Abstract
Abstract. The seeder–feeder mechanism has been observed to enhance orographic precipitation in previous studies. However, the microphysical processes active in the seeder and feeder region are still being understood. In this paper, we investigate the seeder and feeder region of a mixed-phase cloud passing over the Swiss Alps, focusing on (1) fallstreaks of enhanced radar reflectivity originating from cloud top generating cells (seeder region) and (2) a persistent low-level feeder cloud produced by the boundary layer circulation (feeder region). Observations were obtained from a multi-dimensional set of instruments including ground-based remote sensing instrumentation (Ka-band polarimetric cloud radar, microwave radiometer, wind profiler), in situ instrumentation on a tethered balloon system, and ground-based aerosol and precipitation measurements. The cloud radar observations suggest that ice formation and growth were enhanced within cloud top generating cells, which is consistent with previous observational studies. However, uncertainties exist regarding the dominant ice formation mechanism within these cells. Here we propose different mechanisms that potentially enhance ice nucleation and growth in cloud top generating cells (convective overshooting, radiative cooling, droplet shattering) and attempt to estimate their potential contribution from an ice nucleating particle perspective. Once ice formation and growth within the seeder region exceeded a threshold value, the mixed-phase cloud became fully glaciated. Local flow effects on the lee side of the mountain barrier induced the formation of a persistent low-level feeder cloud over a small-scale topographic feature in the inner-Alpine valley. In situ measurements within the low-level feeder cloud observed the production of secondary ice particles likely due to the Hallett–Mossop process and ice particle fragmentation upon ice–ice collisions. Therefore, secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountaintop observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.
Highlights
Mixed-phase clouds (MPCs), which consist of ice crystals and supercooled cloud droplets, play a crucial role in precipitation formation and are responsible for 30 % to 50 % of the precipitation in the midlatitudes (Mülmenstädt et al, 2015)
The ice crystal number concentration (ICNC) is derived from pre-calculated lookup tables containing the measurement variables, together with the corresponding microphysical state that would lead to exactly these measurements
The contour frequency by altitude diagram (CFAD; Fig. 5) of the radar reflectivity (Fig. 5a) indicates a rapid increase in the radar reflectivity near cloud top, suggesting that the ice crystals were formed in the layer between 5000 and 4000 m
Summary
Mixed-phase clouds (MPCs), which consist of ice crystals and supercooled cloud droplets, play a crucial role in precipitation formation and are responsible for 30 % to 50 % of the precipitation in the midlatitudes (Mülmenstädt et al, 2015). Mignani et al (2019) disentangled the surface processes and SIP mechanisms by analyzing single freshly fallen dendritic crystals, which grow between −12 and −17 ◦C, on their INP content They observed an ice multiplication factor of 8 in winter MPCs at the mountaintop station of Jungfraujoch and suggested secondary ice formation as a probable reason for their findings. While most of the studies agree that generating cells have important implications for precipitation formation, less research has focused on the mechanisms that are responsible for the enhanced ice formation and growth within these cells We will approach this problem from an INPcloud perspective by combining INP and ice crystal measurements. The analysis is based on an extensive set of observations including (1) ground-based remote sensing observations from a cloud radar, microwave radiometer and wind profiler, (2) balloon-borne in situ observations, (3) INP measurements, and (4) surface-based precipitation measurements
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