The tropical cyclone risk assesses how often a hurricane or a typhoon brings the site wind able to cause structural damage (at least category 1). It is estimated from 10,000 years of cyclones simulated from those observed between 1980 and 2017; for the future it accounts for slightly less frequent but more intense cyclones, as assessed by the IPCC. In inland Europe the risk is nil; on the Mediterranean coasts it is not assessed, because no published dataset covers medicanes.
Indicator. Return period, in years, of hurricane-force wind: category 1 on the Saffir-Simpson scale, 64 knots of 1-minute mean wind, that is 29 m/s of 10-minute mean wind with the 0.88 factor used by STORM. The 100-year cyclone wind and, as information only, the one of the STORM future models are also reported.
Data. STORM (Bloemendaal et al. 2020): 10,000 years of synthetic tropical cyclones built on the observed IBTrACS tracks 1980-2017, with the maximum 10 m wind (10-minute mean) for 28 return periods from 10 to 10,000 years, on a 0.1° grid (about 10 km). The cell of the site is used or, for coastal and island sites, the nearest one with data within 20 km. Scenarios: SSP1-2.6 / SSP2-4.5 / SSP3-7.0 / SSP5-8.5.
Calculation.
(1) The return period of 29 m/s is interpolated, on a logarithmic scale, between the two STORM return periods that bracket it.
(2) Future: for each basin the median changes assessed by Knutson et al. 2020 for 2 °C of warming are used, the basis of IPCC AR6 chapter 11: cyclone intensity increasing (from +0.9% in the South-West Pacific to +5.5% in the North-East Pacific) and total number decreasing (from -3.8% to -18.1%).
(3) The changes are brought to the warming of each scenario and year with the method of Jewson 2021, the same as in the CLIMADA tool, using the global temperature assessed by IPCC AR6 (20-year means) against the STORM reference period.
(4) Winds are multiplied by the intensity change and return periods divided by the frequency change; then the return period of 29 m/s is recomputed.
(5) The four STORM future models (SSP5-8.5, 2015-2050, Bloemendaal et al. 2022) project an increase about three times stronger: they are shown as information and do not set the class.
Notes: below 10 years STORM says nothing more, so a site that already gets hurricane-force wind every 10 years stays F; the data do not account for surface roughness and terrain.
Where it is not computed.
• Outside the cyclone basins, that is no cyclone in 10,000 simulated years (for example inland Europe) → A.
• Within about 30 km of the Mediterranean (IHO sea areas): medicanes exist, but STORM does not model them and no published dataset covers them → not assessed (N/D).
• Poleward of 40 degrees latitude, where STORM has data: the authors advise against using them, because the model does not simulate the transformation of hurricanes into extratropical storms → not assessed (N/D). The wind of former hurricanes falls under the peak gust risk.
Thresholds → level A–F.
• hurricane-force wind at least once every 10 years → F
• every 10–50 years → E
• every 50–100 years → D
• every 100–1,000 years → C
• every 1,000–10,000 years → B
• less than once every 10,000 years, or outside the basins → A.
Same thresholds as the peak gust risk (ThinkHazard!, Eurocode EN 1991-1-4, STORM); the six-level ladder is a 3Bee choice.
From physical to impact. The physical level (A–F) is combined with the production process's sensitivity through the risk matrix (6 physical levels by 5 sensitivity bands) → impact level; the damage curve converts the level into a damage factor, from which the economic value at risk is derived.
Sources.
• [Bloemendaal et al. 2020, STORM tropical cyclone return periods](Bloemendaal et al. 2020, STORM tropical cyclone return periods)
• [Bloemendaal et al. 2022, STORM under future climate](Bloemendaal et al. 2022, STORM under future climate)
• [Knutson et al. 2020, Tropical Cyclones and Climate Change Assessment, Part II](Knutson et al. 2020, Tropical Cyclones and Climate Change Assessment, Part II)
• [Jewson et al. 2021, Knutson et al. 2020 tropical cyclone projections data](Jewson et al. 2021, Knutson et al. 2020 tropical cyclone projections data)
• [Jewson 2021, Knutson et al. projections converted to risk model baselines](Jewson 2021, Knutson et al. projections converted to risk model baselines)
• [IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate](IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate)
• [IPCC AR6 WGI, Chapter 4: assessed global surface air temperature projections](IPCC AR6 WGI, Chapter 4: assessed global surface air temperature projections)
• [ThinkHazard! methodology report (GFDRR)](ThinkHazard! methodology report (GFDRR))
• [Marine Regions, IHO Sea Areas](Marine Regions, IHO Sea Areas).
Indicator. Return period, in years, of hurricane-force wind: category 1 on the Saffir-Simpson scale, 64 knots of 1-minute mean wind, that is 29 m/s of 10-minute mean wind with the 0.88 factor used by STORM. The 100-year cyclone wind and, as information only, the one of the STORM future models are also reported.
Data. STORM (Bloemendaal et al. 2020): 10,000 years of synthetic tropical cyclones built on the observed IBTrACS tracks 1980-2017, with the maximum 10 m wind (10-minute mean) for 28 return periods from 10 to 10,000 years, on a 0.1° grid (about 10 km). The cell of the site is used or, for coastal and island sites, the nearest one with data within 20 km. Scenarios: SSP1-2.6 / SSP2-4.5 / SSP3-7.0 / SSP5-8.5.
Calculation.
(1) The return period of 29 m/s is interpolated, on a logarithmic scale, between the two STORM return periods that bracket it.
(2) Future: for each basin the median changes assessed by Knutson et al. 2020 for 2 °C of warming are used, the basis of IPCC AR6 chapter 11: cyclone intensity increasing (from +0.9% in the South-West Pacific to +5.5% in the North-East Pacific) and total number decreasing (from -3.8% to -18.1%).
(3) The changes are brought to the warming of each scenario and year with the method of Jewson 2021, the same as in the CLIMADA tool, using the global temperature assessed by IPCC AR6 (20-year means) against the STORM reference period.
(4) Winds are multiplied by the intensity change and return periods divided by the frequency change; then the return period of 29 m/s is recomputed.
(5) The four STORM future models (SSP5-8.5, 2015-2050, Bloemendaal et al. 2022) project an increase about three times stronger: they are shown as information and do not set the class.
Notes: below 10 years STORM says nothing more, so a site that already gets hurricane-force wind every 10 years stays F; the data do not account for surface roughness and terrain.
Where it is not computed.
• Outside the cyclone basins, that is no cyclone in 10,000 simulated years (for example inland Europe) → A.
• Within about 30 km of the Mediterranean (IHO sea areas): medicanes exist, but STORM does not model them and no published dataset covers them → not assessed (N/D).
• Poleward of 40 degrees latitude, where STORM has data: the authors advise against using them, because the model does not simulate the transformation of hurricanes into extratropical storms → not assessed (N/D). The wind of former hurricanes falls under the peak gust risk.
Thresholds → level A–F.
• hurricane-force wind at least once every 10 years → F
• every 10–50 years → E
• every 50–100 years → D
• every 100–1,000 years → C
• every 1,000–10,000 years → B
• less than once every 10,000 years, or outside the basins → A.
Same thresholds as the peak gust risk (ThinkHazard!, Eurocode EN 1991-1-4, STORM); the six-level ladder is a 3Bee choice.
From physical to impact. The physical level (A–F) is combined with the production process's sensitivity through the risk matrix (6 physical levels by 5 sensitivity bands) → impact level; the damage curve converts the level into a damage factor, from which the economic value at risk is derived.
Sources.
• [Bloemendaal et al. 2020, STORM tropical cyclone return periods](Bloemendaal et al. 2020, STORM tropical cyclone return periods)
• [Bloemendaal et al. 2022, STORM under future climate](Bloemendaal et al. 2022, STORM under future climate)
• [Knutson et al. 2020, Tropical Cyclones and Climate Change Assessment, Part II](Knutson et al. 2020, Tropical Cyclones and Climate Change Assessment, Part II)
• [Jewson et al. 2021, Knutson et al. 2020 tropical cyclone projections data](Jewson et al. 2021, Knutson et al. 2020 tropical cyclone projections data)
• [Jewson 2021, Knutson et al. projections converted to risk model baselines](Jewson 2021, Knutson et al. projections converted to risk model baselines)
• [IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate](IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate)
• [IPCC AR6 WGI, Chapter 4: assessed global surface air temperature projections](IPCC AR6 WGI, Chapter 4: assessed global surface air temperature projections)
• [ThinkHazard! methodology report (GFDRR)](ThinkHazard! methodology report (GFDRR))
• [Marine Regions, IHO Sea Areas](Marine Regions, IHO Sea Areas).