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b
C
T
3
/
4
,
H
=
(4.24)
1.09
b
C
T
1
/
4
C
q
1
/
2
,
LE
=
(4.25)
0.55
z
g
T
1
/
2
.
=
b
(4.26)
For large Bowen ratios when the humidity influence can be neglected, scintil-
lometer measurements at one wavelength are sufficient and the heat flux can be
derived from a relation between
C
T
2
and
C
n
2
from Wesely (
1976
) and from Monin-
Obukhov similarity theory (Kleissl et al.
2008
). A comparison of the structure
parameters for temperature and humidity from scintillometer measurements with
simultaneous eddy-covariance measurements is described in Beyrich et al. (
2005
),
showing a consistent behaviour in time and deviations in the order of 20-35%.
4.4.3 Humidity or Water Vapour Flux
Vertical profiles of water vapour fluxes have been derived from ground-based remote
sensing with high-resolution Doppler LIDAR instruments by, e.g. Giez et al. (
1999
),
Bösenberg and Linné (
2002
). These fluxes can also be observed by the simultaneous
operation of a Bragg-RASS for the wind fluctuations and a water vapour DIAL
for the humidity fluctuations (Wulfmeyer
1999
). Figure
4.33
shows an example.
Here fluxes up to about 600 m above ground have been determined with a vertical
resolution of about 70 m.
Water vapour fluxes have been obtained by the operation of a Doppler LIDAR for
the wind fluctuations and a DIAL for the humidity fluctuations as well (Linné et al.
2007
). The eddy-correlation method was applied, and error estimates of
50W/m
2
for latent heat flux were found. Since the sampling error dominates the overall
measurement accuracy, time intervals between 60 and 120 min were required for
a reliable flux calculation, depending on wind speed. Rather large errors may occur
with low wind speeds because the diurnal cycle restricts the useful interval length.
The LIDAR flux profiles are well complemented by tower measurements at 50 and
90 m above ground and by area-averaged near surface fluxes from a network of
micrometeorological stations. Water vapour flux profiles in the convective boundary
layer exhibit different structures mainly depending on the magnitude of the entrain-
ment flux. In situations with dry air above the boundary layer, a positive entrainment
flux is observed which can even exceed the surface flux. Flux profiles which linearly
increase from the surface to the top of the boundary layer are observed as well as
profiles which decrease in the lower part and increase in the upper part of the bound-
ary layer. In situations with humid air above the boundary layer, the entrainment flux
is about zero in the upper part of the boundary layer and the profiles in most cases
show a linear decrease (Linné et al.
2007
).
±
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