Tropical Cyclone Impact and Forest Resilience in the Southwestern Pacific

Round 1

Reviewer 1 Report

Review report on study entitled ‘Tropical cyclones impact and forest resilience in the

Southwestern Pacific’ by Baptiste Delaporte et al.,

This paper investigates the impact of tropical cyclones on forest vegetation by using the normalized difference vegetation index (NDVI) data over the Southwestern Pacific region. In this investigation, the authors used segmented linear models to identify significant breakpoints in the relationship between the reconstructed maximum sustained wind speed  and the observed changes in NDVI. The authors reported that the impact of TCs on forest vegetation was negligible when the TCs winds are below 50 m/s. The authors found second breakpoint, with a sharp increase in damages for TCs having winds > 75 m/s and they suggested that extremely intense TCs can cause extreme damages to forest vegetation. Overall, this is a very well-constructed, quite interesting study. The presentation is clear and the analysis looks good. I am happy with the present manuscript and recommended it for the publication in the Remote Sensing journal.

The correction needs in Figure 4. The x axis and y axis labels are missing. Please add.

Author Response

Thank you for this remark. The correction has been made on figure 4.

Reviewer 2 Report

General comments

This manuscript analyzes the changes in forest vegetation related the tropical cyclone intensity. The 2D wind speed structure of TCs is generated using the Willoughby cyclone model. This study claims a second breakpoint was found from the analysis with a sharp increase in damages for winds > 75 m/s. The analysis and figures are presented nicely. The finding also discussed well. However, this article needs some improvement before it publishes. I recommend accepting this manuscript after major revision.

 

Major comments

1. Table 2 list the frequency of TCs between 2000-2020. In section 2, please provide how many years of TC data consider in the study. It will help to follow the manuscript. Also, provide the NDVI maps that will show the changes during the years considered in this study. For example, show the average NDVI plot for the first five years and the last five years of data used in this study.

 

2. The radius of maximum wind(RMW) in equation 2 says RMW is the only function of latitude of TC center, but fig1 shows the relationship of W_Max and RMW. Please provide more explanation about how the fig1 was generated.

 

3. The fig1 shows the RMW decreases if the category of TC is increased. Please give a brief explain the science behind that. What causes the RMW to reduce when the intensity of TC increase.

 

4. The subfigure labels and captions are different in Figure 4. Please check the correct.

 

5. Please cite the following articles related to this study.

 

1. Jyoteeshkumar Reddy, P., Sriram, D., Gunthe, S.S. et al. Impact of climate change on intense Bay of Bengal tropical cyclones of the post-monsoon season: a pseudo global warming approach. Clim Dyn 56, 2855–2879 (2021). https://doi.org/10.1007/s00382-020-05618-3
2. Mittal, R., Tewari, M., Radhakrishnan, C. et al. Response of tropical cyclone Phailin (2013) in the Bay of Bengal to climate perturbations. Clim Dyn 53, 2013–2030 (2019). https://doi.org/10.1007/s00382-019-04761-w.

Comments for author File: Comments.pdf

Author Response

Below are our responses to each of the comments.

Comment #1:

In section 2.3. an indication of the duration has been added: “over the 20 years studied period (2000-2020)”.

Comment #2 :

We used 20 years of data to study the correlation between NDVI variations and cyclonic winds and focused on NDVI anomalies (Observed relative change in NDVI) as shown in Figure 4a.

Comment #3:

Indeed, Figure 1 concerns the wind speed profile (not the maximum wind speed) in cyclones as a function of the distance to the centre (radius). This concerns essentially equations 2f and 2g. We have therefore changed the label of the y-axis in Figure 1 and added a clarification in the legend of this figure.

Comment #4:

There is no easy response to the reviewer’s question on the physical basis for such inverse relationship, which not intuitive. That negative correlation between Vmax and RMW have been mentionned and in a number of studies aiming at understand the inner core dynamics of TCs. Willoughby and Rahn, 2004 state that ‘This pattern of correlation represents the ‘‘convective ring process,’’ contraction of the eye and sharpening of the wind maximum in response to symmetric heating in the eyewall as described by Smith (1981), Shapiro and Willoughby (1982), and Schubert and Hack (1982).” The latter studies used analytic or layered models of TC circulations in response to diverse sources of heating and momentum such as those provided by latent heat release from ocean evaporation or momentum fluxes from frictional forces at the ocean surface boundary layer. The RMW occurs at the boundary of the cloud free eye and the convective ring off the eye. They show, solving the equations, that complex vertical and radial secondary circulations in the eye (inside the RMW) and outside the RMW. In response to the differential responses of these secondary circulations due to the sources in and off the eye, “the RMW will usually tend to contract in response to convective heating that results in intensification” (Shapiro and Willoughby, 1982, p389) leading to the negative correlation between RMW and maximum winds.

Schubert, W. H., and J. J. Hack, 1982: Inertial stability and tropical cyclone development. J. Atmos. Sci., 39, 1687–1697.

Shapiro, L. J., and H. E. Willoughby, 1982: The response of balanced hurricanes to local sources of heat and momentum. J. Atmos. Sci., 39, 378–394.

Smith, R. K., 1981: The cyclostrophic adjustment of vortices with application to tropical cyclone modification. J. Atmos. Sci. 38, 2021–2030.

Willoughby, H.E., Rahn, M.E., 2004. Parametric Representation of the Primary Hurricane Vortex. Part I: Observations and Evaluation of the Holland (1980) Model. Mon. Wea. Rev. 132, 3033–3048. https://doi.org/10.1175/MWR2831.1

Comment #5:

These corrections have been done.

Comment #6 :

Good point

Reviewer 3 Report

In this study, the authors use a satellite-based vegetation index to understand the impact of TCs on forests. Further, they show using analysis that transition points exist relating the damages to recovery time. The idea itself is simple and interesting. I just have a few suggestions for the authors to consider

 

• No doubt TC wind speed is an important factor that affects the ability of forests to withstand the impact of storms. However, I wonder whether TC translation can also play a role. Slower moving storms typically can cause a lot more damage than faster moving ones, as the impacts from winds sustained over a longer period of time tend to be larger. Have the authors considered this possibility? To account for TC translation, authors can perhaps use established TC metrics such as ACE/PDI or perhaps they can normalize TC wind speed with translation speed. This can potentially address, at least partly, the point you mentioned about the large variation in forest damages for a given wind speed.
• Is there a reason why the authors have picked the South Pacific region for this study? It appears to me that the results in this study are fairly broad and are applicable to many other areas globally. It’d be good to add a few sentences in the introduction as for the motivation for considering the South Pacific region in this study.

Author Response

Comment #1:

Thanks for the interesting comment. Indeed, the speed of the cyclone (and therefore its residence time) is important for the impacts. However, we have chosen to test the relationship between maximum sustained wind speed and impacts on forest vegetation, and to compare this relationship with the Saffir-Simpson scale.

In order to better clarify this aspect, the following sentence has been added to the discussion paragraph (L230): "Finally, large variation in the observed damages for a given maximum sustained wind speed could be explained by different TCs translation speed with slower TCs (longer residence time) should be more destructive than faster ones (e.g., Liu et al., 2020)."

Liu, L., Wang, Y., Zhan, R., Xu, J., & Duan, Y. (2020). Increasing Destructive Potential of Landfalling Tropical Cyclones over China. Journal of Climate, 33(9), 3731–3743. https://doi.org/10.1175/JCLI-D-19-0451.1

Comment #2:

Indeed, this cyclone basin is relatively unstudied compared to most of the studies that are conducted in the North Atlantic and Pacific North West Basin.

A sentence has also been added to the text to justify this study (L54): "Our understanding of the impacts of TCs on forests in the islands of this region is relatively poor as most studies in this region have been conduced in Australia and on a global scale most studies have focused on the most intense cyclones in the North Atlantic Basin and to a less extent in the North West Pacific Basin (Lin et al., 2020)"

Lin, T.-C., Hogan, J. A., & Chang, C.-T. (2020). Tropical Cyclone Ecology: A Scale-Link Perspective. Trends in Ecology & Evolution, 0(0), Article 0. https://doi.org/10.1016/j.tree.2020.02.012

Round 2

Reviewer 2 Report

I thank the authors for giving responses to my comments. I am satisfied with the responses and recommend accepting the paper in the present form.