Heavy snowfall event over the Swiss Alps: did wind shear impact secondary ice production?

Abstract

Abstract. The change in wind direction and speed with height, referred to as vertical wind shear, causes enhanced turbulence in the atmosphere. As a result, there are enhanced interactions between ice particles that break up during collisions in clouds which could cause heavy snowfall. For example, intense dual-polarization Doppler signatures in conjunction with strong vertical wind shear were observed by an X-band weather radar during a wintertime high-intensity precipitation event over the Swiss Alps. An enhancement of differential phase shift (Kdp>1∘ km−1) around −15 ∘C suggested that a large population of oblate ice particles was present in the atmosphere. Here, we show that ice–graupel collisions are a likely origin of this population, probably enhanced by turbulence. We perform sensitivity simulations that include ice–graupel collisions of a cold frontal passage to investigate whether these simulations can capture the event better and whether the vertical wind shear had an impact on the secondary ice production (SIP) rate. The simulations are conducted with the Consortium for Small-scale Modeling (COSMO), at a 1 km horizontal grid spacing in the Davos region in Switzerland. The rime-splintering simulations could not reproduce the high ice crystal number concentrations, produced too large ice particles and therefore overestimated the radar reflectivity. The collisional-breakup simulations reproduced both the measured horizontal reflectivity and the ground-based observations of hydrometeor number concentration more accurately (∼20 L−1). During 14:30–15:45 UTC the vertical wind shear strengthened by 60 % within the region favorable for SIP. Calculation of the mutual information between the SIP rate and vertical wind shear and updraft velocity suggests that the SIP rate is best predicted by the vertical wind shear rather than the updraft velocity. The ice–graupel simulations were insensitive to the parameters in the model that control the size threshold for the conversion from ice to graupel and snow to graupel.

Simulating the seeder–feeder impacts on cloud ice and precipitation over the Alps

“…Conversely, in the absence of supercooled cloud droplets, breakup is expected to yield smaller ice crystals, thereby increasing the likelihood of sublimation below the cloud base. The findings on the role of deposition and riming in the seeder-feeder process have implications for the process's significance over Switzerland: across the Swiss Alps, the liquid water fraction for wintertime mixed-phase clouds is less than ∼ 20 % (Henneberg et al, 2017;Dedekind et al, 2021Dedekind et al, , 2023a. Because of the lower liquid fraction, the interaction between liquid water and ice particles (the riming process) is less efficient than during summertime.…”

“…To understand the effect of the internal and external seederfeeder process on surface precipitation better, we use the non-hydrostatic limited-area atmospheric model of the Consortium for Small-scale Modelling (COSMO version 5.4.1b;Baldauf et al, 2011). This COSMO version has recently been used to study wintertime orographic MPCs in the Swiss Alps (Lohmann et al, 2016;Henneberg et al, 2017;Dedekind et al, 2021Dedekind et al, , 2023a. We use a two-moment cloud microphysics scheme within COSMO with six hydrometeor categories, including hail (Blahak, 2008), graupel, snow, ice crystals, raindrops and cloud droplets (Seifert and Beheng, 2006).…”

“…Also of interest to us is the impact of ice particle breakup through ice-graupel collisions on the seeder-feeder process (e.g., Sullivan et al, 2018;Dedekind et al, 2021Dedekind et al, , 2023a. The enhancement of ice particles through secondary ice production processes, other than the Hallett-Mossop process, has not been studied in light of the seeder-feeder process.…”

Simulating the seeder-feeder impacts on cloud ice and precipitation over the Alps

“…To understand the effect of the internal and external seeder-feeder process on surface precipitation better, we use the nonhydrostatic limited-area atmospheric model of the Consortium for Small Scale Modelling (COSMO version 5.4.1b;Baldauf et al, 2011). This COSMO version has recently been used to study wintertime orographic MPCs in the Swiss Alps (Lohmann et al, 2016;Henneberg et al, 2017;Dedekind et al, 2021Dedekind et al, , 2023. We use a two-moment cloud microphysics scheme within COSMO with six hydrometeor categories, including hail (Blahak, 2008), graupel, snow, ice crystals, raindrops and cloud droplets (Seifert and Beheng, 2006).…”

Effect of Secondary Ice Production Processes on the Simulation of ice pellets using the Predicted Particle Properties microphysics scheme

“…So far, studies on SIP have been conducted by simulating different types of clouds and weather, including orographic mixed-phase clouds (e.g. Dedekind et al, 2023;Georgakaki et al, 2022), polar mixed-phase clouds (Sotiropoulou et al, 2020;Zhao et al, 2021), cold frontal rainbands (Sullivan et al, 2018), cold marine boundary layer clouds (Karalis et al, 2022), stratiform clouds (Zhao and Liu, 2022), and convective weather. Convective weather represents most of this research (Field et al, 2017).…”