Turbulent Transport in the Atmospheric Surface Layer - Transport in the Atmosphere-Vegetation-Soil Continuum - page 69

3
Turbulent Transport in the Atmospheric Surface Layer
3.1 Introduction
This chapter is concerned with exchange processes between the surface and the atmo-
sphere. According to the surface energy balance, during daytime the net input of
energy at Earth's surface ( Q* - G ) is used to supply heat to the atmosphere and to
evaporate water. This heat and water vapour needs to be transported away from the
surface. During night time, on the other hand, as we have seen in Chapter 2 , the avail-
able energy is generally negative and hence the sensible heat is transported downward
(water vapour can go either way). The exact partitioning between the sensible and
latent heat lux (both during day time and night time) is at this stage not crucial and
is dealt with later in Chapter 7 .
How does this transport of heat and water vapour from and to the surface occur?
If we take heat transport as an example, one option could be to transport the heat by
molecular heat diffusion. A typical daytime value for the sensible heat lux could be
100 W m -2 , and the thermal diffusivity of air is around 2·10 -5 m 2 s -1 . Then we can
derive from Eq. ( 1.6 ) that a vertical temperature gradient of more than 4000 K per
meter would be required (note that in this case the transported quantity used in Eq.
( 1.6 ) is enthalpy per unit volume: ρc p T ). It is clear that vertical temperature gradients
of this magnitude do not occur, so there must be another mode of transport. 1 This is
turbulent transport: heat, water vapour (and other gases) as well as momentum are
transported by the movement of parcels of air that carry different concentrations of
heat, water vapour, etc.
As we will see later, atmospheric turbulence is mostly produced by processes
related to Earth's surface: wind shear and surface heating. These production mecha-
nisms have a strong diurnal variation due to the variation in insolation. The part of
the atmosphere in which this diurnal cycle of turbulence production (as well as the
variation in luxes of, e.g., water vapour and CO 2 ) is noticeable is called the atmo-
spheric boundary layer (ABL). The ABL is the turbulent layer between the surface
1 In fact, very close to the surface such temperature gradients do occur, but over a very small distance only.
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