the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Heavy snowfall event over the Swiss Alps: Did wind shear impact secondary ice production?
- 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.
- 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.
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Zane Dedekind et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-429', Anonymous Referee #1, 02 Aug 2022
- AC1: 'AC1', Zane Dedekind, 10 Nov 2022
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RC2: 'Comment on acp-2022-429', Anonymous Referee #2, 09 Sep 2022
General Comment
This study focused on a role of vertical wind shear and turbulence on secondary ice production. The topic is unique and interesting. The hypothesis in this study is reasonable. However, I had difficulty to follow the manuscript probably because of a lack of detailed descriptions of simulations. There is also a lack of observational evidence of secondary ice production. I suggest considering revising the following points before publication.
Specific comments
1. It was difficult follow the entire manuscript, probably because of a lack of descriptions of model simulations (or I cannot find at least). The authors should give detailed, careful explanations of simulations for people who do not have model background. Specifically, I have the following questions:
- For Eqs. 1-3, what does “BR” stand for? What do BR28, BR2.8T, and BR-Sot mean? Because I could not know them, understanding the following descriptions was very difficult for me.
- What does N_BR mean?
- Lines 164-165: To me this sentence does not make sense at all. Need detailed explanations. What is scaled; what are BR, Sot, 2.8T?
- What is RS simulation? I could not easily find the description about the simulation.
- It seems to me that the radar reflectivity from the simulations is very large. For many regions below 5-6 km, reflectivity attained or exceeded 30 dBZ for all simulations. The authors mentioned the size of simulated particles, but still I think too large for snow scattering at X-band, otherwise it was graupel. Was graupel produced in the entire cloud below 5 km? Please give detailed settings of calculation of reflectivity from the simulation data.
2. I also felt a lack of observational evidence of secondary ice production. The authors need to show observational data and explanations of the secondary ice production. Below are my comments.
- For the case, both KDP and ZDR coincidentally increased at the same altitude. A signature of large KDP and large ZDR does not necessarily represent secondary ice production. Rather, it can be interpreted as size growth of individual particles (without secondary ice production). One of good signatures of secondary ice production is large KDP collocated with small ZDR. This can be seen Fig. S3, but less description about this in the text. In addition to such signature, the previous literature also showed other observational evidence such as in-situ data, Doppler spectra, and/or liquid water path. This manuscript did not show such evidence.
- Because of less figures from the observations (there are only reflectivity and hydrometeor classification plots), it was difficult to follow the first and second paragraphs of Sect. 3.1.1.
- RHI scans were performed from the low to high elevation angles (0-90 degrees). Observed Kdp and Zdr should have strong dependency on elevation angles. Did you correct the values for angles?
- I was not sure how the 2DVD data were used other than number concentration. Did the 2DVD show good evidence of secondary ice production?
- Please explain how to estimate NICE and IWC from observation (e.g. Fig. 5, Fig. S3).
- Did you see shear instability?
Technical comments
- Three digits are needed for latitude/longitude of the instrument locations.
- Line 123 “dual-Doppler radar output”: I was confused. Did you perform dual Doppler radar analysis (did you use two Doppler radars)? If so, please provide the second radar information.
- Line 150: The use of consistent unit throughout the manuscript for temperature is better.
- 3.2: What criteria were used for separating the period?
- AC2: 'Reply on RC2', Zane Dedekind, 10 Nov 2022
Zane Dedekind et al.
Zane Dedekind et al.
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