intermittent processes for turbulent transport in this sublayer (Kaimal and Finnigan
1994). The occurrence of these counter-gradients causes a cyclical variation in the
energy from sensible and latent heat stored in the air within the canopy. Under the
counter-gradient process, air parcels from the trunk space penetrate through the
crown into the atmosphere or a gust can enter the atmosphere (Foken 2017). The
cycle begins with a calm period when the atmosphere of the rough sublayer
accumulates substantial amounts of sensible and latent heat as well as carbon
dioxide (Denmead and Bradley 1985). Turbulent eddies then form, and the warmer
atmosphere is replaced with cooler, drier air with lower carbon content. This process lasts for about 30 s, after which the air is heated and humidified until the
occurrence of another discrete phenomenon.
In a forest stand the maximum air temperature happens in the upper crown at
around 1–2 pm. Below the crown, the daytime temperatures are lower. During the
daytime unstable stratification above the canopy prevails. At night time, the minimum temperature associated with radiative cooling occurs at the top crown level.
The minimum temperature in the forest ground occurs a bit later due to the
downward flow of cool air from the crown tops. During night time, stable stratification above forest stands is common (Foken 2017).
Mean velocity and instantaneous fluctuations are strongly attenuated within the
canopies. The mean flow profile is exponential under the canopy, in contrast with
the logarithmic profile above it (Fig. 4.2). A second velocity maximum occurs at
the lower level where the space among trunks is more open than within the crowns.
Airflow exerts drag forces on natural or modified terrain surfaces and bodies. In
agroforests surfaces, drag is caused by the canopy and is imparted by its elements
onto the velocity field. This drag is made up of skin friction and form drag forces
(Shaw 1995). The latter, above the canopy, where momentum absorption takes
place, are stronger under turbulent flow conditions. Over and within agroforest
canopies, examples of a wide range of existing individual and grouped elements
include foliage, trees, crops, forest stands, or even water streams.
Those objects present irregular boundaries of retarded air, forming cascades of
eddies in leeward surfaces. The drag on leaves can be measured in a wind tunnel
under a steady and controlled airflow, e.g., with leaf metallic replicas, allowing to
evaluate how the drag coefficient varies with wind speed and direction. The drag on
real leaves increases due to the influence of cuticles and hair, insofar that for
aluminum specimens, covered with real leaves, that increase was of about 20% at
wind velocities of 1.5 ms
−1 and 50% at 0.5 ms
–1 , due to higher relevance of skin
friction at lower wind speeds.
Wind tunnel studies are theoretically restricted to laminar flow, which is usually
common under these experimental conditions, while in natural conditions turbulent
flow prevails with associated increases in momentum transfer, especially if the
vegetal elements are shaken by airflow. In this context, Monteith and Unsworth
(2013), referred that in real turbulent conditions aerodynamic resistances to
momentum transfer are downgraded by an order of magnitude as much as 1.5.
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
107
1994). The occurrence of these counter-gradients causes a cyclical variation in the
energy from sensible and latent heat stored in the air within the canopy. Under the
counter-gradient process, air parcels from the trunk space penetrate through the
crown into the atmosphere or a gust can enter the atmosphere (Foken 2017). The
cycle begins with a calm period when the atmosphere of the rough sublayer
accumulates substantial amounts of sensible and latent heat as well as carbon
dioxide (Denmead and Bradley 1985). Turbulent eddies then form, and the warmer
atmosphere is replaced with cooler, drier air with lower carbon content. This process lasts for about 30 s, after which the air is heated and humidified until the
occurrence of another discrete phenomenon.
In a forest stand the maximum air temperature happens in the upper crown at
around 1–2 pm. Below the crown, the daytime temperatures are lower. During the
daytime unstable stratification above the canopy prevails. At night time, the minimum temperature associated with radiative cooling occurs at the top crown level.
The minimum temperature in the forest ground occurs a bit later due to the
downward flow of cool air from the crown tops. During night time, stable stratification above forest stands is common (Foken 2017).
Mean velocity and instantaneous fluctuations are strongly attenuated within the
canopies. The mean flow profile is exponential under the canopy, in contrast with
the logarithmic profile above it (Fig. 4.2). A second velocity maximum occurs at
the lower level where the space among trunks is more open than within the crowns.
Airflow exerts drag forces on natural or modified terrain surfaces and bodies. In
agroforests surfaces, drag is caused by the canopy and is imparted by its elements
onto the velocity field. This drag is made up of skin friction and form drag forces
(Shaw 1995). The latter, above the canopy, where momentum absorption takes
place, are stronger under turbulent flow conditions. Over and within agroforest
canopies, examples of a wide range of existing individual and grouped elements
include foliage, trees, crops, forest stands, or even water streams.
Those objects present irregular boundaries of retarded air, forming cascades of
eddies in leeward surfaces. The drag on leaves can be measured in a wind tunnel
under a steady and controlled airflow, e.g., with leaf metallic replicas, allowing to
evaluate how the drag coefficient varies with wind speed and direction. The drag on
real leaves increases due to the influence of cuticles and hair, insofar that for
aluminum specimens, covered with real leaves, that increase was of about 20% at
wind velocities of 1.5 ms
−1 and 50% at 0.5 ms
–1 , due to higher relevance of skin
friction at lower wind speeds.
Wind tunnel studies are theoretically restricted to laminar flow, which is usually
common under these experimental conditions, while in natural conditions turbulent
flow prevails with associated increases in momentum transfer, especially if the
vegetal elements are shaken by airflow. In this context, Monteith and Unsworth
(2013), referred that in real turbulent conditions aerodynamic resistances to
momentum transfer are downgraded by an order of magnitude as much as 1.5.
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
107