the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
15 Feb 2021
15 Feb 2021
Comment on Review of Experimental Studies of Secondary Ice
Production
by Korolev and Leisner (2020)
- 1Department of Physical Geography, University of Lund, Lund, Sweden
- 2CNRM, UMR 3589 (CNRS), Meteo-France, 31057 Toulouse Cedex, France
- 1Department of Physical Geography, University of Lund, Lund, Sweden
- 2CNRM, UMR 3589 (CNRS), Meteo-France, 31057 Toulouse Cedex, France
Abstract. This is a comment on the article Review of Experimental Studies of Secondary Ice Production
by Korolev and Leisner (2020, hereafter termed KL2020
), referring to the discussion about ice fragmentation. We argue that the only two studies characterising fragmentation in ice-ice collisions are not so erroneous as to prevent their use in representing this breakup in numerical models, contrary to the impression given in the review. A scaling analysis suggests that breakup of ice during sublimation can make a significant, albeit lesser, contribution to ice enhancement in clouds.
Review of Experimental Studies of Secondary Ice Productionby Korolev and Leisner (2020), Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-123, in review, 2021.
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Vaughan T. J. Phillips et al.
Status: open (until 12 Apr 2021)
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RC1: 'Comment on acp-2021-123', Andrew Heymsfield, 21 Feb 2021
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I'm openly sharing my identity. I have tried to address the authors' comment/concern of the Korolev and Leisner article which suggests that ice fragmentation during sublimation is not generally an important process for secondary ice production. To address that point, I drew on the Lagrangian spiral descents during the CRYSTAL-FACE field program, where the in-situ aircraft spiraled in a descending pattern, drifting with the wind and descending at a rate of about 2 m/sw. These were thunderstorm anvils, which began at a temperature of about -15C and ended at temperatures from about 6 to 8C. The relative humidity in the region at temperatures above 0C were significantly below 100% and the ice particles were sublimating, not melting. I have 3 figures for the 3 spirals, showing temperature, relative humidity and total ice concentration. I see no evidence of a concentration increase that would suggest ice fragmentation in the sublimating region is significant. Furthermore, in such regions, sublimating ice particle fragments would each rapidly fully sublimate or be collected by other particles. Andy Heymsfield
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AC1: 'Reply on RC1', Vaughan Phillips, 21 Feb 2021
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We thank the reviewer for the interesting comment.
It is unclear if the relative humidity shown is with respect to water or ice. We will assume it is with respect to water.
The laboratory studies about sublimational breakup that we referred to in our comment show that such breakup only occurs at relative humidities with respect to ice of less than about 70-80%. Following the melting scheme by Phillips et al. (2007, JAS), the onset of melting occurs when the surface temperature of the ice during sublimation reaches 0 degC. We predict that this occurs when the ambient air temperature exceeds about 1.5 degC (codes available from us on request), for the three CRYSTAL-FACE aircraft descents shown by the reviewer. At that ambient temperature for onset of melting (1.6 degC), the relative humidity with respect to ice is 80%.
For the three CRYSTAL-FACE descents shown by the reviewer, sublimation below the melting level can only occur between 0 and 1.6 degC, an extremely narrow range of heights where a uniformly high relative humidity with respect to ice was observed (> 80%, we infer).
So, the reviewer's data does not prove the absence of breakup during sublimation generally, since no-one has ever claimed that at such high relative humidities there would be any breakup. In the comment, we actually base our breakup estimate on a scenario with a relative humidity of about 70% or less.
Equally, CRYSTAL-FACE happened long before the shattering bias was discovered for optical probes. Thus, the probe-tips had not been invented then. So the unfiltered ice concentrations (> 30 microns) shown by the reviewer are likely dominated by artificial fragments from impact of snow on the aircraft probe (e.g. Korolev et al. 2011).
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AC2: 'Reply on AC1', Vaughan Phillips, 22 Feb 2021
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Additionally, everyone agrees that the fragments will disappear during sublimational descent. The point we are making in the comment is that it takes long enough for any given fragment to disappear (about a minute perhaps) that there is a quasi-equilibrium concentration that is enhanced due to the balance between elimination and continual emission in the descending parcel.
(BTW, I meant to write "below the freezing level" instead of "below the melting level" in the previous response).
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CC1: 'Reply on AC2', Jacob Carlin, 24 Feb 2021
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In support of Vaughan’s hypothesis, I wanted to chime in with some evidence we have recently come across of potential sublimational SIP. Specific differential phase (KDP) has recently been proposed as a potential marker of SIP zones as it is sensitive to large concentrations of anisotropic fragments, but this has so far been limited to SIP during riming (e.g., Grazioli et al. 2015; Sinclair et al. 2016; Kumjian and Lombardo 2017). However, looking at quasi-vertical profiles (QVPs) we have recently found bands of enhanced KDP situated squarely within the sublimation zone in 9 of 12 cases of strong sublimation, coincident with gradients of Z and RH w.r.t. ice (taken from coincident RAP analyses). We are currently following up and exploring these signatures more quantitatively. However, given the observational laboratory evidence for sublimational SIP (e.g., Oraltay and Hallet 1989; Dong et al. 1994; Bacon et al. 1998), I contextualized these signatures with the existing literature by emulating Fig. 13 of Oraltay and Hallett (1989; reproduced below) and plotting the median KDP as a function of RHi and ice bulb temperature. Note that Oraltay and Hallett (1989) only included down to 50% RHi and -6C for ice bulb temperatures, so my figure is an extrapolation and it is unclear whether their 70% RHi threshold is maintained at much colder ice bulb temperatures. Regardless, it is apparent that these regions of enhanced KDP indicative of high concentrations of highly anisotropic particles are concentrated primarily in the region identified by Oraltay and Hallett (1989) as the favorable zone for sublimational breakup. The plot using mean KDP was similar. These sorts of values are not likely to be due to aggregates which we believe characterize the majority of parent particles in the cases examined, and riming is precluded due to the degree of dry air present. Thus, although the use of KDP is indirect evidence, under proper conditions (e.g., a continual and sufficiently high flux of parent particles into a sufficiently dry layer and favorable particle habits), I believe Vaughan’s hypothesis of an equilibrium being established between fragment generation and fragment consumption to be plausible.
Jacob Carlin
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CC1: 'Reply on AC2', Jacob Carlin, 24 Feb 2021
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AC2: 'Reply on AC1', Vaughan Phillips, 22 Feb 2021
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AC1: 'Reply on RC1', Vaughan Phillips, 21 Feb 2021
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Vaughan T. J. Phillips et al.
Vaughan T. J. Phillips et al.
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