Estimation of roughness lengths through gust factor analysis

The aerodynamic roughness length is a parameter that quantifies the degree of friction that terrain elements have on the wind near the earth's surface. It is commonly used in the development of vertical profiles of the wind speed. However, uncertainty exists in the precision of current roughness length estimation schemes. A formula has been developed by Wieringa (1973) that calculates the roughness length from gust factors (ratio of the peak wind gust to the mean wind speed over a period). This method has an advantage in that it relies on readily available local wind data such as analog record charts found at the NWS. This study employed the Wieringa formula to evaluate its applicability and ease of use with a number of different types of wind records.

First of all, the formula was applied to data collected at the Texas Tech Wind Engineering Research Field Laboratory (WERFL), where wind speed records are archived from five different vertical levels. Results indicated that both the gust factors and the roughness lengths obtained were higher than those from previous studies.

Secondly, because both meteorologists and engineers are interested in vertical wind profiles in high winds, an attempt was made to obtain wind records from several hurricane passages. Four reliable hurricane wind records from anemometers located in the vicinity were acquired: Hurricane Bob (1991), Hurricane Iniki (1992), and two from Hurricane Bonnie (1998). The Wieringa formula requires specifications of the anemometry and since each record was produced by a different anemometer/recorder system, they provided a thorough assessment of the formula.

Calculated roughness lengths were plotted and compared against the known topography of the site. The results showed a large variability according to the upstream fetch of the wind in the record. Another conclusion was that the gustiness of the wind during a hurricane is highly dependent upon the terrain and it is difficult to ascertain how turbulence caused by convection affects the gust factor and thus the roughness length.