Dear Dr. Wang
I have received three reviews of your revised paper. Whilst one is positive and recommends to accept the paper, the first reviewer remains very critical and recommends rejection. I therefore asked a third expert to review the paper, and this feedback is also rather critical and asks for really major revisions. Obviously several statements and conclusions in the paper are not yet well substantiated and a lot of additional work is required to better disentangle the various mechanisms that are playing a role for this interesting event. Honestly, I was not sure whether "major revisions" or "reject but resubmit" would be the more appropriate decision in this case. I think that a lot of work is required to turn this into a fully convincing paper - and I leave it up to you whether you prefer to do this within the next weeks (in the form of "major revisions") or whether you prefer to have more time and resubmit this study later as a new submission.
With best regards,
Heini

Dear Dr. Wang

As you will see from the comments of reviewers 2 and 3, there are still different views on the merits of your publication. While reviewer 3 mentions that his/her comments "have been satisfactorily addressed" and the quality of the manuscript has improved, reviewer 2 still holds the opinion that "just like the previous versions of the manuscript, this revision still fails to convey the importance of the back-building (BB) process from both observational and modeling perspectives."

I noticed that you now also mention the role of other processes (e.g., orographic forcing) for the formation of the MCSs, but I still also have the impression that you hold tightly to the idea that BB is the key process. It seems to me that this is very difficult to prove for such a complex case. However, I agree with rev. 3 that your manuscript has further improved and that your analyses and hypotheses are interesting. Maybe, as the role of BB cannot be finally proven, also the contrary is not possible - it is not easy to rule out that BB plays a role. Therefore I decided that rejection would be the wrong decision.

However, before accepting your paper for publication, I strongly suggest that you further de-emphasize the importance of the BB process, clearly acknowledge the importance of other processes, and clearly state that the important role ascribed to the BB process is, at this point, a hypothesis yet to be entirely proven or disproven. Further analysis is required to test this hypothesis. And that a clear separation of the role of different processes (BB, orography, frontogenesis, ...) can be extremely difficult in real case situations.

Reviewer 3 further provided the following helpful input:
"I agree completely with reviewer 2 that the primary mechanism for the repeated development of convective cells is the lower-tropospheric convergence linked to the prefrontal terrain-channeled southwesterly flow in the vicinity of northeastern Taiwan. It appears to me that the BB process may have promoted the development of cell B2 immediately upstream of cell B1 but was not the primary process resulting in the development in of B2. Indeed, the authors state that the role of old cells "can explain why the spot roughly 15-30 km upstream from the western end of quasi-linear MCSs in the subtropics can often have advantages over other locations for new cell initiation" (page 21, lines 5-10). The BB mechanism described by the authors could perhaps help to explain why the region immediately upstream of cell B1 was particularly favorable for the development of B2, but is insufficient to explain why cell B2 developed.

Of course, the null hypothesis here is that the influences of cell B1 were merely coincidental and were inconsequential for the development of B2. Given that sufficient forcing (i.e., lower-tropospheric convergence) to trigger convection was already in place, B2 could have developed immediately upstream of B1 simply by random chance. The fact that other strong convective cells (e.g., cell C1) developed even in the absence of a preexisting cell (and the influences thereof) perhaps supports the null hypothesis. In my view, it is not possible with the tools and datasets used in this study to quantify the roles of the various processes for the development of convection in this case; thus, it is not possible to disprove the null hypothesis. This should be acknowledged by the authors in the paper."

I therefore ask you to further modify your manuscript in the light of these hopefully useful remarks.

With best regards,
Heini Wernli

Dear Chung-Chieh

Many thanks, I now accept your paper for publication.

Personally, I still think that the formulation "... clear BB behavior occurred in the quasi-stationary MCSs and contributed to the extreme rainfall in northern Taiwan in this event, among other factors linked to frontal forcing and topographic effects" is, of course, not incorrect, but not perfectly "balanced". Why not first say something like "Extreme rainfall events are typically highly complex and involve a range of different physical processes. In Taiwan, topographic forcing is likely to play an important role, as well as for the case under consideration aspects related to frontogenesis. However, in this study, the focus is on the specific BB process, which also played an important role." I am quite sure that a formulation like this would have been appreciated by the critical first reviewer.

I thank you for the fruitful iterations and for submitting your work to ACP.
With best regards,
Heini