Climate models and weather predictions still suffer from an insufficient understanding of cloud processes such as moist convection and cloud formation . The vast range of spatio-temporal scales of atmospheric and cloud dynamics, which is due to its turbulent nature, is a major challenge in resolving such processes . However, turbulence is conjectured to be at the core of many small-scale processes in clouds such as mixing and rain initiation in warm clouds [19, 26]. Fundamental questions are:
• How does turbulent mixing due to turbulent entrainment inside a cloud evolve as a function of time and space? Is the mixing predominantly homogeneous or inhomogeneous?
• How does turbulence affect relative droplet velocity statistics and distribution of droplets in space?
• If globally present in clouds, how important is mixing and inertial clustering?
• How do cloud microphysics vary in time and space?
Providing answers to these questions by in-situ measurements requires the smallest relevant scales of the turbulent flow and cloud features to be resolved. Therefore, the Mobile Cloud Observatory (MCO) specially developed an airborne scientific instrument, the Max Planck Cloudkite+ (MPCK+) as shown in Fig. 2.1. The interested reader is referred to  for a detailed description of both the helikite, the MPCK+ and the operational procedure whereas only a short summary is presented here.
The MPCK+ is specially developed for measuring atmospheric turbulence and cloud microphysical quantities simultaneously. Therefore, the MPCK+ is equipped with instruments that measure cloud droplet sizes, shape and spatial distributions as well as the atmospheric state (air temperature and relative humidity) and wind (Fig. 2.1). The remotely controlled MPCK+ is powered by a battery that lasts for about 30 min with all imaging instruments running and several hours with only non-imaging instruments running. The imaging instruments are the Particle Image Velocimetry (PIV) and holography system (Fig. 2.1), which sample 1.6 L/s and 1.7 L/s of air, respectively. Decisive for the MPCK+ design is to fully capture the coupling between cloud microphysics and turbulence. This is why the overlap of sampling volumes between, especially, the holography (droplet sizes and three-dimensional spatial distributions) and the PIV (two-dimensional droplet velocities and two-dimensional droplet spatial distribution) is crucial. This overlap is illustrated in Fig. 2.1 by the green
Figure 2.1 Visualization of the Max Planck Cloudkite + (MPCK+) and its main scientific instruments: the combined holography and PIV unit. The PIV is operated at 15 Hz with a probing volume of 23 cm×15 cm×0.3 cm (green laser sheet) whereas the holography is operated at 75 Hz with a probing volume of 1.5 cm×1.5 cm×10 cm (violet laser column).
The Fast Cloud Droplet Probe (Fast CDP) measures cloud droplet sizes in the range 1.5 µm to 50 µm. The Laser Doppler Velocimetry sensor (LDV) was not operated during EUREC4A . The wind speed is measured by a pitot tube at 100 Hz. Fluctuations of the wind speed and air temperature are measured by a hot- and cold-wire, respectively, at ∼10 kHz. Relative humidity and air temperature are measured at <10 Hz.
Figure 2.2The 250 m3 helikite at the airfield next to the Max Planck Institute for Dynamics and Self-organization (A) and at the rear of RV Maria S. Merian during EUREC4A (B). A:
Besides the scientific instrumentation, the main components of the setup are the winch, the main line, the backup line and the 250 m3 helikite. Initially, it was planned to mount the MPCK+ at the keel of the 250 m3 helikite. B: The MPCK+ is hang from the 250 m3 helikite at the front during EUREC4A .
laser sheet (PIV, wavelength 532 nm) and violet laser column (Holography, wavelength 355 nm).
The MPCK+ is carried by a tethered helikite consisting of a 250 m3 helium-filled balloon and a kite where the keel is 15 m long (Fig. 2.2). The helikite can lift a net payload of 50 kg at an altitude of 1 km above mean sea level (MSL). At the airfield next to the Max Planck Institute for Dynamics and Self-organization (Fig. 2.2A), the 250 m3 helikite is held by a main tether (“main line”) and a backup tether (“backup line”). The main tether is guided by a main pully and spooled by a large winch, which is anchored to the ground. Originally, the MPCK+ was planned to be mounted at the keel of the 250 m3 helikite. During the EUREC4A field campaign, the MPCK+ was hung from the helikite at the front in order to simplify the attachment and de-attachment process (Fig. 2.2B). Being an airborne instrument that is pulled by a research vessel, it is possible to sample individual clouds of interest with the MPCK+. Another advantage of tethered aerostats is the low relative wind speed. Together with the high sampling rate of the imaging instruments, the low relative wind speed allows for measurements of cloud features at very high spatio-temporal resolution while the smallest resolved scales are comparable to the Kolmogorov scales of the turbulent flow. As an example, the cloud edge can be resolved with an accuracy below the low decimeter scale at relative wind speeds of ∼10 m/s. Furthermore, it is possible to continuously sample the cloud volume with both PIV and holography if the relative wind speed <1 m/s.
To summarize, the MPCK+ is a mobile instrument that can be operated in remote regions of the globe. The MPCK+ is equipped with a, to my knowledge and up to now, unique PIV/holography combination to acquire highly resolved measurements inside clouds. The MPCK+ measurements allow for unraveling cloud droplet - turbulence interactions, e.g. the existence of spatial clustering, which is crucial for understanding
rain formation in warm clouds and their radiative characteristics.