15 Jul 2022
15 Jul 2022
Status: this preprint is currently under review for the journal ACP.

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

Zane Dedekind1,, Jacopo Grazioli2,, Philip H. Austin1, and Ulrike Lohmann3 Zane Dedekind et al.
  • 1Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Earth Sciences Building, 2207 Main Mall, Vancouver, BC, V6T 1Z4, Canada
  • 2Environmental Remote Sensing Laboratory (LTE), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
  • 3Institute of Atmospheric and Climate Science, ETH Zurich, Switzerland
  • These authors contributed equally to this work.

Abstract. Intense dual-polarization Doppler signatures in conjunction with strong vertical wind shear were observed by an X-band weather radar during a winter high 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 will show that ice-graupel collisions are a likely origin of this population. 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 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 suggested 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 conversion rate size restriction from ice to graupel and snow to graupel.

Zane Dedekind et al.

Status: open (until 26 Aug 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-429', Anonymous Referee #1, 02 Aug 2022 reply

Zane Dedekind et al.

Zane Dedekind et al.


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Short summary
Simulations allowing ice particles to collide with one another producing more ice particles represented surface observations of ice particles accurately. An increase in ice particles formed through collisions was related to sharp changes in the wind direction and speed with height. Changes in wind speed and direction can therefore cause more enhanced collisions between ice particles and alter how fast and how much precipitation forms. Simulations were conducted with an atmospheric model COSMO.