Fourth Study Conference on BALTEX Scala Cinema Gudhjem

( z )
∂H
H = H f − z f ⋅
∂z
0 (4)
with flight level zf .
Thus the combination of low-level flights with an inverse
model (LLF+IM) allows the determination of the areaaveraged
turbulent surface fluxes from square-shaped flight
patterns at only one low altitude (e.g. at 100 m or
less)without any supporting data from other systems.
4. Verification in LES
Simulated flight measurements over homogeneous and
heterogeneous terrain in a LES (Schröter et al. 2000) were
consulted to verify the LLF+IM method. Area- and timeaveraged
turbulent fluxes were derived directly from the
LES model. These 'true' data were then used to quantify
statistical and systematic errors of the inverse method.
Within the LES model virtual measurement flights at five
altitudes within the artificial CBL were performed using a
3D box pattern, Both the mean fluxes and the extrapolated
(using LLF+IM) surface fluxes agreed very well with the
given 'true' data.
Figure . 2 : Sensible heat flux measured with Do 128
and Helipod during the LITFASS-98 joint field
experiment.
5. LITFASS-98 Measurements
During the LITFASS-98 experiment (Beyrich et al. 2002)
airborne measurements using the helicopter-borne
turbulence probe Helipod (Bange and Roth 1999) and the
research aircraft Do 128 (Corsmeier et al. 2001) were
performed to determine area-averaged vertical turbulent
fluxes. Both systems are equipped with sophisticated sensors
to measure turbulent fluctuations and fluxes very precisely.
The experimental site near the meteorological observatory at
Lindenberg / Germany was characterized by its strong
heterogeneity. Both airborne systems flew synchronously
3D box patterns. The area-averaged fluxes (Fig. 2) agreed
very well at common flight levels. The linear extrapolation
to the ground led to values within the statistical uncertainty
of ground-based measurements.
The analysis of only the lowest flight levels yield even better
results: Using the LLF+IM method (blue lines) very similar
vertical gradients of the heat flux were calculated for both
airborne systems. With only a small discrepancy of less than
10 Wm -2 to each other, the extrapolated lines calculated with
LLF+IM matched the ground-based micro-meteorological
stations and the scintillometer, as well as the upper tower
measurements.
-68-
6. Tharandt 2001 Experiment
During the Tharandt experiment in the framework of the
STINHO-1campaign a low-level grid pattern was flown
with the Do 128 research aircraft. The site consisted
mainly of a large forest surrounded by grassland and
agriculture. The difference in surface temperature was
clearly visible in the infra-red measurements performed by
the Do 128. Due to its larger surface roughness it was
expected that the forest developed larger sensible heat flux
than the surrounding grass, even though the surface
temperature of the latter was significantly higher. To
prove this thesis the LLF-IM method was applied to the
airborne measurements at about 150 m above the ground
(Fig. 3). Although the heat fluxes measured at the flight
altitude differed only by a few Wm -2 from each other, at
the surface the flux above the forest was about 10 Wm -2
larger than above the surrounding grassland. This was due
to the clearly larger vertical flux gradient that belonged to
the forest. At about 350 m above the ground the
extrapolated flux profiles of grassland and wood were
united. Whether this is a kind of a turbulent flux blending
height will be subject to further research.
Figure . 3 : Sensible heat flux measured with the Do
128 aircraft over Tharandt forest and grassland in
2001.
References
Bange, J. and R. Roth, 1999: Helicopter-Borne Flux
Measurements in the Nocturnal Boundary Layer Over
Land - a Case <strong>Study</strong>. Boundary- Layer Meteorol., 92,
295-325, 1999..
Beyrich, F., H.-J. Herzog, and J. Neisser: The LITFASS
Project of DWD and the LITFASS- 98 Experiment:
The Project Strategy and the Experimental Setup.
Theor. Appl. Climatol., 73, 3-18, 2002.
Corsmeier, U., R. Hankers, and A. Wieser: Airborne
Turbulence Measurements in the Lower Troposphere
Onboard the Research Aircraft Dornier 128-6, D-
IBUF. Meteor. Z., N. F., 4, 315-329, 2001.
Grunwald, J., N. Kalthoff, F. Fiedler, and U. Corsmeier:
Application of Different Flight Strategies to
Determine Areally Averaged Turbulent Fluxes. Beitr.
Phys. Atmosph., 71, 283-302, 1998.
Schröter, M., J. Bange, and S. Raasch: Simulated Airborne
Flux Measurements in a LES Generated Convective
Boundary Layer. Boundary-Layer Meteorol., 95, 437-
456, 2000.
Tarantola, A.: Inverse Problem Theory. Elsevier,
Amsterdam, 613 pp, 1987.
Wolff, M. and J. Bange: Inverse Method as an Analysing
Tool for Airborne Measurements. Meteor. Z., N. F., 9,
361-376, 2000.