Observing the Microstructure of Boundary Layer Clouds using a Holographic Imager on a Tethered Balloon

Predicting the formation, dissipation and location of fog and boundary layer clouds represents a challenge for state-of-the-art numerical weather prediction (NWP) models. NWP models have major difficulties in predicting boundary layer clouds because of a poor understanding and representation of the interaction of numerous physical processes spanning a wide range of spatial and temporal scales (e.g. nucleation, droplet growth, radiative processes, turbulence, mesoscale circulation) and the lack of high-resolution observational data in the planetary boundary layer (PBL).

To overcome the gap in observations within the PBL between ground-based measurements and manned airplanes, we have developed a novel measurement platform with our holographic cloud imager (HOLIMO) on a 175 m³ tethered balloon (HoloBalloon). First vertical profiles of the phase-resolved cloud properties of boundary layer clouds up to 700 m altitude above the surface were obtained over the Swiss Plateau in February 2018. The vertical profiles provide detailed information about the microphysical cloud structure and the meteorological conditions (T, RH, 3D wind). This information is crucial to identify and understand the key physical processes and their interactions during the fog formation, development and dissipation phase, as the droplet size distribution and liquid water content depend on the phase of the fog life cycle. The ability of our holographic instrument to measure the three-dimensional spatial distribution of cloud particles allows investigating the small-scale cloud structure on a mm-scale and thus studying entrainment and turbulent mixing at the cloud edges.

This is the first time that a holographic cloud imager is deployed on a tethered balloon. Digital in-line holography has the advantages of a well-defined and velocity-independent detection volume and enables a three dimensional view of the cloud structure. In combination with the low inflow velocities of a tethered balloon system, a high spatial resolution below 1 m is possible, which enables valuable insights into the small-scale microstructure of clouds. Thus, this measurement platform cannot only be used to improve our understanding of fog microphysics but also in general to address some fundamental open questions in cloud physics.