the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Jun 2023
Roles of Tropical Waves in the Formation of Global Tropical Cyclone Clusters
Abstract. This study examines the role of tropical dynamics in the formation of global tropical cyclone (TC) clusters. Using theoretical analyses and idealized simulations, it is found that global TC clusters can be produced by the internal dynamics of the tropical atmosphere, even in the absence of landmass surface and zonal sea surface temperature (SST) anomalies. Our analyses of a two-dimensional InterTropical Convergence Zone (ITCZ) model capture indeed some planetary-scale stationary modes whose zonal and meridional structures can support the formation of TC clusters at the global scale. Additional idealized simulations using the Weather Research and Forecasting (WRF) model confirm these results in a range of aqua-planet experiments. Specifically, the examination of two common tropical waves including the equatorial Rossby (ER) wave and the equatorial Kelvin (EK) wave shows that ER waves could develop and maintain a planetary-scale stationary structure for a range of zonal wavenumbers [5–11], while EK waves do not. This numerical result is consistent with the ITCZ breakdown model and reveals some forcing structures that can support stationary "hot spots" for global TC formation. The findings in this study offer different insights into the importance of tropical waves in producing global TC clusters beyond the traditional explanation based on zonal SST anomalies.
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RC1: 'Comment on egusphere-2023-974', Anonymous Referee #1, 26 Jun 2023
General comments:
This study explored how the planetary stationary waves can modulate the TC genesis location. It is presented that the equitorial Rossby waves can act as an incubator of TCs while equitorial Kelvin waves do not have a clustering effect. The topic is very interesting and can add new insigts into explaining TC genesis ‘hot spot’. However, the current version has some confusing places which need to be addressed before publication.
Specific comments:
- Section 2 is mostly repeating the equations in W19 with very little information about how this part is different from W19. If they are exactly the same, then there is no need to repeat it again as if it is a new result. If not, please clarify the differences and emphasize what’s new.
- The connection between theoretical and modeling part is not stated clearly. The current layout gives me a very abrupt transition from Section 2 and Section 3. For example, what is the connection between the imposed ER wavenumber in WRF simulations and theoretically derived K bound; what are the assosiated magnitude of mean flows, Rossby number, Ekman number and Rayleigh number in the WRF simulations and the corresponding theoretical value of PSW wavenumber from the theory?
Technical corrections:
- L121-124: expand this part to clarify the differences between the theoretical analysis in this manuscript and W19.
- L127: subscription missing in the streamfunction expression: ‘LU_0’ -> ‘L_yU_0’
- L123: a new character ‘I’ is introduced, which is exactly the same as A in W19. Why not adopt the same character?
- L150: ‘max’ on the exponent is a little misleading, I though it was taking some maximum value…
- L168-169, L363: a~0.06 and K~12 in W19, please be consistent.
- L250 and below: V20 is not defined. And I only found Vu et al. 2021.
- In the table 2, the last two rows are mismatched.
- L271-272: I think the purpose of the paper is to show with no land it is possbile to have PSW by earth internal dynamics, but here it reads very contradictory.
- L275: ‘no significant impacts’ on what?
- The criteria in Table 3 are used according to Vu et al. 2021, should state this in text ‘refer to Vu et al. 2021 for more details’
- For the numerical results, are the EK results the same as the ones in Vu et al. 2021?
- Do the ER/EK waves imposed in the WRF model have seasonality? Can you show the seasonality?
- Figure 5: it’s very interesting to see the TC numbers differ in different ER K experiments while the number keeps quite stable in EK experiments. Can you discuss a little bit why?
- L339: missing ‘ in ‘don’t have’
Citation: https://doi.org/10.5194/egusphere-2023-974-RC1 -
AC1: 'Reply on RC1', Chanh Kieu, 04 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-974/egusphere-2023-974-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2023-974', Paul Roundy, 29 Jun 2023
The authors apply theoretical and ideolized tropical channel modeling to show that initialization inncluding normal mode Rossby waves leaves stationary wave patterns behind in their model over long aquaplanet integrations, but Kelvin wave initializations do not. The stationary wave features act as genesis regions for tropical cyclones. The analysis and results are intriguing, and I think merit publication for academic interest, but I have several concerns about the algorithm, interpretation, and results.
1. Although vertical normal modes are a popular way to build simple models of convectively coupled waves, in the real atmosphere and in numerical models, they may not exist independent of the effects of coupling of waves to convection (which drives overturning circulations limited in vertical extent at the tropopause). Real waves, even convectively coupled ones, typically propagate vertically and the tropopause is not a limit to their movement. This fact implies that initializing a model with idealized normal mode waves will result in the model having to move toward a state consistent with its internal dynamics. This point might not refute the authors' overall arguments because initializing the model with a wave disturbance of the same type but more consistent with the model's native form of the wave, might still result in a similar outcome to what they showed.
2. The authors' analytical solution suggests that stationary waves might occur in the tropics, if the assumptions of the simple model apply in nature. Yet nature can yield stationary waves through other mechanisms. The leading one is probably forcing by regional SST anomalies, interaction with topography, etc., which their model set-up would not include (as the authors already explain). Other possilble sources of stationary waves include waves that would otherwise propagate, but whose propagation is balanced by advection. In a model environment that includes a steady background state flow, it's conceivable that such signals could occur with stability. In nature, this kind of steady basic state is implausible, because sea surface temperature patterns vary over time. I recommend that the authors analyze their model basic state for conditions that could lead to such stationary advection-balanced propagation for Rossby waves. It may be that the model background flow explains why Rossby waves can become stationary in the model but Kelvin waves cannot. In order for Rossby waves to be stationary under conditions of balance by advection, they must be non dispersive. This would place a control on which scales of the waves would be favored by this mechanism to become stationary.
Citation: https://doi.org/10.5194/egusphere-2023-974-RC2 -
AC2: 'Reply on RC2', Chanh Kieu, 04 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-974/egusphere-2023-974-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Chanh Kieu, 04 Aug 2023
Status: closed
-
RC1: 'Comment on egusphere-2023-974', Anonymous Referee #1, 26 Jun 2023
General comments:
This study explored how the planetary stationary waves can modulate the TC genesis location. It is presented that the equitorial Rossby waves can act as an incubator of TCs while equitorial Kelvin waves do not have a clustering effect. The topic is very interesting and can add new insigts into explaining TC genesis ‘hot spot’. However, the current version has some confusing places which need to be addressed before publication.
Specific comments:
- Section 2 is mostly repeating the equations in W19 with very little information about how this part is different from W19. If they are exactly the same, then there is no need to repeat it again as if it is a new result. If not, please clarify the differences and emphasize what’s new.
- The connection between theoretical and modeling part is not stated clearly. The current layout gives me a very abrupt transition from Section 2 and Section 3. For example, what is the connection between the imposed ER wavenumber in WRF simulations and theoretically derived K bound; what are the assosiated magnitude of mean flows, Rossby number, Ekman number and Rayleigh number in the WRF simulations and the corresponding theoretical value of PSW wavenumber from the theory?
Technical corrections:
- L121-124: expand this part to clarify the differences between the theoretical analysis in this manuscript and W19.
- L127: subscription missing in the streamfunction expression: ‘LU_0’ -> ‘L_yU_0’
- L123: a new character ‘I’ is introduced, which is exactly the same as A in W19. Why not adopt the same character?
- L150: ‘max’ on the exponent is a little misleading, I though it was taking some maximum value…
- L168-169, L363: a~0.06 and K~12 in W19, please be consistent.
- L250 and below: V20 is not defined. And I only found Vu et al. 2021.
- In the table 2, the last two rows are mismatched.
- L271-272: I think the purpose of the paper is to show with no land it is possbile to have PSW by earth internal dynamics, but here it reads very contradictory.
- L275: ‘no significant impacts’ on what?
- The criteria in Table 3 are used according to Vu et al. 2021, should state this in text ‘refer to Vu et al. 2021 for more details’
- For the numerical results, are the EK results the same as the ones in Vu et al. 2021?
- Do the ER/EK waves imposed in the WRF model have seasonality? Can you show the seasonality?
- Figure 5: it’s very interesting to see the TC numbers differ in different ER K experiments while the number keeps quite stable in EK experiments. Can you discuss a little bit why?
- L339: missing ‘ in ‘don’t have’
Citation: https://doi.org/10.5194/egusphere-2023-974-RC1 -
AC1: 'Reply on RC1', Chanh Kieu, 04 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-974/egusphere-2023-974-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2023-974', Paul Roundy, 29 Jun 2023
The authors apply theoretical and ideolized tropical channel modeling to show that initialization inncluding normal mode Rossby waves leaves stationary wave patterns behind in their model over long aquaplanet integrations, but Kelvin wave initializations do not. The stationary wave features act as genesis regions for tropical cyclones. The analysis and results are intriguing, and I think merit publication for academic interest, but I have several concerns about the algorithm, interpretation, and results.
1. Although vertical normal modes are a popular way to build simple models of convectively coupled waves, in the real atmosphere and in numerical models, they may not exist independent of the effects of coupling of waves to convection (which drives overturning circulations limited in vertical extent at the tropopause). Real waves, even convectively coupled ones, typically propagate vertically and the tropopause is not a limit to their movement. This fact implies that initializing a model with idealized normal mode waves will result in the model having to move toward a state consistent with its internal dynamics. This point might not refute the authors' overall arguments because initializing the model with a wave disturbance of the same type but more consistent with the model's native form of the wave, might still result in a similar outcome to what they showed.
2. The authors' analytical solution suggests that stationary waves might occur in the tropics, if the assumptions of the simple model apply in nature. Yet nature can yield stationary waves through other mechanisms. The leading one is probably forcing by regional SST anomalies, interaction with topography, etc., which their model set-up would not include (as the authors already explain). Other possilble sources of stationary waves include waves that would otherwise propagate, but whose propagation is balanced by advection. In a model environment that includes a steady background state flow, it's conceivable that such signals could occur with stability. In nature, this kind of steady basic state is implausible, because sea surface temperature patterns vary over time. I recommend that the authors analyze their model basic state for conditions that could lead to such stationary advection-balanced propagation for Rossby waves. It may be that the model background flow explains why Rossby waves can become stationary in the model but Kelvin waves cannot. In order for Rossby waves to be stationary under conditions of balance by advection, they must be non dispersive. This would place a control on which scales of the waves would be favored by this mechanism to become stationary.
Citation: https://doi.org/10.5194/egusphere-2023-974-RC2 -
AC2: 'Reply on RC2', Chanh Kieu, 04 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-974/egusphere-2023-974-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Chanh Kieu, 04 Aug 2023
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