Secondary ice production processes in wintertime alpine mixed-phase clouds

Paraskevi Georgakaki,Étienne Vignon,Georgia Sotiropoulou,Athanasios Nenes,Anne-Claire Billault-Roux,Alexis Berne

doi:10.5194/acp-22-1965-2022

Abstract

Abstract. Observations of orographic mixed-phase clouds (MPCs) have long shown that measured ice crystal number concentrations (ICNCs) can exceed the concentration of ice nucleating particles by orders of magnitude. Additionally, model simulations of alpine clouds are frequently found to underestimate the amount of ice compared with observations. Surface-based blowing snow, hoar frost, and secondary ice production processes have been suggested as potential causes, but their relative importance and persistence remains highly uncertain. Here we study ice production mechanisms in wintertime orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the Jungfraujoch site in the Swiss Alps with the Weather Research and Forecasting model (WRF). Simulations suggest that droplet shattering is not a significant source of ice crystals at this specific location, but breakups upon collisions between ice particles are quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the ice–ice collisional breakup mechanism is primarily associated with the occurrence of seeder–feeder events from higher precipitating cloud layers. The enhanced aggregation of snowflakes is found to drive secondary ice formation in the simulated clouds, the role of which is strengthened when the large hydrometeors interact with the primary ice crystals formed in the feeder cloud. Including a constant source of cloud ice crystals from blowing snow, through the action of the breakup mechanism, can episodically enhance ICNCs. Increases in secondary ice fragment generation can be counterbalanced by enhanced orographic precipitation, which seems to prevent explosive multiplication and cloud dissipation. These findings highlight the importance of secondary ice and seeding mechanisms – primarily falling ice from above and, to a lesser degree, blowing ice from the surface – which frequently enhance primary ice and determine the phase state and properties of MPCs.

Highlights

Understanding orographic precipitation is one of the most critical aspects of weather forecasting in mountainous regions (Roe, 2005; Rotunno and Houze, 2007; Chow et al, 2013)
Our sensitivity simulations revealed that the Droplet freezing and shattering (DS) mechanism is ineffective in the two considered alpine mixed-phase clouds (MPCs), even under the higher updraft velocity conditions associated with the NW winds case study, owing to the lack of large drops required for the process
Including a description of the BR mechanism is essential for reproducing the ice crystal number concentrations (ICNCs) observed in the simulated orographic clouds, especially at temperatures higher than ∼ −15 ◦C, where ice nucleating particles (INPs) are generally sparse

Summary

Introduction

Understanding orographic precipitation is one of the most critical aspects of weather forecasting in mountainous regions (Roe, 2005; Rotunno and Houze, 2007; Chow et al, 2013). Orographic clouds are often mixed-phase clouds (MPCs) containing simultaneously supercooled liquid water droplets and ice crystals (Lloyd et al, 2015; Lohmann et al, 2016; Henneberg et al, 2017). MPCs are persistent in complex mountainous terrain because the high updraft velocity conditions generate supercooled liquid droplets faster than can be depleted by ice production mechanisms (Korolev and Isaac, 2003; Lohmann et al, 2016). Ice crystals falling from a high-level seeder cloud into a lower-level cloud (external seeder–feeder event) or a lowerlying part of the same cloud (in-cloud seeder–feeder event) can trigger cloud glaciation and enhance precipitation over mountains (e.g., Roe, 2005; Reinking et al, 2000; Purdy et al, 2005; Mott et al, 2014; Ramelli et al, 2021). Analysis of satellite remote sensing over the 11-year period, between April 2006 and October 2017, suggests that seeding events are widespread over Switzerland, occurring with a frequency of 31 % of the total observations in which cirrus clouds seed lower mixed-phase cloud layers (Proske et al, 2021)

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