Hurricane Cost is Largely Controlled by the Vertical Wind Shear

It is challenging to identify metrics that best capture hurricane destructive potential and costs. Although it has been found that the sea surface temperature and vertical wind shear can both make considerable changes to the hurricane destructive potential metrics, it is still unknown which plays a more important role. Here we present a new method to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the integrated power dissipation, kinetic energy and angular momentum of hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are much better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also the cost of hurricanes. The physical cause of this is likely to be the important role of the wind shear controlling the size and intensity of the initial vortex and ultimately the size of the US landfalling hurricane.

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Currently, there are several well-known metrics to infer the destructive potential of hurricanes. The accumulated cyclone energy (ACE) and power dissipation index (PDI) are good representatives of these measures, as they are able to consider the hurricane frequency, intensity and duration. The important role of sea surface temperature (SST) in hurricane intensity has been identified using PDI and ACE. However, the limitation of these metrics is that they do not take into account the spatial extent of the hurricane wind structure, namely, any size effects.

The size effect is crucial to understanding the hurricane destructive potential and cost. The vertical wind shear is one of the most important atmospheric variables affecting hurricane size and wind structure evolution. However, it has been unclear whether the SST or vertical wind shear plays a more important role in the ultimate damage. To answer this question we need metrics of hurricane destructive potential that take into account the hurricane intensity and wind structure at the same time. To date it has not been possible to conduct such an analysis because it requires continuous historical profiles of near-surface wind speed from hurricane center to an outer storm limit.

To overcome this obstacle, we use a new analytical model (“the λ model”) to reconstruct the hurricane historical wind profiles for 1988-2014. The λ model is highly effective because it requires no scaling parameters. It constructs a wind profile from only the minimum surface pressure, the latitude of hurricane center and one measure of wind radius. With the reconstructed wind profiles, we calculate three “integrated metrics”: the integrated power dissipation (IPD), the integrated kinetic energy (IKE) and the integrated angular momentum (IAM). These metrics are based on the whole wind structure at landfall so the hurricane intensity and size effect are both considered at the same time.

Our results show that the IPD of individual hurricanes at landfall is well correlated with the adjusted hurricane cost. However, the IPD itself is only weakly related to hurricane intensity. Neither maximum wind speed at landfall nor PDI correlate as well with the hurricane cost as IPD does. There is also a good correlation between the hurricane cost and the other integrated metrics IKE and IAM. For the intensity only driven metric ACE, the weak correlation is similar to PDI. The wind structure at landfall is therefore a crucial factor of the destructive potential of individual hurricanes, rather than just the intensity or the duration.

We next compare the annually accumulated IPD at landfall to the annually accumulated PDI of all hurricanes for 1988-2014. To explain the long-term changes in IPD and PDI, we also look at the annual variations in SST and vertical wind shear within the main development region of hurricanes (MDR). Our results show that the SST is somewhat positively related to IPD (R²=0.36), but the vertical wind shear shows a remarkably stronger anti-correlation (R²=0.73). In terms of hurricane cost, the annually accumulated IPD shows a much better correlation than the PDI does. Since the annually accumulated IPD shows good correlations with both long-term hurricane cost and environmental factors, it is plausible to establish a link between the cost and SST or vertical wind shear in the MDR directly. It is surprising that the annual hurricane cost is largely controlled by the vertical wind shear in the MDR (R²=0.64). In contrast, the correlation between the cost and SST is much weaker (R²=0.26) and more uncertain.

We also investigate the sensitivity of the annually accumulated hurricane cost and five hurricane destructive potential metrics (IPD, IKE, IAM, PDI and ACE) to the SST and vertical wind shear in the MDR. The sensitivity to SST in the MDR is 222%/^oC (for cost), 155%/^oC (for IPD), 150%/^oC (for IKE), 146%/^oC (for IAM), 103%/^oC (for PDI) and 101%/^oC (for ACE). On the other hand, decreasing the vertical wind shear in the MDR by 1.0 m/s is linked to the increase in hurricane cost of 126%, IPD of 86%, IKE of 83 %, IAM of 81%, PDI of 46% and ACE of 41%.

The physical explanation in this strong connection between the wind shear and damage likely lies in the evolution of hurricane wind structure and their final size. The hurricane size can be significantly influenced by the size and intensity of the initial vortex over the MDR that develops to a US landfalling hurricane later. A large and strong initial vortex cannot be generated under strong vertical wind shear in the MDR because it inhibits genesis and subsequent intensification. When the initial vortex is larger it encourages horizontal angular momentum flux into the hurricane to drive its growth. This explains the remote link of the vertical wind shear in the MDR and the annually accumulated IPD, IKE and IAM at US landfall.