1. In my view, there is insufficient evidence in the paper to support the authors’ claim that ‘the cold-pool mechanism’ was absent in this case. The authors need to point to specific evidence that the cold-pool mechanism was entirely absent in this case. Analyses of near-surface temperature or potential temperature fields could be included in the paper to verify the absence of a surface cold pool in the vicinity of the back building convection. Might it be possible that the initial training line/adjoining stratiform MCS established a cold pool and an associated boundary in the vicinity of northern Taiwan that subsequently served as a focus for the initiation of convective cells within the back-building MCS?
2. The authors provide a very detailed analysis of the various terms of the vertical momentum equation for developing and maturing cells within the back-building convective line. This analysis documents processes associated with the development of convective cells, but it does not explain how the convective cells were actually initiated in the upstream portion of the convective line. It is not clear precisely how the forcing (i.e., lower-tropospheric lifting) necessary to initiate new convective cells in the upstream portion of the convective line was established and how that forcing was manifested. The authors have provided various statements throughout the manuscript vaguely asserting that the convection was linked to forcing related to the approaching front and to low-level convergence associated with terrain-channeled flow without providing any supporting evidence. Moreover, the authors state in the paper, “Given all these driving mechanisms and forcings (at meso-β scale or larger), the BB process at convective (meso-γ) scale was also a contributing factor leading to the extreme rainfall, especially during the later 6 h period after 18:00 UTC” (page 10, lines 8–11). This statement makes it seem as though the back-building process was somehow independent of the forcing at the meso-β scale. Is it possible that the back-building processes was predominantly governed by processes acting at the meso-β scale? Perhaps, new cells developed 15–30 km upstream of old cells simply because that was where lower-tropospheric convergence and lifting were strongest?
3. I suggest including some additional analyses from the CReSS model simulation to illustrate the favorable mesoscale configuration – and the forcing therein – that contributed to new cell development within the back building convective line. For example, you could provide a series of maps showing potential temperature, convergence, and wind on some level in the lower troposphere (e.g., 950 hPa) during the time period of the back building. Such maps could highlight surface boundaries linked to, e.g., convectively generated cold pools or the Mei-yu front, which may have been involved in the development and maintenance of convection.
4. Somewhere in the introduction, please provide a brief description of the Mei-yu season and how extreme precipitation can be produced in connection with the Mei-yu front. Such information would likely be useful for readers who are unfamiliar with the meteorology of southeastern Asia.
5. The impact and scientific significance of the paper would be enhanced if the authors were to include a discussion of how the results of their study relate to those of prior studies of back-building MCSs. Such a discussion could help to put the results of this study into a broader context and to emphasize the implications and importance of the results.
P1,L14: What do you mean by ‘across the scale’? Which scale specifically?
P1,L17: What do you mean by ‘more favorable’? More favorable than what?
P2,L2: Perhaps better to say ‘location’ rather than ‘spot’
P2,L6: Consider inserting ‘convective’ after organized.
P2,L23: ‘upwind’ is ambiguous here; perhaps you mean ‘upstream relative to the system motion vector’
P2,L25: The authors seem to imply here that TL/AS and BB MCSs are completely distinct. In fact, there is often overlap between these two types of MCSs. Heavy-rain-producing MCSs can often feature characteristics of both types. Case studies by Schumacher et al. (2011) and Peters and Schumacher (2015) illustrate this.
P2, L27-30: Back building of convection can also occur in conjunction with lifting along a quasi-stationary frontal boundary as well (e.g., Schumacher et al. 2011).
P3,L11-14: Although environments in the tropics and subtropics may not always be conducive to the formation of strong surface cold pools in association with convection, cold pools can often develop in conjunction with subtropical and tropical MCSs and can often play an important role in MCS organization and evolution. Also, cold pools need not be strong in order to impact the evolution of an MCS. In my view, it is erroneous to downplay the importance of cold pools in subtropical and tropical MCSs.
P4,L10: What do you mean by ‘across the scale’? Which scale specifically?
P4,L28: Replace ‘resulted’ with ‘resulting’
P6,L14-15: More precisely, positive (negative) values of a laplacian of some variable correspond to local minima (maxima) of that variable, right?
P7,L17: Here and elsewhere in the paper, how were frontal positions determined?
P7,L20: Replace ‘as well as’ with ‘as does’
P7,L22: Insert ‘possibly’ before ‘produced’
P7,L24: I am really confused. I do not understand physically how moisture flux convergence can influence the formation of the near-surface wind pattern in Fig. 2a. Please explain. Also, although moisture flux convergence is mentioned multiple times in the paper, the authors do not actually show moisture flux convergence calculations.
P7,L25-26: This statement is vague. How exactly is this pattern ‘particularly conducive’? Please explain.
P8,L5-6: There is no antecedent for ‘the barrier jet’. Which barrier jet?
P8,L12: Note, however, that the sounding indicates that the environment is subsaturated; perhaps this suggests the possibility for cold pool formation?
P9, L21: What is meant by ‘more active’ here?
P10,L2-6: In my view, you have not shown sufficient evidence to support this statement. Also, it is not clear what is meant by ‘forcings of the approaching front as well as the terrain-influenced low-level convergence and moisture flux convergence’, nor is it clear how such forcings were related to the development and maintenance of convection. The authors have not provided any diagnostics to quantify frontal or terrain-influenced forcing.
P11,L30: ‘observations’ instead of ‘observation’
P12,L19-20: What is meant by ‘forcings of the front and terrain’ and how were these forcings favorable for new cell development?
P12,L22-24: The authors describe the acceleration and deceleration of the LLJ in Fig. 10b, which to me implies that the wind within the LLJ is changing with respect to time. This is, however, not shown in Fig. 10b. Rather, Fig. 10b shows spatial variations of the wind at a single time. Please revise.
P13,L18: Replace ‘details’ with ‘detail’
P14,L24: It is not accurate to say that moisture flux convergence drives MCS development. My understanding is that moisture flux convergence is 'associated with' or 'a signature of' MCS development.
P14,L26–P15,L2: I find the reasoning in these lines of text to be difficult to follow. Please clarify.
P15,L10-11: If the downdraft regions are characterized by positive buoyancy, how are the downdrafts being forced?
P15,L19: According to eq. 2, changes in buoyancy are largely governed by changes in 𝜃v’. Given that changes in 𝜃v’ can only arise due to diabatic processes (e.g., latent heating, evaporative cooling), it is not clear to me how adiabatic cooling or warming can result in buoyancy changes. Please explain.
P15,L25: Is melting of hydrometeors not important in this case?
P17,L6: Perhaps this is a naive question, but are other diabatic processes (e.g., radiation, melting) unimportant here?
P17,L21-22: Insert ‘possibly’ before ‘enhanced’; the authors have not explicitly shown that evaporative cooling was involved.
P17,L29-30: Even without the ‘three advantages’ associated with a nearby cell, C1 still appears to develop into an intense convective cell (Fig. 9). Perhaps this suggests other factors are more important for convective cell development?
P18,L12-13: Again, in my view there is insufficient evidence to support this claim.
P18,L22-23: There is insufficient evidence to support this statement. The authors have not provided any diagnostics to quantify frontal or terrain-influenced forcing.
P18,L28: It seems to me that the old cell may have influenced the development of the new cell but did not play a role in the actual triggering/initiation of the new cell.
Schumacher, R. S., T. J. Galarneau Jr., and L. F. Bosart, 2011: Distant effects of a recurving tropical cyclones on rainfall in a midlatitude convective system: A high-impact predecessor rain event. Mon. Wea. Rev., 139, 650–667, doi:10.1175/2010MWR3453.1.
Peters, J. M., and R. S. Schumacher, 2015: Mechanisms for organization and echo training in a flash-flood-producing mesoscale convective system. Mon. Wea. Rev., 143, 1056–1083, doi:10.1175/MWR-D-14-00070.1.