Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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The Helicopter-Borne Turbulence Probe Helipod in LITFASS Field
Campaigns: Strategies and Results
Peter Zittel, Thomas Spieß, and Jens Bange
Aerospace Systems, TU Braunschweig, Hermann-Blenk-Str. 23, 38110 Braunschweig – Germany. E-mail: p.zittel@tu-bs.de
1. Introduction
The LITFASS experiments (Lindenberg Inhomogeneous
Terrain – Fluxes between Atmosphere and Surface: a Long
– Term <strong>Study</strong>) within the German research programs
VERTIKO and EVA-GRIPS aimed at the spatial and
temporal variability of the turbulent fluxes of heat and
humidity above heterogeneous terrain. As experimental site
a 20 km x 20 km terrain near MOLindenberg was chosen,
which contains forest, fields, grassland, and lakes. The site
was considered to be typical for Central Europe and the
BALTEX region. The helicopter-borne turbulence probe
Helipod (Fig. 1; e.g. Bange and Roth 1999) performed a
total of 105 hours of measurements in 2002 and 2003. The
data sampled on various flight patterns can be used as a
reference for ground stations, remote-sensing systems, and
numerical models.
Figure 1. The helicopter-borne turbulence measurement
system Helipod is an autonomously operating sensor system,
attached to a 15 m rope. It operates at 40 m/s air speed and
measures wind vector, temperature, and humidity at a rate of
100 Hz.
2. Flight Pattern
The turbulent structure of the atmospheric boundary layer
(ABL) was examined using several flight patterns like the
low-level Grid (flat convection and reference data), Big Grid
/ Extended Big Grid (area–averaged turbulent fluxes,
calibration of German Air Force Tornado infrared images),
Vertical Grid, 3D-Box, Catalog, Stations (reference for the
ground stations), and Fishbone (calibration of numerical
models). Additionally vertical profiles up to 1500 m were
performed. The flights were carried out in various situations
(e. g., well-mixed layer, flat convection in the developing
ABL, ground-based inversion) all above heterogeneous
terrain.
Catalog flights (Fig. 2) were performed at noon and the
early afternoon at different altitudes in the ABL above
different surface types like grassland, forest, lake, and a mix
of all. The length of the legs was approximate 8 to 14 km.
This flight pattern was used to determine turbulent fluxes
and their length-scale (MR-Decomposition and wavelet
analysis) above homogeneous section.
The flight pattern 3D-Box (Fig. 3) was flown at three
heights during the early afternoon in the convective ABL.
The length of these legs was 10 km. The 3D-Box was used
to determine area-averaged turbulent fluxes and other
statistics and spectra at different heights. Additionally,
vertical latent and sensible heat flux were determined with
the inverse method (Bange et al., 2002, Zittel et al., 2003)
for comparison (Fig. 7).
Figure 2. The flight pattern Catalog above homogeneous
patches within the heterogeneous terrain. Along the flight
path the surface temperature is plotted.
Figure 3. The flight 3D-Box: A square-shaped flight
pattern of 10 km x 10 km with square-shaped flights
between 140 m and 700 m altitude.
The flight pattern Vertical Grid (Fig. 4) was flown at noon
and early afternoon at several altitudes within the ABL.
The 12 km legs were oriented in the mean wind direction
in order to compare Helipod and LIDAR flux
measurements within the boundary layer.