Turbulent Transport in the Atmospheric Surface Layer - Transport in the Atmosphere-Vegetation-Soil Continuum - page 91

receiver (= concentration or density). Thus gas analysers do not measure the speciic
concentration q X , as was used in the deinition of the luxes in the previous section, but
the density ρ X . This problem can be solved in two - equivalent - ways.
The irst method is to calculate q X irst from the gas analyser data and temperature,
before calculating the lux. In the second method one calculates the lux directly from
the measured density:

F = = + www ρ ( ′ )( ρρ ρ ρ + = + ) w

w ′′

(3.14)

X X

X X X X

The term involving the mean vertical wind is not equal to zero. It is called the Webb
term (Webb et al., 1980 ). From the fact that the vertical mass lux of dry air with den-
sity ρ d is zero (see Eq. ( 3.12 )), the mean vertical wind speed can be deduced:

w ρ

ρ

′ ′ ′′ w

T

T

ρ ρ w ′ ′

w ρ == + 0 w ⇒=− w

d

≈ d d

d

d

(for the last step, see Eq. ( 3.6 )). Especially for gases with a mean concentration that
is high relative to the luctuations in the concentration (e.g., CO 2 ) the Webb term is
an important contribution to the total lux of Eq. ( 3.14 ). Physically this mean vertical
velocity (Webb velocity) can be understood as follows (for a situation with positive
surface heat lux). Turbulent motions transport relatively cool air downward. At the
surface the air is heated by the surface sensible heat lux. As a result the density
decreases and the air parcel expands. This expansion of the air close to the surface
pushes the entire air column upward.
Gas analysers can either be open path analysers or closed path analysers. In the
case of an open path analyser, the absorption measurement takes place at the same
location as where the vertical wind speed is measured: the optical path is located close
to the measurement path of the sonic anemometer. Currently, open path analysers are
available only for H 2 O, CO 2 and CH 4 . For closed path analysers, air is sampled at
the location of the vertical wind speed observations and transported through a tube
towards a gas analyser that is located elsewhere. This has the advantage that the gas
analyser is not exposed to unfavourable weather conditions such as rain and dew that
may disturb the measurements (e.g., Heusinkveld et al., 2008 ). Furthermore, it allows
for the use of gas analysers (e.g., spectrometers) that can detect a large range of gases.
Special care needs to be taken to correct for the decorrelation between the vertical
wind speed and the gas concentration due to time delays and signal broadening in the
transport tube towards the gas analyser (e.g., Moncrieff et al., 1997 )

Question 3.10: From the assumption th a t there is no mean mass lux into or from the
surface, the mean vertical wind speed w can be computed. Giv en a situation where
the mean temperature is 300 K and the kinematic heat lux w ′′ is 0.1 K m s -1 (thus
the sensible heat lux ρ cwT

p ′′ is approximately 120 W m -2 , assuming a mean density
of 1.2 kg m -3 ). How large is the mean vertical wind speed?