Putting the spotlight on small cloud droplets with SmHOLIMO - a new holographic imager for in situ measurements of clouds

Abstract

The microstructure of liquid and mixed-phase clouds is characterized by the cloud droplet size distribution (CDSD), which influences the cloud evolution and its interaction with radiation. However, state-of-the-art cloud probes still face challenges because they require either platforms that move at constant speed or inlets that can directly alter the actual CDSD. Therefore, precise and accurate in situ measurements of CDSDs, especially of cloud droplets smaller than 6 μm, are still lacking. This can lead to uncertainties in the microphysics and thus in weather and climate models, which are based on parameterizations often derived from these measurements. We present a new in situ instrument, the Small Holographic Imager for Microscopic Objects (SmHOLIMO), specifically designed to measure a broad spectrum of the CDSDs, i.e., from 3.7 to ≈100 μm, with a sample volume rate of 2.5 cm3s-1. Thereby, SmHOLIMO pushes the resolution limit towards the limits seen with forward-scattering probes, while still maintaining the advantages of open-path holography, i.e., a well-defined sample volume (operation at variable wind speed); no need for an inlet; independence of particle size, phase, refractive index, and shape; and the potential of spatial analyses. After calibrating SmHOLIMO in the laboratory, the instrument was deployed in the field, on a tethered balloon system, probing a dissipating low stratus. We demonstrate its ability to measure the cloud microphysical properties at high spatio-temporal resolution. Furthermore, we compare the SmHOLIMO observations to those of another holographic imager (resolution: 6 μm) and to co-located remote sensing measurements. We unequivocally show the importance of SmHOLIMO's skills to capture the lower tail of the CDSD, which significantly affects the derived quantities of cloud droplet mean diameter (up to 1.6 times smaller), number concentration (up to 4 times higher), and cloud optical depth (up to 2.7 times higher). SmHOLIMO's high-resolution in situ data of cloud droplets will help us to better interpret observations and to refine the representation of clouds in climate and weather models.