An overview of the airflow distortion at anemometer sites on ships

Wind speed measurements obtained from ship-mounted anemometers are biased by the presence of the ship itself distorting the flow of air to the anemometer. Recent studies have used numerical models to simulate the flow around very detailed representations of individual research ships in order to quantify the effects of flow distortion at the anemometer locations. Such anemometers are generally in well-exposed positions, typically on a mast in the bows of the ship. In contrast, very little is known about the possible effects of airflow distortion on the wind speed measurements from fixed anemometers on Voluntary Observing Ships (VOS). This is because (1) the several thousand or so merchant vessels vary significantly in shape and size and it would be impractical to study each individual ship, and (2) the anemometer location is not usually known. This paper describes initial results from a study of flow distortion over 'typical' tankers and bulk carriers. A method to describe the shapes of these VOS and container ships is presented. The influence of distortion on the flow above the bridges of VOS is shown to be significant, with possible biases in the measured wind speed of between +11% and -100%, depending on the anemometer location. Recommendations for the siting of anemometers are made. Copyright (c) 2005 Royal Meteorological Society.