Wind direction measurements on moored coastal buoys

Abstract

Wind direction measurements from anchored floating objects (buoys) are disturbed by the movement of an agitated sea surface. This article describes corrected wind direction measurements that account for a rotated and tilted anemometer, in which the rotations around the Euler angles ‘‘roll,’’ ‘‘pitch,’’ and ‘‘yaw’’ are simultaneously measured on coastal buoy ‘‘Vida’’ in Slovenian waters of the Gulf of Trieste. The wind data analysis is composed of a short-term analysis during a 4-h summer storm and a long-term analysis of wind direction for an 8-yr period. In the short-term analysis, the time evolution of the spectral composition of the anemometer’s translation and wind speeds revealed the dominance of 3–4-s oscillations that are mostly caused by waves. Anemometer velocity may account for up to 5% of apparent wind speed. In the long-term analysis, wind direction measurements were validated against those from two other stations in the Gulf of Trieste, as well as from the Acqua Alta tower in front of the Venice lagoon; all three are located in Italian waters. Short-term analysis of a summer storm and long-term measurements from four stations show that correcting the wind direction with the buoy’s orientation accounts for about 108–128 of the error in wind direction.