Turbulent Transport in the Atmospheric Surface Layer - Transport in the Atmosphere-Vegetation-Soil Continuum - page 89

Table 3.2 Relation between the nonturbulent surface luxes and various deinitions
of turbulent luxes, and the context in which they are used. Transport of heat and
water vapour are used as examples.

Heat Water vapour Application

Quantity Unit Quantity Unit

Surface energy
lux

H W m -2 L v E W m -2 Energy

balance

L ρ ′′ W m -2

Turbulence
energy lux

ρ θ c p ′′ W m -2 wq Energy

balance

v

Surface
mass lux

E kg m -2 s -1 Water

balance

Turbulent mass
lux

ρ w ′′ kg m -2 s -1 Water

balance

Surface
kinematic lux

K m s -1 kg kg -1 m s -1 Scaling

( Section 3.5 )

H
ρ p

ρ

Turbulent
kinematic lux

w ′ θ

K m s -1

w ′′

kg kg -1 m s -1 Scaling

( Section 3.5 )

c + u

c - u

u

'tail wind' 'head wind'

Figure 3.9 Sonic anemometer (in this case a Campbell Sci CSAT, left). Each pair of
arms contains a sound source and microphone at both sides. The travel time of the
ultrasonic sound pulse depends on the speed of sound ( c ) and the wind speed ( u ).
With head wind the travel time will be longer than in still air, and with tail wind it
will be shorter.

3.4.2 Eddy-Covariance Method
The deinition of the turbulent lux as the covariance of the transported quantity and
vertical wind speed directly provides a way to determine luxes from observations.
When the luctuations of, for example, temperature and vertical wind speed are mea-
sured simultaneously, one could determine the sensible heat lux from the covari-
ance of the two signals. This is the idea behind the eddy-covariance method in a
nutshell. The usual setup is to use a sonic anemometer to measure the wind speed
in three orthogonal directions, in combination with one or more gas analysers that
measure the concentrations of water vapour and CO 2 on the basis of absorption of