Characterization and comparison of aerodynamic roughness lengths using ground-based photography and sonic anemometry

In this study, two methods used to estimate surface terrain conditions (z0) surrounding Automated Surface Observing System (ASOS) stations in tropical cyclone-prone regions were analyzed. The first method utilized ground-based photography from the National Oceanic and Atmospheric Administration Hurricane Research Division's Tropical Cyclone Wind Exposure Documentation Project (WEDP). The second applied a modified effective z0 method using ASOS wind data. Comparisons reveal that WEDP z0 estimates are typically larger in magnitude and do not account for changes in upstream z0. Variability in the z0 estimates showed a distinctly skewed nature in the probability distributions, which may have some physical meaning. It was also shown that standardized maximum 1-min sustained wind speeds using both the WEDP and MM2010 median z0 estimates for short fetch lengths can have differences as large as 15%. The MM2010 z0 estimates were also compared with single weighted mean z0 estimates to be incorporated in the next version of the U.S. wind loading standard and suggest that a single weighted mean z0 value does not effectively capture the variability of the terrain with respect to wind direction.