Turbulence in clouds covers a large range of spatial scales, which contribute to the different cloud processes. Starting with scales comparable to the cloud dimension itself (~ 100 - 1000 m) down to the dissipation scale (~1 mm), where kinetic energy dissipates into heat, yields a broad range of spatial scales of roughly six orders of magnitude. This makes turbulence measurements quite difficult and prevents that all scales can be resolved in a single experiment at the same time.

The influence of atmospheric turbulence on the different cloud processes is a key topic in the current discussion on cloud physics. For example, on large scales unsaturated environmental air is mixed into the clouds at their edges and at cloud top (so-called "entrainment"). On smaller spatial scales further turbulent mixing finally leads to the homogenization of the affected cloud areas. The mixing processes have significant influence on the thermodynamic properties of the cloudy air in which cloud droplets develop and grow to larger sizes finally ending as drizzle or rain drops. On even smaller scales comparable with the size of cloud droplets itself turbulence interacts directly with the droplets which results in an increased droplet collision efficiency and has, therefore, consequences on further droplet growth and the onset of precipitation.

Since most turbulence measurements in clouds are done by aircraft with typically high true air speed (TAS), the spatial scales which can be resolved are in the order of meters. Assuming a TAS of 100 m/s, sampling frequencies in the order of 100 kHz are needed for turbulence sensors to resolve the dissipation scale. This is, even for laboratory experiments, a challenge and currently not possible on aircraft. Furthermore, often the different sensors are spread on the aircraft instead of being collocated which makes correlations of different parameters on small scales questionable.

To overcome this limitations the cloud turbulence measurement system ACTOS (Airborne Cloud Turbulence Observation System) has been developed for high-resolution measurements in clouds. ACTOS is an autonomous measurement payload including sensors for turbulence (three-dimensional wind vector, temperature, and humidity) but also for cloud microphysical parameters such as droplet number concentration and droplet size distribution. The photo shows ACTOS in its current version in front of a helicopter. In this combination measurements in clouds are possible up to a height of about 3000 m. ACTOS is carried by the helicopter with a 140 m long rope fixed at the external cargo hook and can be dipped into the clouds from above. Therefore, with a TAS of 15 m/s (in this special configuration) high-resolution turbulence measurements unbiased from the helicopter downwash are possible. In addition, there are fewer complications for fast-response sensors which often occur on fast-flying research aircraft.