Operational Implications for NEXRAD WSR-88D Wind Speed Reduction Factors

As hurricanes approach the coastline, Air Force and NOAA reconnaissance aircraft missions focus on flying parallel to the coastline to assist operational forecasters with issuing pre-landfall warnings and advisories. These warnings, in turn, are relayed to emergency managers and the general public in order to preserve life and property. However, once a hurricane makes landfall, reconnaissance missions cease and the established methodologies employed to reduce flight-level wind speeds to the surface are no longer valid. To fill this data void, a modified Velocity Azimuth Display (VAD) technique is employed to the coastal Next-Generation Radar (NEXRAD) network to generate horizontal wind profiles. The VAD wind profiles are then coupled with archived Automated Surface Observing System (ASOS) and historical Texas Tech University Hurricane Research Team (TTUHRT) surface deployment wind data to evaluate wind speed reduction factors (WSRF).

Overland WSRFs are generated using a fifteen minute window centered on the timestamp representative of the middle of the volume coverage pattern (VCP) employed in landfalling hurricanes that come within 150 km of coastal NEXRAD WSR-88D sites. Eulerian measurements below 10 m are adjusted to the 10 m observation height before the mean wind speed is standardized to open exposure. Standardized, 10 m mean wind speeds are adjusted to a 5s gust using the Durst Curve and divided by the VAD mean hurricane jet wind speed in each time window. In non-hurricane jet wind regimes, the radar bin that contains the highest wind speed magnitude is used in place of the hurricane jet wind speed. It is hypothesized that the magnitude of the maximum wind speed contained in the VAD wind profile represents the upper bound of the maximum momentum expected to reach the surface during landfall.

Preliminary comparisons show that overland WSRFs differ by distance from measurement site to storm center, storm-relative azimuth, wind speed magnitude, and reference height. To account for storm-relative dependencies, the WSRFs are grouped into 90o storm-relative quadrants assuming the interpolated storm heading is north. Following the storm-relative binning, the WSRFs are binned a second time using a custom distance binning scheme with a radius of influence less than or equal to 150 km. The distance dependent WSRFs are applied to each respective storm-relative quadrant hurricane jet/VAD maximum wind speed and compared against a second adjustment method that uses the mean WSRF for an entire distribution of WSRFs for a given storm and WSR-88D site. Preliminary percent difference calculations reveal that the distance-dependent adjustment technique outperforms the single mean WSRF adjustment method; however, the mean WSRFs for each distance bin exhibit a fair amount of storm-to-storm variability at each radar site.