Multiscale Perspectives on an Extreme Warm-Sector Rainfall Event over Coastal South China

Round 1

Reviewer 1 Report

The west coast of south China is a known storm center mainly due to the topographic up lifting of the humid & warm marine moisture.  Observational analysis on the evolution and mesoscale features associated with various storm generation and processes such as the warm-sector extreme rainfall are lack in literature. This study reveals multiscale processes of the warm-sector extreme rainfall using various sources of detailed observation and is a significant contribution to understand the features of extreme rainfall generation, raindrop size and evolution etc.    

 

Minor comments:

 

1)      Fig.1b., add the location of the radar, re-sort the order of the three stations from south to north and change the symbols’ color to match the line color of fig.1c&d, and re-arrange the order of the three stations to match fig1b .

 Fig.5, lack unit for the vertical height

Author Response

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Reviewer 2 Report

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Comments for author File: Comments.pdf

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Reviewer 3 Report

This study investigates a heavy rainfall event that hit South China in 2017, focusing on the environmental conditions and microphysical characteristics. At first, the authors point out that a boundary-layer jet coupled with a synoptic low-level jet plays a significant role in generating a moist environment favorable for the generation of the precipitation system. Under the moist environment, mesoscale convergence lines initiate convection with an aide of topography. The precipitation system is maintained by quasi-stationary outflow boundaries. From microphysical analyses with radar observations, the authors claim that the warm-rain process characterizes the precipitation system causing the rainfall event.

Although the analysis results sound interesting, the manuscript includes many unreasonable interpretations. In particular, the authors conclude that the precipitation system is mainly produced by the warm rain process, even though much evidence of the cold rain process is shown. The discussion about the roles of double LLJ in the initiation of the precipitation system is also unclear. The detailed comments are listed below.

Major comments

1. Discussion about the roles of the double LLJ on the initiation of convection is unclear. The authors show that the evolution of the precipitation system causing the rainfall event corresponds to that of the double LLJ (L262-265). On the other hand, the authors also claim that the convective initiation occurs in 2100LST 21 (L313-321), before the coupling of the LLJs. The description seems to deny the contribution of the double LLJ to the initiation of the precipitation system. How does the double LLJ contribute to the initiation of the precipitation system?

 

2. Section 5: The explanation of the roles of the MOBs in the production of the precipitation system seems to be contradictory. The authors point out that the MOBs contribute to generating the precipitation system by a continuous lifting of the moist air (L337-339). On the other hand, the authors also claim that the moist environment produced by the double LLJ reduces the cooling and surface outflow due to weak evaporation (L321-322). This indicates that the moist environment is detrimental to generating the precipitation system by reducing the production of the MOBs.

 

3. Although the authors claim that the changes in the BLJ and precipitation amount relate to moisture amount (L289-291), the significance of the changes is unknown. How much does the moisture amount change before and after the enhancement of the BLJ? The authors show the information about the moisture amount in Figs. 6b and 6c, both need to revise. Why the absolute humidity shown in Fig. 6b exceeds 1000 g m-3. Because absolute humidity normally means the amount of water vapor in a unit volume of moist air, its value of over 1000 g m-3 is impossible. Additionally, we cannot read how much the moisture amount is before the rainfall event from Fig. 6c due to the same color scales below 90%.

 

4. A high value of CAPE is observed (Fig. 6a). Such a high CAPE value is favorable for generating precipitation systems like thunderstorms and supercell storms (e.g., Bluestein and Parker 1993) characterized by the cold rain process. Is the high CAPE environment consistent with the conclusion that the precipitation system is mainly produced by the warm rain process, shown in section 6? Although the authors pointed out that the CAPE value decreases at night, how much is the CAPE value at around the rainfall time? Can you estimate it from ERA5 data if there is no observation?

 

5. Discussion about the microphysical characteristics seems to be unreasonable. The authors claim that extreme rainfall is mainly produced by the warm rain process based on large LWC compared with IWC (L418-420). However, the hydrometeor classification shown in Fig. 9 designates that about 30% of a mixture of rain and hail contains around the surface, clearly indicating the contribution of the cold rain process. Moreover, the authors claim in L420-421 that 40dBZ echo-top height during the S3 period is about 6 km, indicating weak convection based on Fig. 1e. However, the 40 dBZ echo-top height over 8 km is observed at around 03LST in Fig. 1e so that the figure is not necessarily indicating weak convection. Additionally, CFAD in the S3 period (Fig. 9c) shows a 40 dBZ echo-top height reaching 10 km. These echo-top heights are consistent with convections in the continental regions (e.g., Liu and Zipser 2013), suggesting the contribution of the cold rain process. Why do the authors not discuss these results?
I suggest adding analysis using observation data whose echo-top heights do not reach the freezing level. A comparison of precipitation amount associated with echo-top heights lower than the freezing level with that with echo-top heights higher than the freezing level can clarify whether the warm rain process is dominant or not.

 

Minor comments 

1. L147-L152, Fig. 9: I suggest not using abbreviates for hydrometeor categories. It seems to hinder the ease of reading.

2. Section 4: How do the LLJ and BLJ couple? Do independent two jets couple? Or do the LLJ result from the BLJ lifted by the topography of the coast?

3. L267: Where is Figure 5c?

4. L320: What does “joint effect of the BLJ enhancement and the topographic uplift” means? Please add more specific explanations.

5. L422-427: The vertical deviations of D_m and log10N_w seem to be small, so the significance is unknown. If possible, please add discussion with previous studies showing vertical profiles of D_m and log10N_w and show that these vertical profiles are consistent with those in precipitation systems with a typical warm rain process.

6. Fig. 11 caption: It is difficult to distinguish gray solid border, gray dotted border, and black dotted line. Where is the black dotted line?

References

Bluestein, H. B., and S. S. Parker, 1993: Modes of isolated, severe convective storm formation along the dryline. Monthly Weather Review, 121, 1354-1372, https://doi.org/10.1175/1520-0493(1993)121<1354:MOISCS>2.0.CO;2

Liu, C., and E. Zipser, 2013: Regional variation of morphology of organized convection in the tropics and subtropics. Journal of geophysical research atmosphere, 118, 453-466, https://doi.org/10.1029/2012JD018409

Author Response

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Round 2

Reviewer 3 Report

My previous major concerns have been addressed. Although I have a few minor comments listed below, I can recommend the publication of this paper

Specific comments 

1.  Section 6.2: It seems less reasonable to discuss the 40 dBZ echo top height with a mean value. Since the frequency of weak convections is normally high, the average 40 dBZ echo top height easily takes lower values due to the high frequency of weak convections. I suggest adding quantitative information, for example, numbers of pixels with the 40 dBZ echo top heights higher and lower than 6 km.

2.  L146: Wu et al. (2018a) -> Wu et al [55]

3.  L390 blow -> below ?

Author Response

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