Tropical Cyclone Risk Models - Circular Symmetry Revisited

From a catastrophe modeling standpoint, accurate simulation of tropical cyclone (TC) winds in terms of the maximum, the distance at which they occur, and how they decay outward from the radius of maximum winds are important features for correctly modeling the damage from TCs. Whether those maximum winds occur on the left, right, front, or back of the storm is also an important consideration. Because of the number of events they must simulate, which typically run in the hundreds of thousands, many catastrophe models are inherently parametric in nature and therefore must make simplifying assumptions. For example, they typically assume that TCs have a circularly symmetric rotational wind field and that the forward speed imposes any horizontal asymmetry which leads to the strongest winds being on the right-hand side for northern hemisphere storms.

Observational and modeling studies show that wind asymmetries can stem from embedded mesoscale circulations like tornadoes, environmental wind shear, and extratropical transitioning. The latter is a typical feature of storms affecting Japan. Over sixty percent and nine of the top ten loss causing events that have affected Japan undergo extratropical transition within 500 km of the coastline. The transitioning process typically leads to the formation of fronts which can significantly skew the symmetry. In many cases, the evolution leads to the formation of strongest winds on the left hand side, which can conflict with the assumption that strongest winds from a TC are on the right hand side and which can lead to inaccuracies in the modeled damage for a particular storm and also for the overall risk of a particular region.

This study examines the climatological frequency with which typhoons affecting Japan exhibit maximum winds on the left hand side. This study also examines the different types of synoptic scale environments under which a left-hand maximum wind occurs. Both of these features are important to understand so that wind field asymmetries can be incorporated more accurately into the parametric wind field calculation of TC catastrophe models. Finally, this study provides a quantitative example of a typhoon event where not accounting for such asymmetries has a significant impact on the insured loss calculation.