|Review of “Ice particle sampling from aircraft-Influence of the probing position on the ice water content.” By Afchine et al. |
Recommendation: Requires revision for publication
The authors have made a number of changes in the revised manuscript, and added some CFD simulations, all of which enhance the quality of the manuscript. However, I still think that the authors are not adequately acknowledging the uncertainties associated with their results for the reasons stated below:
1) The authors categorically state that “the wing IWC is derived from the measurements of PSD_Ice, that should be only weakly influenced by flow perturbation effects.” Although it is believed that this is the case, the quality of the wing IWC will depend on exactly where the wing probes are mounted. Typically these probes are mounted as far beneath and ahead of the leading edge of the wing as possible, and there may be enhancements or shadowing of concentrations depending on the exact mounting location. Is this something the CFD simulations can address? This uncertainty in the wing IWCs must be better acknowledged rather than assuming that the wing IWC is a reference value. Further, their statement that “most favorable for an undisturbed sampling on aircraft is most likely the position under an aircraft wing” is not necessarily true as many aircraft mount the probes so that the probes are positioned ahead of the aircraft wing to get a more clear flow. The authors seems to acknowledge this to some extent later on when they state that “they are in most cases mounted below the aircraft wings with sufficient distance to the wing and aircraft body”, but they do not define what sufficient is.
2) Perhaps the greater uncertainty the calculation of the wing IWC is the use of an M-D relation to derive the IWC from a measured size distribution. This manuscript does not adequately acknowledge the uncertainty in the calculation of the IWC from a size distribution. The authors state that “a reliable agreement between IWCs from two different instruments mounted at two different positions is a reasonable indication for an applicable IWC measurement.” But, if the basis of the choice of m-D coefficients is to get better agreement with the IWCs made in another location, one is merely obtaining what one assumed in the derivation of the m-D relations. Thus, it is not surprising that the IWC_PSD agrees with the FISH IWC given that this agreement was used to justify the choice of m-D relations used: this is a bit of circular logic. This point is also made on page 13 where it states that “the good agreement between the two measurements … shows the validity of the m-D relation used to calculate the IWC from the PSD.” I also have to make a comment on Figure 8, which the authors use to state that the IWCs derived from PSDs are not very sensitive to the choice of m-D relation: note that 8 orders of magnitude are included on the vertical axis so visually there appears to be little dependence. However, some of the differences are not that small. The writing and analysis needs to be more quantitative so that it can be determined what a large and small difference is.
3) I don’t think the authors accurately describe how ice crystal bouncing might be affecting the calculation of the IWC. The authors state that shattering “does not play a significant role for the calculation of the IWC since the ice fragments contribute to the integrated mass of PSD_ice in the same way as the original large crystal.” However, this statement is not true. The shattered artifacts are typically generated from large ice crystals hitting the tips of the probes which are outside of the sample volume, with the small remnants then being swept into the sample volume. I agree that given the use of the Korolev tips, the impact of the shatter artifacts on IWC is most likely minimal.
Despite these limitations, I still think that the manuscript makes a contribution to our scientific understanding of probe position on measured IWC. However, I think the uncertainties in the work need to be better acknowledged. Further, if a more quantitative analysis of the uncertainties could be made the manuscript would read better. Further, use of terms like “significant”, “small” and “large” should be avoided and replaced with more quantitative explanations of what the differences are.