|Review of the first revised version of “Atmospheric rivers in CMIP5 climate ensembles downscaled with a high resolution regional climate model” by Gröger et al.|
I acknowledge the great effort made by the authors in improving their manuscript and responding to my comments. The manuscript was much nicer to read and clearer but I still have some minor comments listed below following the manuscript order.
Abstract, Lines 19-20: I think the authors should add here the downscaling of 9 historical simulations and of the ERAI reanalysis as well.
- The time period 2070-2099 should be written too for the three RCP.
- In the caption, I find “the original ERAI hindcast data” a bit misleading. Could the authors replace it with the ERAI reanalysis?
Section 2.2, Line 184: The title is also misleading because the authors write about the hindcast produced with the ERAI reanalysis. I suggest to remove this small title as it is not needed.
- Line 206: the unit of the gravitational acceleration should be [m s^-2] and not “[m^3/(kg * s^2)]”.
- The authors could also mention here the unit of the IVT as well (kg m^-1 s^-1).
- Point 8 from line 245 onwards: The authors should mention if they use an absolute value or a percentile of IVT as threshold to define the AR mask. From Fig. 3, the mask seems to be defined with IVT above the 85th percentile of each grid point.
Section 2.5, Line 304: I believe the word “heavy” should not appear here.
- Lines 338-339: Maybe refer to Fig. 3 at the end of the sentence “However, … and lifetime.”.
- Line 350: after “warming”, I would add “compared to the other models”.
- In the paragraph made by lines 348-351: since the authors emphasise the different behaviour of MIROC, they could maybe also add that RCA-MIROC is the model with the lowest number of AR detected in all simulations (historical and future), as far as I can see in Table 3.
Table 3: some white spaces are missing between the numbers and parenthesis in column 3 for RCP2.6.
Section 3.3, Line 367: The reference to Fig. 2 is not needed here and also a bit confusing.
Section 3.4, Lines 421-425: do the authors mean that there might not be only one IVT mask for some time steps?
- Lines 438-439: “ENSM” -> MEAN. I believe the references to the figures are wrong. It should be: 0.86 for %AMP (Fig. 5b), 0.82 for %TP (Fig 5d), and 0.92 for %95P (Fig. 5c). I suggest to switch the last two to get the figures in alphabetical order (b, c, and d).
- Line 451: Between the two sentences “…hindcast.” and “However, …”, I would also point out that RCA-MEAN and RCA-ERAI highly overestimate the number of AR in September compared to ERAI but underestimate it in October. This was mentioned in the previous version of the manuscript (see lines 748-760 of the tracked-changes version). The authors do not need to provide an explanation for this behaviour.
Figure 7: “Note no realization are available for RCA-IPSL, RCA-CAN and RCA-CNRM.” Please add “that” between “Note” and “no” and mention that it is for rcp26 only at the end.
Figure 8: “Difference between 2070-2099 minus 1970-1999” -> Difference between 2070-2099 and 1970-1999.
The caption should also mention that the figure displays the ensemble mean difference.
- Line 502: “nearly everywhere.” -> nearly everywhere for RCP45 and RCP85.
- Line 511: “The changes” -> The relative changes.
Figures 9 and 10: both figures have a different projection than the rest of the maps (Figs. 5, 8, and 12).
Figure 9: as for Fig. 8 mention in the caption that this is an ensemble mean of the relative change.
Figure 10: same comment as for Figs. 8 and 9: this is an ensemble mean.
Section 4.3, Line 538: Yes, there is a reduction over Norway but also over many European mountainous regions, such as the Pyrenees, Massif Central, Alps, mountains in the Balkans. The authors could also mention the high absolute values In Scandinavia in panels (a) and (c) of Fig. 10 which seem to be linked to the lack of sea ice in the Gulf of Bothnia in the future.
- Line 556: What do the authors mean with “models’ solutions”? Should it be responses instead of “solutions”?
- Line 566: I would remove “highly” because there is some variation among the models.
- Line 659: “Fig. 8a” -> Fig. 12a.
- Line 575: “along the Norwegian coast”: This statement can be extended to the whole Scandinavia.
- Line 580: “smaller” than when? I believe it should say smaller in the future (compared to the historical period).
- Line 591: add RCA-ECE here as well.
- Line 592: “decrease, occurring in the UK; was” -> decrease over UK was.
Figure 12: please add in the caption that the figure is valid for scenario RCP85.
- Line 617: “a robust climate change signal”: I would not call it “robust” but high.
- Lines 625-627: instead of or in addition to the low-level jet stream, I would mention the storm tracks with a reference to Zappa et al. (2013) their Fig. 2a, which I think is going in the direction of the present study’s results.
- Line 635: “distinct N-S gradients”: to me, the Iberian peninsula rather shows E-W gradients in the ERAI reanalysis. RCA-ERAI shows gradients in both directions because of the orography.
- Lines 636-637: This sentence should be rewritten as it is not clear. Suggestion: The AR imprint on the analysed indices in the ERAI dataset was weak over Iberia but stronger in distant parts of Eastern Europe compared to the downscaled RCA-ERAI (Fig. 5).
- Line 678: “is coarse” -> is still relatively coarse.
Line 79: “(Albano et al., 2017; )” Is there a reference missing? Otherwise, should be (Albano et al., 2017).
Line 83: “Held and Soden ,2006” -> Held and Soden, 2006
Lines 112,113,114: put commas at the end of the three lines.
Line 220: “bin specific” -> bin-specific
Lines 230 and 231: remove the commas in the two references (after Villarini and after al.).
Line 322: “equator to pole” -> “equator-to-pole”
Line 327: delete one “and the”.
Line 390: “Downscling” -> Downscaling
Line 442: “occurs” -> occur
Line 538: “a~20%” -> a ~20%
Line 539: “Fig.10a” -> Fig. 10a
Line 588: “standard deviations were” -> standard deviation was. The authors write about only one standard deviation here, the one displayed in Fig. 12c.
Line 627: “Pasquier et a., 2018” -> Pasquier et al., 2018
This study discusses the ARs over the North Atlantic from ensembles of 24 global climate simulations following the greenhouse gas scenarios RCP2.6, RCP4.5, RCP8.5 downscaled using a regional climate model (RCA-NEMO) with 0.22° resolution and the results are compared with against ER-I reanalysis data. The study finds that ARs would become more frequent and more intense in a future warmer climate especially in the higher emission scenarios under the assumption of RCP2.6. They also propagate further inland to eastern Europe in a warmer climate.
Though I am yet to complete the review, here is a major comment on the detection of atmospheric rivers. Authors have mentioned that they employed Lavers et al. (2012, 2013) method to detect ARs based on the 5 degrees binning along 10 degrees west. Though it is a well-known approach, one might see the spatial "patchy" and "noise" at a given time step in the AR detection (figure 1 in Lavers et al., 2013). To be specific, one might expect that a high-resolution data detection algorithm could retain values over a few grid points that satisfy the binned threshold but do not satisfy the AR criteria. This noise in turn would cause bias in comparing the long term (climatology etc) spatial variability among different models. Also, the authors mentioned that the ARs with 18 hours of persistence were considered. But I do not see any description of finding persistence. Hence, authors are encouraged to provide more details on these issues in the manuscript.