Beyrich, F.

Bruin, H.A.R., de

Kohsiek, W.

Lüdi, A.

Meijninger, W.M.L.

Name not available

Scintillometer fluxes of sensible and latent heat over a heterogeneous area - a contribution to LITFASS-2003

de Bruin, H.A.R.

NARCIS 

Wageningen Yield 

9.2     SCINTILLOMETER FLUXES OF SENSIBLE AND LATENT HEAT OVER A HETEROGENEOUS 
AREA – A CONTRIBUTION TO LITFASS-2003 
 
 
Wouter M.L. Meijninger1, Frank Beyrich2, Andreas Lüdi3, Wim Kohsiek4 and Henk A.R. de Bruin1 
1 Meteorology and Air Quality Group, Wageningen University, Wageningen, The Netherlands 
2 German Meteorological Service, Lindenberg, Germany 
3 Institute of Applied Physics, Bern University, Switzerland 
4 Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands 
 
 
1. INTRODUCTION 
 
Surface fluxes of sensible heat (H) and 
evaporation (LvE) are important in many 
atmospheric processes and can be measured 
with reasonable accuracy over homogeneous 
areas. However, surface fluxes that are 
representative for large natural landscapes 
(comparable to the grid box size of numerical 
models or satellite remote sensing pixels) are 
more difficult to determine because at these 
scales the earth’s surface is always 
heterogeneous. 
The scintillation technique is one of the 
few techniques that can provide fluxes at scales 
of several kilometers (10km). For example the 
combination of a large aperture optical 
scintillometer (LAS) and a microwave 
scintillometer (MWS), also known as the two-
wavelength method (Andreas, 1989), can 
provide the fluxes of both H and LvE at kilometer 
scales. 
So far there have been only a few 
experiments with (optical) scintillometers 
operated over heterogeneous terrain (see e.g. 
Beyrich et al., 2002; Meijninger et al., 2002a) 
and only one experiment that systematically 
investigated the two-wavelength method using 
mircowave scintillometers (Meijninger et al., 
2002b). 
In this study we will present the results 
of scintillometer measurements carried out 
during the LITFASS-2003 experiment 
(Lindenberg, Germany) using three large 
aperture scintillometers (LAS) and one 
microwave scintillometer (MWS). The main goal 
of this study is to investigate the performance of 
the  
 
 
Corresponding author’s address: W.M.L. 
Meijninger, Meteorology and Air Quality Group, 
Wageningen University, Duivendaal 2, 6701 AP, 
Wageningen, The Netherlands; email: 
wouter.meijninger@wur.nl 
 
scintillometers in providing area-representative 
fluxes over a complex heterogeneous area. For 
that we will compare the scintillometer results 
against aggregated eddy-correlation (EC) 
measurements using a footprint model (following 
Meijninger et al., 2002a,b). In addition we will 
also discuss briefly some aspects that require 
further attention, such as uncertainty in the 
stability functions, outer scale effects, saturation 
and the correlation between temperature and 
humidity (RTq) that plays an important role in the 
two-wavelength method. 
 
2. EXPERIMENTAL SETUP 
 
The orography of the Lindenberg area 
(20×20km2) has been formed by inland glaciers 
during the last ice age and is characterized by 
slight, irregular undulations and a number of 
small lakes. The land use consists of 43% forest, 
45% farmland, 7% open water and 5% villages, 
making it an ideal complex area for testing the 
scintillation technique. 
 
 
Figure 1: The Lindenberg area and the 3 
scintillometers paths (1: Forest-LAS, 2: XLAS, 3: 
LAS-MWS). The white areas are open water, dark 
areas farmland and light gray areas forest.  
One LAS together with the MWS were 
installed over a path of 4.7km (effective height 
43.6m). The source area of these scintillometers 
consists roughly of 10% forest and 90% 
farmland. A second LAS was installed over a 
homogenous forest (path length 3km, effective 
height 46.8m). Finally, an extra large aperture 
scintillometer (XLAS) was set-up over a path 
length of 10.8km (effective height 69.7m). The 
source area of the XLAS consists roughly of 
70% forests and 30% farmland. The 
scintillometer paths are shown in Figure 1. 
 
3. THEORY 
 
3.1 The Scintillation Method 
 
The LAS and XLAS scintillometers 
operate at a near-visible wavelength (λ = 930 
nm), making them most sensitive to turbulent 
temperature fluctuations. The microwave 
scintillometer operates at a wavelength of 3.2 
mm (94 GHz). At this wavelength also turbulent 
water vapor fluctuations are important. By 
applying the Monin-Obukhov Similarity Theory 
(MOST) the fluxes of sensible heat (H) can be 
derived from the LAS and XLAS measurements 
and from the LAS-MWS combination (two-
wavelength method) also the latent heat flux 
(LvE). In the MOST procedure we used local 
wind speed data to determine the friction velocity 
and net radiation plus soil heat flux data for the 
moisture correction (LAS and XLAS only). For 
the two-wavelength method we used in-situ RTq 
data from an EC system (at same height as 
LAS-MWS). 
There are a number of expressions for 
the stability function that is required in the MOST 
procedure (Andreas, 1988; De Bruin et al., 
1995). For example in the free convective limit 
the function proposed by De Bruin gives a 16% 
higher flux than using the function proposed by 
Andreas. In this study we have taken the 
average of both expressions to derive the 
average fluxes of sensible heat and latent heat 
plus their uncertainty (see error bars in figures in 
section 4). Furthermore we included 
uncertainties of all required input parameters 
and data in the error bars. 
 
3.2 Footprint Analysis 
 
In order to validate the fluxes of sensible 
heat and latent heat of the scintillometers that 
are installed over mixed areas (i.e., the LAS-
MWS combination and the XLAS) against the in-
situ EC measurements we used an analytical 
footprint model. Considering the atmospheric 
conditions (wind direction and atmospheric 
stability) we determined the source area of the 
scintillometers (see Meijninger et al., 2002a,b). 
Finally, by combining the source area with the 
spatial weighting function of the scintillometer 
and the land use map (Figure 1) we have 
determined the relative contribution of each 
surface type, namely forest, open water and 
farmland (as a function of wind direction and 
stability). 
 
4. RESULTS 
 
4.1 Forest LAS 
 
In Figure 2 the sensible heat fluxes of the LAS 
are plotted against the EC measured fluxes. In 
the figure all data of the experiment are shown 
for both daytime and nighttime periods. As 
mentioned before, the error bars show the 
uncertainty of the LAS fluxes, which is mostly 
related to the consensus in the stability functions 
and uncertainty in the used zero-displacement 
height. The figure shows that the LAS gives 
slightly higher values than the EC method. 
 
-100
0
100
200
300
400
500
600
-100 0 100 200 300 400 500 600
H EC [W m-2]
H
LA
S 
[W
 m
-2
]
1:1
y = 1.22x
R2 = 0.95
Figure 2: Sensible heat flux of the Forest-LAS 
(plus tolerances) plotted against the EC measured 
fluxes of sensible heat (Day number 139-168). 
In Figure 3 the results of one of the 
Golden days (Day number 153) are shown. 
Again it can be seen that the LAS gives slightly 
higher fluxes than the EC system. Interesting to 
see is that both Figures show that also during 
night-time (stable) periods the LAS performs 
very well.  
-100
0
100
200
300
400
500
600
700
800
30 300 530 800 1030 1300 1530 1800 2030 2300
Time [UTC]
Fl
ux
 d
en
si
ty
 [W
 m
-2
]
Rn G H (LAS) H (EC)
 
Figure 3: Diurnal cycle of the sensible heat flux 
measured by the LAS and EC system plotted 
against time together with the net radiation (Rn) 
and soil heat flux (Gs) (Golden day number 153). 
 
4.2 LAS-MWS 
 
Unfortunately the MWS of the LAS-MWS 
system broke down during the experiment. 
Therefore we can present only the first 10 days. 
In Figure 4 the sensible heat flux results of the 
LAS-MWS system (unstable conditions only) are 
plotted against the area-averaged fluxes of the 
estimated source area. The latter results are 
derived from the in-situ EC measurements using 
a footprint model. 
-50
0
50
100
150
200
250
-50 0 50 100 150 200 250
H average  [W m-2]
H
LA
S-
M
W
S 
[W
 m
-2
]
1:1
y = 1.07x
R2 = 0.81
 
Figure 4: Sensible heat flux (plus tolerance) of the 
LAS-RWS combination plotted against the area-
averaged flux of sensible heat (Day number 139-
149, unstable periods only). 
It can be seen that the both results 
agree very well. In Figure 5 the latent heat fluxes 
are compared. This Figure shows that the LAS-
MWS system gives higher fluxes than the 
aggregated EC measurements. It must be noted 
that most EC systems show a gap in the energy 
balance closure while the LAS-MWS system 
does not (see Figure 6). Since the MWS is most 
sensitive to eddies in the order of the first 
Freznel zone ( 4m≈= LF λ ) it is also 
possible that the MWS is more sensitive to outer 
scale effects (that might be related to surface 
heterogeneities) than the LAS 
( 0.06m≈= LF λ ). The influence of 
absorption at 94 GHz is very small and therefore 
not considered here (A. Lüdi, pers. 
communication) 
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350
L v E source-area  [W m-2]
L v
E
LA
S-
M
W
S 
[W
 m
-2
]
1:1
y = 1.41x
R2 = 0.75
Figure 5: Latent heat flux of the LAS-MWS 
combination (plus tolerances) plotted against the 
area-averaged flux of latent heat (Day number 139-
149, unstable periods only). 
-50
50
150
250
350
450
550
-50 50 150 250 350 450 550
R n  - G s  [W m-2]
( H
 +
 L
vE
) L
A
S-
M
W
S 
[W
 m
-2
]
1:1
y = 0.96x
R2 = 0.66
Figure 6: Energy balance closure of the LAS-MWS 
system compared to the available energy (Rn– Gs). 
4.3 XLAS 
 
In Figure 7 the XLAS fluxes are 
compared with the aggregated EC 
measurements. It can be seen that the XLAS 
fluxes systematically underestimate the EC 
fluxes. This phenomenon is known as saturation 
of the XLAS signal. Here it is a combined result 
of the long path length of the XLAS and the high 
sensible heat flux of the forest (up to 600 W.m-2). 
 
-50
50
150
250
350
450
-50 50 150 250 350 450
H sourec-area [W m-2]
H
XL
A
S 
[W
 m
-2
]
XLAS
XLAS corrected 1:1
y = 1.02x
R2 = 0.72
 
Figure 7: Non-corrected and corrected sensible 
heat flux of the XLAS (plus tolerances) plotted 
against the area-averaged flux of sensible heat 
(Day number 139-168). 
 
As long as the saturation stays small 
(i.e. no super saturation occurs) it is possible to 
correct for the saturation effect. Here we used 
the correction of Kohsiek et al. (2002), which is 
based on the saturation criterion of Frehlich and 
Ochs (1990): 
 ( ) XLASXLAScorrXLAS HHH 002.01_ +=             (1) 
 
In Figure 7 the corrected XLAS fluxes 
are shown. It can be seen that the corrected 
fluxes agree much better with the averaged EC 
fluxes. Unfortunately, there is still no consensus 
about the saturation point, and the following 
correction (Kohsiek et al., 2002). For example, 
according to the saturation criterion of Frehlich 
and Ochs the LAS of the LAS-MWS system and 
the Forest-LAS should also experience 
saturation. However, the results don’t show this 
(Figure 2 and 4), indicating that the criterion of 
Frehlich and Ochs might be too strict 
 
5. CONCLUSIONS & FUTURE PLANS 
 
The results demonstrate that the 
scintillometers give fluxes of sensible and latent 
heat that are comparable to the aggregated EC 
fluxes and that the scintillation method can be 
applied over heterogeneous areas (see also 
Beyrich et al., 2004, see 9.1). Aspects that we 
will focus on are the impact of RTq on the LAS-
MWS system (using the LES results of 
Uhlenbrock et al., 2004, see 9.3), saturation of 
the optical scintillometers, the influence of outer 
scale effects on the MWS, and the application of 
scintillometers during night-time (stable) periods. 
 
ACKNOWLEDGEMENTS 
 
This study was supported by the Dutch 
Science Foundation (NWO project 813.03.007). 
 
REFERENCES 
 
Andreas, E.L.: 1988, Estimating Cn2 over 
Snow and Sea Ice from Meteorological Data, J. Opt. 
Soc. Am. 5, 481-495. 
Andreas, E.L.: 1989, Two-Wavelength Meth-
od of Measuring Path-Averaged Turbulent Surface 
Heat Fluxes, J. Atmos. Oceanic. Tech. 6, 280-292. 
Beyrich, F., De Bruin, H.A.R., Meijninger, 
W.M.L. and Schipper, F.: 2002, Experiences from 
One-Year Continuous Operation of a Lareg Aperture 
Scintillometer over a Heterogeneous Surface, 
Boundary-layer Meteorol. 105, 85-97. 
De Bruin, H.A.R., Van den Hurk, B.J.J.M. 
and Kohsiek, W.: 1995, ‘The Scintillation Method 
Tested over a Dry Vineyard’, Boundary-Layer 
Meteorol. 76, 25-40. 
Frehlich, R.G. and Ochs, G.R.: 1990, Effects 
of Saturation on the Optical Scintillometer, Appl. 
Optics 29, 548-553. 
Kohsiek, W., Meijninger, W.M.L. and De 
Bruin, H.A.R.: 2002, Long Range Scintillometry, 15th 
Symposium on Boundary Layer and turbulence, 15-19 
July ‘02, Wageningen, The Netherlands, pp. 125-128. 
Meijninger, W.M.L., Hartogensis, O.K., 
Kohsiek, W., Hoedjes, J.C.B., Zuurbier, R.M. and De 
bruin, H.A.R.: 2002, Determination of Area-Averaged 
Sensible Heat Fluxes with a Large Aperture 
Scintilometer over a Heterogeneosu Surface – 
Flevoland Field Experiment, Boundary-Layer 
Meteorol. 105, 37-62. 
Meijninger, W.M.L., Green, A.E., Harto-
gensis, O., Kohsiek, W., Hoedjes, J.C.B., Zuurbier, 
R.M. and De Bruin, H.A.R.: 2002b, Determination of 
Area-Averaged Water Vapour Fluxes with Large 
Aperture and Radio Wave Scintillometers over a 
Heterogeneous Surface – Flevoland Field Experiment, 
Boundary-Layer Meteorol. 105, 63-83. 


Scintillometer fluxes of sensible and latent heat over a heterogeneous area - a contribution to LITFASS-2003

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