The helicopter-borne ACTOS for small-scale cloud turbulence observations Holger Siebert , Katrin Lehmann , Manfred Wendisch, and Raymond Shaw Leibniz-Institute for Tropospheric Research (IfT), Leipzig, Germany Dept. of Physics, Michigan Technology University, Houghton, MI, USA 1. Motivation Clouds play a major role in the Earth system and are relevant to many aspects of climate and daily weather forecast. The dynamics of clouds span a wide range of spatial scales from the macroscopic cloud extension itself down to the Kolmogorov microscale (typically in the mm range for atmospheric conditions). Since cloud microphysical properties on larger scales are controlled by processes taking place on smaller scales, measure- ments with high spatial and temporal resolution are es- sential for a better understanding of cloud processes. The majority of airborne in-situ observations of clouds have been made by fast-flying research aircraft which limits the spatial resolution of most parameters to the meter scale or so. To overcome this limitation the Air- borne Cloud Turbulence Observation System (ACTOS) has been developed which was originally designed for the use beneath a tethered balloon (Siebert et al. 2003, 2006b). Due to the low true airspeed (TAS) of such a balloon-borne system the spatial resolution of the mea- surements is much higher compared with aircraft data. In this paper the new helicopter-borne version of ACTOS is introduced (see also Siebert et al. (2006a)). However, compared to balloon–borne measurements, which also meet the slow–flying criterion, a helicopter is even more advantageous due to its longer cruising range and pos- sible ceiling. A helicopter is more flexible in time and space than a balloon and can be chartered at different airfields. Furthermore there are fewer limitations with re- spect to possible payload (weight, size, available electri- cal power). siebert@tropos.de Besides a technical overview of ACTOS the pos- sibilities of using a helicopter for cloud research are discussed by showing measurement examples which clearly demonstrate the unique capabilities of the new system. 2. Experimental Setup ACTOS is an autonomous measurement payload which can, in principle, be carried by different platforms such as balloons, blimps, Zeppelin, or helicopters. The system is equipped with sensors to perform high-resolution mea- surements of meteorological standard parameters such as wind vector, air temperature, and humidity but also cloud and aerosol microphysical properties such as liq- uid water content ( ) and number concentrations of interstitial aerosol particles in boundary layer clouds. To keep the load for the carrier platform as low as pos- sible, a light-weight frame made from carbon-fiber and aluminum was designed. The total weight of ACTOS in- cluding the instruments is 200 kg. ACTOS is equipped with an autonomous power supply and data acquisition to be completely independent from its carrier platform. A data link between ACTOS and the helicopter cabin was installed to ensure on-line monitoring of standard param- eters during the flights. 2a. Sensor Equipment ACTOS was designed to provide collocated measure- ments of several types of parameters. All sensor outputs are sampled with a joint real-time data acquisition sys- tem to ensure precise temporal correlation between the 1