The perspective below: ground-level reconnaissance in landfalling hurricanes

A growing body of evidence suggests that small-scale rain bands (Gall et al. 1998), boundary layer rolls (Wurman and Winslow 1998) and other convective features contribute to or modulate the gustiness of these winds. It is also speculated that these convective features are responsible for causing the isolated swaths of damage to buildings and trees observed during damage assessments (Wakimoto and Black 1994). These events have been successfully recreated in high-resolution full-physics “moist” mesoscale numerical models (Lau et al. 2004) and “dry” hurricane boundary-layer models (Kasheta and Chang 2002), but comprehensive field observation of “wind” or “damage” streaks has not occurred during a landfalling hurricane. The data that is available do point to the transport of large eddies to the earth's surface, however. First, hurricane surface wind power spectra (Schroeder and Smith 2003) contain additional low-frequency (large eddy) energy not found in extratropical models. Second, large integral length scales—estimates of the average physical dimensions of turbulent eddies—also appear with some periodicity during landfall. These length scales are on the order of 3-4 times larger than observations from extratropical models and are expected to be found much higher in the atmosphere. Third, the ratios of longitudinal turbulence intensity to friction velocity are unusually high compared to winter storm and thunderstorm data, which indicates that these gust structures do not contribute to shear stress at the surface. These observations are consistent with predictions from high Reynolds number turbulent boundary layer research (Högström et al. 2002) that suggest surface-layer turbulence is determined by attached eddies that largely originate in the shearing motion immediately above the surface layer. Further, these conditions meet all of the descriptions of inactive turbulence concept described by Townsend (1961) and Bradshaw (1967), yet to the authors' knowledge, no clear link between hurricane convective features and this phenomenon has been established.

This paper investigates these issues through recent field research conducted by the Florida Coastal Monitoring Program (FCMP). The FCMP is a unique joint venture—led by wind engineering faculty at FIU, Clemson University and the University of Florida—that deploys mobile weather stations to measure ground-level wind speeds, instruments single-family housing to quantify wind pressure loading and conducts damage assessments to evaluate the performance of low-rise structures and the building codes and standards that guided their construction. The FCMP has collected 50+ observations in 20 named storms in Alabama, Florida, Louisiana and North Carolina since 1998, including Dennis, Katrina, Rita and Wilma in 2005.

The current research infrastructure includes six 10-m mobile weather stations designed to withstand gust loading and debris generated by a strong Category 5 hurricane. The data acquisition system measures 3D wind speed and direction at 5- and 10-m and collects temperature, rainfall, barometric pressure, and relative humidity data at the tower's base. Research personnel can deploy a tower in less than 30 minutes and within the hour, begin transmitting summary data from anywhere in the field to emergency managers and meteorologists in real-time. These research activities support the Hurricane Research Division (HRD) and the National Hurricane Center (NHC). During landfall, the HRD compares and quality controls reconnaissance aircraft data—which include flight-level, GPS sonde and stepped frequency microwave radiometer wind speed estimates—to wind speed data collected from FCMP weather stations erected near or at the coast. The NHC uses FCMP data to evaluate conditions at landfall and to verify its forecasts.

References

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