Comparison of in situ UAV Turbulence Measurements with Tower and Scintillometer Data in LITFASS-2009

During the field campaign LITFASS-2009 (1 to 22 July, 2009, near Lindenberg, Germany) several instruments were applied to measure the atmospheric boundary-layer (ABL) structure and turbulent transport during daytime over heterogeneous terrain under cloudy conditions. Main focus was the representativeness of tower and large-aperture scintillometer (LAS) measurements in comparison with airborne measurements on a comparatively small scale (1 to 5 km horizontally). Although it is known that statistics of such measurements are usually poor, for many meteorological applications it is necessary to obtain ABL structure and vertical turbulent fluxes on a very local scale.

We like to present first results of the recently completed measurements and focus on

1) vertical profiles up to 1200 m of wind, temperature and humidity obtained from aircraft, tower (up to 90 m) and remote sensing data;

2) vertical turbulent flux profiles in the lower ABL and surface layer obtained from tower and airborne measurements;

3) turbulent surface fluxes and turbulent structure parameters obtained from ground stations, LAS and aircraft.

Technically, we like to focus on the UAV instrumentation. The meteorological mini aerial vehicle (M²AV) T200 'Carolo' is a self-constructed model plane with two electric engines and a wingspan of two metres. The maximum take-off weight is 6 kg, including 1.5 kg of payload. It is hand- or catapult-launched which makes handling and operation easy.

With an endurance of approximately 50 minutes, the range accounts for 60 km at a cruising speed of 22 m/s.

For the mounting of the meteorological sensors a nose boom was constructed to minimise the aircraft's influence on the

measurements and to get the sensors positioned close to each other. The meteorological instrumentation consists of

- a five-hole probe (5HP) for fluid-dynamic measurements in the coordinate system of the aircraft;

- GPS and INS for navigation and attitude measurements necessary to transform the fluid-dynamic measurements into the earth's coordinate system and to calculate the turbulent wind vector;

- two temperature sensors (fast and slow);

- a humidity sensor (humicap);

- a laser and a pressure altimeter.

Turbulence measurements are sampled at 100 Hz, resulting in a resolution (after anti-aliasing filters) of about 33 Hz (except for humidity data). Structure functions and power spectra demonstrate that Kolmogorov's inertia range of locally isotropic turbulence is represented without systematic errors at least up to this frequency.

All measurements are stored aboard the M²AV and post-processed after landing. The operator is able to communicate with the aircraft via a data link within 5 km range, i.e. for status request and for update of way points, altitude etc.

The research UAV is controlled by an autopilot - it is not remotely controlled by a human pilot on the ground - and operates therefore automatically. This allows for measurement flights in the lower ABL over larger distances outside the range of sight and in remote areas. E.g., in 2007 measurements with M²AVs over the ice edge in Antarctica were performed as well as in the stable ABL at night. Also the autopilot is able to keep altitude and track much more precisely than a human pilot can do usually.