Poster

Incorporating Climate Variability and Numerical Weather Prediction Technology into Hurricane Risk Assessment

The AHC is a new hurricane event set that brings unparalleled technological sophistication to the science of hurricane risk assessment.

● Event occurrence frequencies are climate dependent.
● All hurricane events are simulated with NWP technology.
● Individual event winds are easily accessible by the user.
● Employs a 150+ year historical record augmented with paleo data.

Climate Effects

The AHC allows one to examine how hurricane risk varies due to fluctuations in climate associated with the El-Niño / Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). These climate signals influence the number of hurricanes that form, the likelihood of major hurricanes, and which geographic areas are most at risk.

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Ratio of Category 3-5 hurricane landfall frequency for two sets of ENSO-NAO conditions relative to the long-term average frequency. A value of 2 indicates that the landfall frequency is twice the long-term average; a value of .5 indicates that the landfall frequency is half the long-term average.

Numerical Weather Prediction Technology

NWP technology forms the basis for modern weather forecasting worldwide. The AHC incorporates one of the premier models employed by the National Hurricane Center in their hurricane forecasts. The AHC is the first hurricane risk assessment tool to use NWP technology to capture the complex wind patterns that develop as landfalling storms interact with coastline, land topography, and variations in vegetation and land use. Better representation of over-land hurricane winds means better loss estimates on an event-by-event basis, and better assessment of overall hurricane risk. The AHC is the only hurricane catalog where every hurricane event is generated using an NWP model.

The AHC enables users to see the detailed wind fields that control their exposure.

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RealTrack™ NWP Simulation of Hurricane Isabel. The left figure shows surface wind speed and wind vectors shortly after landfall. The right figure shows maximum wind speed experienced at each location during Isabel’s history.

Extended Historical Record

The AHC incorporates 150 years of hurricane data, providing the most accurate and complete statistical basis for hurricane risk assessment. In addition to the record provided by the US National Hurricane Center (NHC), the AHC incorporates near-shore lake-bottom-sedimentary records which have recorded thousands of years of extreme hurricane events. This extensive data record, combined with modern extreme-value statistical theory, yields the most accurate assessment of extreme event frequencies available today.

Climate Variability

Changes Hurricane Risk

The AHC is the first hurricane risk assessment tool to explicitly account for climate induced variability in hurricane activity. Through careful analysis of long term hurricane and climate records, AEF scientists have established a clear link between Atlantic basin hurricane activity and year-to-year climate variability. The AHC provides quantitative assessments of how climate influences the number of hurricanes that form, the likelihood of major hurricanes, and which geographic areas are most at risk.
The climate phenomena known as the El-Niño / Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) strongly influence hurricane activity in the Atlantic basin.

In the AHC, the climate is divided into a 3x3 matrix composed of high, neutral, and low conditions for both the NAO and ENSO indices. AEF scientists advise clients on which climate is most appropriate for their risk assessment based on both current and forecast conditions. Forecast information is provided as soon as credible predictions become available. ENSO forecasts will be provided in early winter, and updated ENSO and NAO information will be supplied in the spring.

The effect of the climate state on hurricanes varies on a regional basis and as hurricane intensity changes. Major hurricanes – category 3 and higher – are more dramatically affected by climate than weaker storms. For example, the Carolinas show only minor occurrence frequency variability for weaker storms, but major storm occurrence rates in the Carolinas are more than 70% higher under high risk conditions and about 50% lower in low risk conditions.

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Ratio of hurricane landfall frequency for two sets of ENSO-NAO conditions relative to the long-term average frequency. A value of 2 indicates that the landfall frequency is twice the long-term average; a value of .5 indicates that the landfall frequency is half the long-term average.

Every hurricane event in the AHC is assigned multiple return periods that correspond to the different possible climates. Local wind exceedance profiles will vary according to which climate is specified, and hypothetical hurricane seasons constructed from AHC events will reflect the geographic landfall frequency characteristics associated with the different climate scenarios.

Numerical Weather Prediction Technology Produces

the Most Accurate Hurricane Winds Available Today

          Numerical Weather Prediction (NWP) employs three-dimensional computer models to solve the physical equations that describe the evolution of the atmosphere over time. The AHC incorporates one of the premier NWP models employed by the National Hurricane Center in their hurricane forecasts. This NWP technology assures that the over-land winds and hurricane tracks contained in the AHC have an unprecedented level of realism. The AHC is the only hurricane catalog to employ NWP in the generation of storm events.

          The RealTrack™ NWP technology utilized in the AHC is unrivaled in its ability to simulate the detailed wind patterns that arise as hurricanes interact with the land-sea interface, topographic features, and variations in vegetation and land use.

          RealTrack™ NWP technology properly simulates the change in storm structure that occurs when a hurricane makes impact with the coastline.

Hurricane Andrew (1992) Open Exposure 10 m Winds, 0900 UTC 24 Aug

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RealTrack™ NWP Simulation of Hurricane Andrew at Landfall: The left figure shows RealTrack™ simulated 1-minute sustained winds, and the right shows observational analyses from NOAA’s Hurricane Research Division.

RealTrack™ NWP technology captures the details of the interaction between hurricanes and topographic features.

The Hurricane of 1938 (track indicated in gray) traversed an area of New England with highly varied topography, shown in the figure on the upper right. AEF’s RealTrack™ simulation of this storm (lower right) shows the complex wind structures that this topography induced. For this illustration, all variations in vegetation and land use were removed, so that the wind patterns visible in the lower figure are entirely the result of topographic interactions. The physical processes that produced these wind patterns cannot be captured without the use of an NWP model.

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RealTrack™ NWP technology produces hypothetical hurricane tracks that are consistent with the physical laws that govern the movement of complex storm systems. Traditional catastrophe risk models use statistical methods to generate hypothetical hurricane tracks. The AHC is the only hurricane catalog where every hurricane event is based on a track generated by an NWP hurricane model or a historical storm track. This insures that every event in the AHC follows a physically plausible path.

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Hurricanes can move in complicated patterns that are difficult to reproduce with statistical methods. The left figure shows selected tracks from the historical record, and the right shows selected hypothetical tracks produced by RealTrack™ NWP technology.