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Abstract

On the 30th of January 2021, a research aircraft equipped with Optical Array Probes (OAPs), a Counterflow Virtual Impacter (CVI), and W-band cloud radars, performed a research flight in snow conditions above the La Chaux-de-Fonds airport where ground-based weather radars and a Multi-Angle Snowflake Camera were simultaneously operated. 28 flight legs were performed between 1500m and 3600m in the same cloud system, which originated from the passage of a cold front, with varying cloud top height between 8000m and 6000m. The unique sampling strategy applied in the first 15 legs and the use of an innovative shape recognition algorithm based on convolutional neural networks applied to OAP (2DS and PIP probes) data, allows for the investigation of columnar growth, aggregation, riming, and melting in the temperature range between -10 and +1 °C. Efforts are made to compute columnar growth rates which were compared with relevant laboratory experiments from the literature. The size distributions of compact, graupel-like particles and highly rimed aggregates across the explored flight altitudes are analyzed and provide critical information on the influence of riming. The presence of millimetric hexagonal plates and dendrites, formed around –15 °C, shed light on the microphysical processes happening above the sampled temperatures. Finally, the question of aggregation is explored through the evaluation of the influence of turbulence and ice crystal concentrations.