (Shaw et al.
1990). During transitional periods of instability, periodic waves tend to form in a
similar way to the flow over isolated hills (Chap. 5) with a natural frequency of the
order of Brunt–Väisälä, N BV , given by the expression:
N BV ¼
g
h
@h
@z
0:5
ð4:7Þ
At night, trunk spaces are unstable, while the upper levels are thermally stable
with little turbulence, as shown in Fig. 4.1 (Kaimal and Finnigan 1994). This leads
to the formation of dew at the top of the crown due to radiative cooling, while the
environment surrounding the trunks is dry because of turbulent mixing. At higher
levels corresponding to d height (Chap. 2) where fluxes vary according to the
gradient direction, the Ri f parameter can be used to characterize thermal stability.
4.3 Turbulent Transport of Kinetic Energy
Turbulent flow in plant environments is associated with processes governing
momentum, heat, and mass exchanges between the atmosphere and forest canopies
(Raupach and Thom 1981). These exchange processes regulate microclimate in
canopies and have a special impact on carbon dioxide and water vapor fluxes,
2
0
(θ - θ hc ) (K)
w' θ'
0
1
1
Daytime
Night-time
-1
-1
-2
-3
2
a)
b)
3
1
z/h
c
0
Fig. 4.1 Typical profiles within and above the forest canopy in day and night-time conditions for
a potential temperature, and b sensible heat, (h c = canopy height) (after Kaimal and Finnigan
1994)
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
111