Fine-scale thermodynamic and microphysical measurements in clouds are important for understanding turbulent mixing processes (e.g., entrainment) and cloud evolution (e.g., droplet spectra broadening). Experimental data on this topic with sub-meter spatial resolution are still rare due to the high true airspeed of typical research aircraft. To overcome this limitations the use of a tethered-ballone borne system has been suggested.

A new measurement payload carried by such a tethered balloon was developed: The system is called ACTOS (Airship-borne Cloud Turbulence Observation System): It was developed for in-situ measurements of dynamic, thermodynamic and cloud microphysical properties with sampling frequencies of at least 100 Hz, that is, a spatial resolution in the order of 10 cm at typical wind speed of 10 m/s is possible. This platform is equipped with a threedimensional ultrasonic anemometer, an ultrafast thermometer for temperature fluctuation measurements in clouds, a humidity measurement system based on a Lyman-alpha hygrometer with a special pre-impactor inlet and a Particle Volume Monitor (PVM) to determine the liquid water content in clouds. A 400 qm, hydrogen filled tethered balloon is used to carry the system up to a maximum height of 1200 m. A general description of the platform, sensor equipment and the balloon will be presented in the first part.

The balloon and ACTOS have been deployed during the BBC (Baltex Bridge Campaign) performed in Cabauw (The Netherlands) in September 2001: The results of these measurements with the improved version of the system are reported here. Beside general tests during this campaign data from measurements in a lifted fog layer are presented as a case study. This fog layer in between 300 and 500 m was present for several hours which enabled more than 20 vertical profiles. Data from the Cabauw tower and rawinsondes complete data set.