Shallow Cumulus Clouds Under a High-Speed 3D Microscope: Results from Airborne Holography Measurements During EUREC4A

We present the results of holographic in-situ measurements in shallow cumulus clouds collected with the tethered aerostat Max Planck Cloudkite (MPCK) during the EUREC4A campaign.

The MPCK holography system achieves a total volumetric sample rate of 100cm^3 per meter flight providing unprecedented insights into three-dimensional cloud structure. We achieve this through the high sampling frequency of the MPCK holographic setup of 75 Hz, combined with the low true airspeed of the aerostat, which leads to a separation on the order of about 10 cm between consecutive holograms. Each hologram has a measurement volume of 10 cm^3 and samples a high number of cloud droplets. Nominally, we can resolve droplets down to 6µm in diameter, however, detection is reliable for droplets of 8µm diameter or larger.

Cloud microphysics properties such as droplet size distribution, liquid water content, and clustering can be calculated at sub-meter spatial resolution. With this, we get a well-resolved horizontal snapshot of the microphysics of a cloud. To process the vast amount of holographic data collected, we have developed new analysis techniques based on convolutional neural networks and verified size and concentration measurements using a novel method.

We present the new processing and verification methods and reveal based on our holographic data the three-dimensional structure of shallow cumulus clouds including the degree of clustering and inhomogeneity.