the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jul 2023
Simulating sea level extremes from synthetic low-pressure systems
Abstract. In this article we present a method for numerical simulations of extreme sea levels using synthetic low-pressure systems as atmospheric forcing. Our simulations can be considered as estimates of the high sea levels that may be reached when a low-pressure system of high intensity and optimal track passes the studied region. We test the method using sites located along the Baltic Sea coast and simulate synthetic cyclones with various intensities and tracks. To model the effects of the cyclone properties on sea level, we simulate internal Baltic Sea water level variations with a numerical two-dimensional hydrodynamic model, forced by an ensemble of time-dependent wind and air-pressure fields from synthetic cyclones. The storm surges caused by the synthetic cyclones come on top of the mean water level of the Baltic Sea, for which we used a fixed upper estimate of 100 cm. We find high extremes in the northern Bothnian Bay and in the eastern Gulf of Finland, where the sea level extreme due to the synthetic cyclone reach up to 3.5 meters. In the event that the mean water level of the Baltic Sea has a maximal value (1 meter) during the cyclone, highest sea levels of 4.5 meters could thus be reached. We find our method to be suitable for use in further studies of sea level extremes.
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RC1: 'Comment on nhess-2023-102', Anonymous Referee #1, 21 Jul 2023
Review on “Simulating sea level extremes from synthetic low-pressure systems” by Jani Särkkä, Jani Räihä, Mika Rantanen, and Matti Kämäräinen
The manuscript investigates sea level extremes in the Baltic Sea through numerical simulations of synthetic low-pressure systems. The authors conducted simulations based on historical records and compared the results with actual data. However, there are several areas where the manuscript can be improved before it is considered for publication:
Cyclone Model Validation
The article heavily relies on synthetic cyclones, which are artificially created. This approach introduces several assumptions, and synthetic cyclones may not accurately represent the complex dynamics and characteristics of real cyclones. This limitation raises questions about the reliability of the results. The discussion on the model's validity should be expanded in Section 2 instead of conclusion. To enhance the manuscript, the authors can:
- Justify the chosen intensity of the cyclone (pressure anomaly ∆P of -40 hPa and minimum radius R of 1000 km) by comparing it to extreme extratropical storm events observed in the Baltic Sea. What is the implication of the radius R and how is it different from the radius of the maximum wind of a tropical storm?
- Explain the characteristics of extratropical storms, compare with the model and discuss the limitation of the model
- If possible, discuss their similarities and differences compared to tropical storms and compare the synthetic cyclone model to existing hurricane models (e.g., Holland 1980).
Numerical Simulations
The manuscript lacks details about the numerical simulations. It would be helpful to include:
- The computational domain used in the simulations.
- The number of simulations performed, including the range of origins, speeds, and directions
Clarification on the handling of initial conditions
- Currently, the cyclone's origin at 10°E with a 1000 km radius suggests that it affects the Baltic Sea at the start of the computation. The manuscript should explain how initial conditions were handled. Also consider originating the cyclone further away from the Baltic Sea for more accurate results.
The grid size in the simulations
- The current spacing of 0.1 degrees in the meridional direction and 0.2 degrees in the zonal direction may be too coarse for predicting sea level elevations at tide gauge stations.
- The correlation between sea level prediction and grid size should be investigated, considering L159 “The higher spatial resolution Averkiev and Klevannyy (2010) use near St. Petersburg is likely the reason for the 1.5 meter difference between their simulated maximum and our result.“ If this statement is valid, it means that the sea level elevation may be higher at Oulu and Riga with finer grid simulations.
Minor Comments
- Throughout the text, it seems that the intensity of the cyclone is fixed, but in several places (e.g. L4, L127), “various intensities” are mentioned. The authors need to correct this.
- Paragraph L50-54 seems unnecessary in the introduction and can be removed.
- Terms such as "large," "small," and "short" need to be defined more precisely, particularly in L194-197.
- L125 - “As we calculate only short-term sea level changes, our results represent the sea level fluctuation with respect to the sea level preceding the arrival of the cyclone.“: unclear
- The mention of an observation in 1984 at Oulu in L144 should include an explanation of what caused that anomaly. For example, what was the intensity (pressure and radius) of the related storm?
Figures
- Figure 2 should include labeled x and y axes.
- Figure 3 does not provide valuable information
Tables:
- Table 1: Pressure anomaly and radius are fixed for all simulations, thus they are not necessary.
Citation: https://doi.org/10.5194/nhess-2023-102-RC1 - AC1: 'Reply on RC1', Jani Särkkä, 28 Nov 2023
-
RC2: 'Comment on nhess-2023-102', Anonymous Referee #2, 08 Oct 2023
- AC2: 'Reply on RC2', Jani Särkkä, 28 Nov 2023
Status: closed
-
RC1: 'Comment on nhess-2023-102', Anonymous Referee #1, 21 Jul 2023
Review on “Simulating sea level extremes from synthetic low-pressure systems” by Jani Särkkä, Jani Räihä, Mika Rantanen, and Matti Kämäräinen
The manuscript investigates sea level extremes in the Baltic Sea through numerical simulations of synthetic low-pressure systems. The authors conducted simulations based on historical records and compared the results with actual data. However, there are several areas where the manuscript can be improved before it is considered for publication:
Cyclone Model Validation
The article heavily relies on synthetic cyclones, which are artificially created. This approach introduces several assumptions, and synthetic cyclones may not accurately represent the complex dynamics and characteristics of real cyclones. This limitation raises questions about the reliability of the results. The discussion on the model's validity should be expanded in Section 2 instead of conclusion. To enhance the manuscript, the authors can:
- Justify the chosen intensity of the cyclone (pressure anomaly ∆P of -40 hPa and minimum radius R of 1000 km) by comparing it to extreme extratropical storm events observed in the Baltic Sea. What is the implication of the radius R and how is it different from the radius of the maximum wind of a tropical storm?
- Explain the characteristics of extratropical storms, compare with the model and discuss the limitation of the model
- If possible, discuss their similarities and differences compared to tropical storms and compare the synthetic cyclone model to existing hurricane models (e.g., Holland 1980).
Numerical Simulations
The manuscript lacks details about the numerical simulations. It would be helpful to include:
- The computational domain used in the simulations.
- The number of simulations performed, including the range of origins, speeds, and directions
Clarification on the handling of initial conditions
- Currently, the cyclone's origin at 10°E with a 1000 km radius suggests that it affects the Baltic Sea at the start of the computation. The manuscript should explain how initial conditions were handled. Also consider originating the cyclone further away from the Baltic Sea for more accurate results.
The grid size in the simulations
- The current spacing of 0.1 degrees in the meridional direction and 0.2 degrees in the zonal direction may be too coarse for predicting sea level elevations at tide gauge stations.
- The correlation between sea level prediction and grid size should be investigated, considering L159 “The higher spatial resolution Averkiev and Klevannyy (2010) use near St. Petersburg is likely the reason for the 1.5 meter difference between their simulated maximum and our result.“ If this statement is valid, it means that the sea level elevation may be higher at Oulu and Riga with finer grid simulations.
Minor Comments
- Throughout the text, it seems that the intensity of the cyclone is fixed, but in several places (e.g. L4, L127), “various intensities” are mentioned. The authors need to correct this.
- Paragraph L50-54 seems unnecessary in the introduction and can be removed.
- Terms such as "large," "small," and "short" need to be defined more precisely, particularly in L194-197.
- L125 - “As we calculate only short-term sea level changes, our results represent the sea level fluctuation with respect to the sea level preceding the arrival of the cyclone.“: unclear
- The mention of an observation in 1984 at Oulu in L144 should include an explanation of what caused that anomaly. For example, what was the intensity (pressure and radius) of the related storm?
Figures
- Figure 2 should include labeled x and y axes.
- Figure 3 does not provide valuable information
Tables:
- Table 1: Pressure anomaly and radius are fixed for all simulations, thus they are not necessary.
Citation: https://doi.org/10.5194/nhess-2023-102-RC1 - AC1: 'Reply on RC1', Jani Särkkä, 28 Nov 2023
-
RC2: 'Comment on nhess-2023-102', Anonymous Referee #2, 08 Oct 2023
- AC2: 'Reply on RC2', Jani Särkkä, 28 Nov 2023
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