Toward Objective, Standardized Intensity Estimates from Surface Wind Speed Observations

Abstract

Extreme wind climatology and event-specific intensity assessments rely heavily on surface wind field observations. The most widely used platforms sited at airports are the Automated Surface Observing System (ASOS) and its predecessor, the Automated Weather Observing System (AWOS). The terrain immediately surrounding most of these stations may be nominally characterized as aerodynamically very smooth because of the runways and flat expanses of grass that define most airport layouts. Outside of most airports, a wide spectrum of marine, open, suburban, and heavily built-up terrain conditions are present. The results of this research indicate that the wind speeds recorded by AWOS/ASOS are deeply sensitive to this terrain. Prior research has shown that direct usage of the raw surface data can introduce surface-layer wind speed errors on the order of 30%?40% due to terrain effects. Similar values are observed for gust speeds in this paper, when averaging technique and anemometer response characteristics are considered. A solution is developed to automatically compute the directional effective roughness length (z0) values using simple averages of peak-to-mean wind speed ratios (gust factors). Adjustments are also made to cup anemometer data to correct for gust attenuation caused by frequency response and block averaging. A new effective surface roughness database is offered that can be used to convert a raw wind speed measurement (sustained or gust) to any predefined aerodynamic metadata (height, terrain, and average period) to serve the needs of operational and research users.