Going with the flow: state of the art marine meteorological measurements on the new NERC research vessel

Abstract

Research ships obtain high-quality meteorological measurements above the ocean surface. These measurements include wind speed and direction, air and sea surface temperature, and atmospheric humidity. Although the sensors are usually well maintained and located in well-exposed sites, such as on a foremast in the bows of the ship, errors can still occur in the measurements. For example, the wind speed measurements will be biased by the distortion of the flow of air to the anemometer caused by the presence of the ship itself. Depending on the ship’s orientation to the wind, the airflow may be raised substantially as it flows over the ship, accelerated or decelerated and become highly turbulent downwind of masts and funnels. Improving the accuracy of the wind speed measurements from research ships is important for research studies of the physical and chemical interactions between the atmosphere and the ocean; for example, studies which examine the rate at which heat, momentum and carbon dioxide are exchanged between the atmosphere and the surface waters. Understanding these exchanges will contribute to a better understanding of the world’s climate. For this reason, airflow distortion effects were taken into account in the design of the UK’s new research vessel, the RRS James Cook. The RRS James Cook (Figure 1) was delivered to the UK’s Natural Environment Research Council (NERC) in the summer of 2006, and replaced NERC’s RRS Charles Darwin which was decommissioned in June 2006. The RRS James Cook is 90 m in length and operates worldwide from the tropics to the edge of the ice sheets. The ship’s design enables it to work in higher seastates and stronger winds than the NERC’s other research vessel, the RRS Discovery. The RRS James Cook can carry 54 scientists and crew and spend up to 50 days at sea before needing to refuel. During the design stage of this vessel, scientists from the National Oceanography Centre, Southampton, (NOCS) made a number of recommendations to the naval architects to guide the production of a ship capable of obtaining the most accurate wind speed measurements possible. NOCS staff advised that the new ship should have a raked, or streamlined, superstructure to minimize the effects of airflow distortion, and that the mast in the bows (where the anemometers are located) be as tall and as far forward as possible. Once the ship was designed, the numerical model VECTIS (Ricardo, 2001) was used to simulate the airflow over the RRS James Cook for wind directions blowing within 30 degrees of bow-on. This range of wind directions was chosen since it is the most commonly encountered (especially when the ship is on passage or on station) and it is also the same range that is used for many research studies of atmosphereocean interaction. The model simulations were three-dimensional and were solved using computational grids of up to 3 million cells. Each wind direction took about a week to complete. The number of cells used was higher in regions of interest (e.g. the foremast) and lower in other areas. Figure 2 shows the numerical model of the ship along with a single slice of wind speed data for a bow-on wind direction. When well away from the influence of the ship the wind speed has a logarithmic vertical profile, i.e. the wind speed near the sea surface is close to zero and the speed increases with height. When the wind approaches the ship, it begins to distort. Figure 2 shows that the flow of air is severely decelerated in some regions (shown by the blue arrows), particularly