Landfalling Tropical Cyclone Winds

Mapping global Tropical Cyclone (TC) wind speed probabilities is challenging due to i) a lack of historical TC wind data and ii) the strong influence of coastal mountain ranges on local winds. This collaboration between C3WE and Willis Towers Watson developed a novel and computationally efficient approach to simulating landfalling TC winds anywhere on Earth. It captures local features such as high winds over coastal hills and lulls over rough terrain. A two-step process models the gradient-level wind field using a parametric wind field model fitted to TC track data, then brings the winds down to the surface using a numerical boundary layer model. The physical wind response to variable surface drag and terrain height produces substantial local modifications to the smooth wind field provided by the parametric wind profile model. For a set of U.S. historical landfalling TCs the simulated footprints compare favorably with surface station observations. Figure 1 shows an example simulation of the overland winds for Hurricane Maria (2017) over Puerto Rico. The snapshot of the wind speeds and the storm footprint both show wind speed-up over high terrain and slow-down over the urban area of San Juan.

A dataset of 714 global historical overland wind footprints has been created to provide new views of historical events. This dataset is being used to advance our understanding of overland TC wind speed probabilities in regions of complex terrain. A preliminary analysis shows strong regional variability in the inland wind speed decay rates, as influenced by regional terrain characteristics. The dataset is also being used by the reinsurance industry to evaluate the wind field module of TC catastrophe models. These modules rely on site coefficients to represent terrain effects rather than physical modeling. In addition, this event set is being used to stress test reinsurance structures to ensure companies have adequate protection, and may demonstrate protection levels to regulators.

Simulated 1-minute sustained wind speeds at 10 meters above the surface for Hurricane Maria (2017) over Puerto Rico. (a) A snapshot at the time of landfall with arrows indicating the wind vectors. (b) Storm lifetime maximum wind speed, or ‘footprint’ (filled contours). Terrain elevation is contoured every 300m and the hurricane track is shown by the thick black line with recorded maximum wind speeds shown by the colored circles every 6 hours along the track.
Figure: Simulated 1-minute sustained wind speeds at 10 meters above the surface for Hurricane Maria (2017) over Puerto Rico. (a) A snapshot at the time of landfall with arrows indicating the wind vectors. (b) Storm lifetime maximum wind speed, or ‘footprint’ (filled contours). Terrain elevation is contoured every 300m and the hurricane track is shown by the thick black line with recorded maximum wind speeds shown by the colored circles every 6 hours along the track.

Reference

  • Done, J. M., Ge, M., Holland, G. J., Dima-West, I., Phibbs, S., Saville, G. R., and Wang, Y.: Modelling Global Tropical Cyclone Wind Footprints, Nat. Hazards Earth Syst. Sci. Discuss., accepted, 2019.