Turbulence measurements with balloons
Measurement-principle with two combined systems
Low resolution wind measurements (vertical resolution: 10 m) result from the GPS wind-finding system of a normal radiosonde. An anemometer onboard the payload measures the flow around the payload, i.e. the vertical wind shear between the wind at the balloon height (v1) and the mostly deviant wind at the height of the payload gondola (v2). The combination of systems is visualized in the top Figure on the right. At IAP we measure this wind shear with a hot-wire-anemometer and deduce turbulence parameters from a spectral analysis of the wind fluctuations.
The measurement principle of hot wire anemometers (also called constant temperature anemometers, CTA) is based on the cooling effect caused by atmospheric air flow passing the wire. The cooling effect is balanced by the electrical current to the wire using a Wheatstone bridge. Thus the wire is held at a constant temperature (e.g. 245 °C in our case). The change in the current shows up as a voltage variation at the anemometer output and directly depends on the wind fluctuations. These data are sampled at a rate of 2000 Hz which refers to a vertical resolution of 2.5 mm assuming a balloon ascent rate of 5 m/s. The platinum plated wire (Type 55P03, Dantec Dynamics) is 5 μm in diameter and 1.25 mm long (Fig. 1).
The measurement axis of the anemometer is horizontal, to measure horizontal wind fluctuations (Fig. 2). The data are saved on board and also transmitted to a ground antenna.
Since December 2007 we successfully launched several balloons at Kühlungsborn (54 °N, 12 °E). In October 2008, 2009 and 2011 the sensor has been flown as part of the BEXUS balloon payload from Kiruna (67 °N, 21 °E). The lower Figure on the right shows an example of the voltage fluctuations obtained during the BEXUS 6 flight for the altitude region between 16.3 km and 16.6 km measured by the hot wire anemometer. We removed the spline trend from the signal to eliminate low frequency disturbing effects caused e.g. by the movement of the gondola. One can clearly distinguish layers of turbulence from regions were no disturbances are observed. This implies that the observed fluctuations are due to atmospheric velocity fluctuations and are not an instrumental artefact. The small remaining fluctuations in the non-turbulent region are due to instrumental noise.