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Mapping of Topographic Effects on Maximum Sustained Surface Wind Speeds in Landfalling Hurricanes

Craig Miller (cam@blwtl.uwo.ca)

Boundary Layer Wind Tunnel Laboratory Department of Civil and Environmental Engineering University of Western Ontario

While the effect of large-scale topography, such as that found on Hispaniola in the Caribbean, on the overall structure and intensity of hurricanes passing over such topography is reasonably well understood, forecasters at the Tropical Prediction Centre/National Hurricane Centre (TPC/NHC) currently have no means available to them to assess the impact of small-scale topography, whose maximum height is less than the depth of the boundary layer, on near surface wind speeds in hurricanes making landfall over such terrain. For topography of this nature it is found that wind speeds near the crests of hills and ridges show marked increases in wind speed when compared to the equivalent wind speed measured at the same height above flat terrain, with increases of over 90% being observed in some field studies. Clearly this speed-up effect has major implications for forecast surface wind speeds in tropical cyclones making landfall on islands such as Puerto Rico and the U.S. Virgin Islands in the Caribbean, or the Hawaiian Islands and Guam in the Pacific, where topographic effects on surface wind speeds are likely to be significant.

Using a linear model for boundary layer flow over topography in combination with the U.S. Geological Survey’s National Elevation Dataset (NED) the effects of small-scale topography on surface wind speeds at a height of 10 m for Puerto Rico and the US Virgin Islands have been mapped. In principle, provided suitable digital terrain data is available, the same approach can also be used to map topographic speed-up effects for other regions of interest to the TPC/NHC if a need is indicated. The original project proposal called for a set of maps showing contours of speed-up factors to be delivered to the TPC/NHC that could then be used by forecasters to assess the effects of topography on maximum sustained surface wind speeds in hurricanes making landfall in Puerto Rico or the US Virgin Islands. This, however, required forecasters to make decisions about the appropriate wind directions to use as well as calculations of the likely wind speeds in the event of a landfalling hurricane.

In an attempt to try and streamline the process a program built around an open source Geographic Information System (GIS) package has been developed that allows forecasters to select an island, a track heading, the position of the track in relation to the island being considered and the maximum sustained wind speed before plotting the forecast wind field using either 5 kt wind speed bands, or Saffir-Simpson hurricane categories. Overlays showing major population centres, roads, etc. can also be superimposed on the forecast wind field.

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