Campaign location and flight strategy
The field experiment took place in the Swiss city of La Chaux-de-Fonds, at 1020 m above sea level in the Jura mountain range. This location was identified based on practical and climatological constraints, to ensure that the aircraft and ground-based instruments could sample the appropriate temperature range (-10 to +2 °C) during some snowfall events.
The ground-based instruments were deployed in the city airport “Les Éplatures” between December 2020 and March 2021. In the second half of January 2021 (22/01 – 30/01), the ground measurements were complemented with flights from the scientific aircraft Safire ATR-42, equipped with in-situ probes and additional remote sensing instruments. The aircraft was stationed in Dijon (France), about 20 minutes of flight away from the campaign site.
The flight strategy was defined to sample in the predefined temperature range and capture as much as possible the variability of snowfall properties in the cloud/precipitation column. Taking into account also operational constraints, the selected strategy was to fly short legs (15-20 km long) at constant altitude in the snow column to document the vertical variability from -10°C level down to IFR (Instrument Flight Rules) minima.
Ground-based platform: remote sensing and in-situ
The ground-based instruments were located in three sites, in the vicinity of la Chaux-de-Fonds. In the main site (site #1), a total of 5 research radars were deployed, operating at X-, K- and W-band, either zenith-profiling or performing hemispherical scans in the direction of the landing track: BASTA-mobile (94 GHz profiler), WProf (dual-polarization 94 GHz Doppler spectral profiler), BALI (scanning mini-BASTA at 94 GHz coupled with a 808 nm lidar), MRR (24 GHz profiler), ROXI (9.48 GHz Doppler spectral profiler). In a secondary site 500 m away along the track, the Multi-Angle Snowflake Camera was deployed together with a 3D sonic anemometer, to capture images of snowflakes in free fall near the ground level.
Site #3 was located 4.8 km to the North-East: there, a scanning X-band polarimetric radar was performing RHIs (range-height indicators) in direction of the main site during precipitation events.
An instrumental payload was selected and integrated on the aircraft allowing for both in-situ measurements and remote sensing of snowfall conditions.
The in-situ payload consists of cloud microphysics instruments installed outside the aircraft on six under-wing stations and on fuselage hard points. These probes sample clouds and precipitation and provide high quality in-situ measurements of snow microphysics such as Total, Ice or Liquid Water Content (TWC/IWC/LWC), Particle Size Distribution (PSD) and particle concentrations with high spatial resolution (typically 100 m) along the aircraft trajectory as well as detailed measurements of snow particles’ physical properties (individual size and shape).
- The imaging probes (optical array probes) allowed to observe hydrometeors across the full size spectrum, with 2D-S and CIP covering smaller sizes, and PIP and HVPS capturing particle sizes up to 6.4 mm and 19.2 mm, respectively.
- Hot-wire probes measured snow bulk properties: a ROBUST probe (TWC), a Nevzorov probe (discriminates between snow and liquid water content) and the LWC-300 (LWC only).
- The CDP-2 scattering probe was installed for the characterization of cloud droplets both in terms of size distribution and bulk data.
- A counterflow virtual impactor was specifically adapted to measure ice and liquid water content in snowfall conditions with large hydrometeors.
A snow accretion monitoring system, conceived for the campaign, was integrated on the aircraft. It consists of a de-iced cylinder and a dedicated camera to record snow accretion during the flights and collect data for validation of numerical tools within ICE GENESIS.
The second core instrument deployed on the ART42 is the combination of the upward- and downward-looking multi-beam 95 GHz Doppler cloud radar RASTA and the side facing bistatic Doppler cloud radar BASTA. Thanks to its 3-beam configuration, RASTA allows to retrieve the three-dimensional (3D) wind field, as well as microphysical and radiative properties of clouds (IWC, visible extinction, particle size, terminal fall speed, and concentration).