The helicopter-borne ACTOS for small-scale cloud turbulence observations

A new helicopter-borne measurement payload for cloud turbulence observations is introduced. Since most in-situ measurements in clouds are made by fast-flying research aircraft the spatial resolution of turbulence data is limited to the order of meter. As a consequence the autonomous measurement payload ACTOS (Airborne Cloud Turbulence Observation System) was developed for the use beneath of a tethered balloon. This setup was successfully used for several years and deployed in many international field campaigns. The spatial resolution of the wind vector, temperature, and liquid water content measurements in clouds was below 10 cm. Additionally, cloud droplet measurements are made to estimate the droplet size distribution and particle number concentration. However, the ceiling of the balloon was limited to approximately 1500 m - often too low for boundary layer clouds. To increase the flexibility and the ceiling of the setup, ACTOS was modified in 2005 for the use beneath a slow-flying helicopter. With this setup measurements in cumulus clouds and in stratiform cloud layers are possible. To circumvent the influence of the down-wash of the rotor blades ACTOS is fixed to the helicopter's external cargo hook with a 140 m long rope. In this paper the technical details and instrumentation of the new ACTOS are discussed but also the requirements and capabilities for using a helicopter for cloud research. Results from the first helicopter-borne cloud experiment illustrate the unique capabilities of the new system.