1,721,037 research outputs found
HiWASE: calibration of surface salinity measurements
Full text linkBetween 1978 and 2009 the Norwegian weather ship Polarfront made continuous meteorological measurements at Station Mike (66oN 2oE). In September 2006, as part of the HiWASE project the ship’s existing measurement systems were complemented by the AutoFlux system to measure the transfers of momentum, heat and CO2 between the atmosphere and the ocean. Surface salinity was measured using a thermosalinograph (TSG) as part of the AutoFlux system. The TSG data were calibrated by comparison to surface CTD measurements, Nansen surface bottles and underway bottle samples. The corrected TSG salinity data has a residual difference from the calibration data, which is generally less than ±0.1 psu except for the summer months when this increases ±0.2 psu. This is sufficient for this study since salinity was only used for the calculation of CO2 solubility in the surface water. The corrected salinity data show a sharp decrease in salinity of about 1 psu during July and August each year. The salinity measured during this time is highly variable and must be used with caution. The data are available from the British Oceanographic Data Centre, UK (http://www.bodc.ac.uk/)
Airflow distortion at instrument sites on the RV "Tangaroa"
Full text linkAccurate wind speed measurements from anemometers on research ships are required to obtain high quality air-sea flux measurements. However, the measurements can be biased by the distortion of the airflow over the ship, i.e. the wind speed can either be accelerated or decelerated by the presence of the ship. The computational fluid dynamics software VECTIS is used here to numerically simulate the airflow over the RV Tangaroa. The airflow distortion at ten anemometer sites has been quantified for a wind speed of 10 ms -1 blowing a) directly over the bows of the ship, b) from ±15 degrees and c) ±30 degrees off the bow. The wind speed errors ranged from decelerations of about 5 % at well-exposed bow locations to decelerations of close to 70 % in the turbulent wake region downwind of the ship’s superstructure. Three anemometers located above the bridge top experienced wind speed increases of between 4 % and 16 % of the free stream, or undistorted, wind speed
Airflow distortion at anemometer sites on the OWS Polarfront
Full text linkAccurate wind speed measurements from anemometers on research ships are required to obtain high quality air-sea flux measurements. However, the measurements can be biased by the distortion of the airflow over the ship, i.e. the wind speed can either be accelerated or decelerated by the presence of the ship and the flow of air can be displaced vertically over the ship's superstructure. The computational fluid dynamics software VECTIS was used to numerically simulate the airflow over the Ocean Weather Ship Polarfront. The airflow distortion at six anemometer sites has been quantified for a wind speed of 10 ms-1 blowing a) directly over the bows of the ship and b) over the ship’s starboard beam. The wind speed errors ranged fromdecelerations of about 1 % for an airflow directly over the bow to accelerations of 10 % for the beam-on flow
Validation of the VECTIS steady-state solver
Full text linkWind speed measurements are obtained from anemometers located on research ships. Even though the anemometers are usually positioned in well-exposed locations the presence of the ship’s hull and superstructure distorts the airflow to the anemometer and biases the wind speed measurements. Previous studies have shown biases of up to 10 % for bow-on flows, and that the biases generally increase for other wind directions. Corrections for the effects of the flow distortion are vital, as these data are used for satellite validation and in climate related studies. Therefore, the computational fluid dynamics (CFD) package VECTIS is used to numerically simulate the airflow over ships and derive corrections for this effect.A VECTIS simulation of one ship at one wind direction currently takes approximately one month to perform on a typical UNIX workstation. Therefore, it would be impractical to study the airflow over a large number of research ships and/or a large number of wind directions. A faster method (the “steady-state solver”) for VECTIS simulations has been available for some time, but requires significant increases in computational speed and memory which have only recently become widely available. This report presents a comparison of VECTIS simulations using the steady-state solver with both previous VECTIS studies and in situ wind speed measurements.Use of the steady-state solver requires a higher mesh density but also cuts model convergence times from weeks to days, allowing fine-resolution models to be run without impractical time constraints. The results of this study show that in regions where the flow distortion is high, the increased mesh density results in significant improvement in the comparison between modelled and in-situ wind speeds
Airflow distortion at instrument sites on the RRS James Clark Ross during the WAGES project
Full text linkWind speed measurements obtained from anemometers mounted on ships are prone to systematic errors caused by the distortion of the airflow around the ship's hull and superstructure. This report describes the results of simulations of the airflow around the RRS James Clark Ross made using the computational fluid dynamics (CFD) software VECTIS. The airflow distortion at anemometer sites used during the WAGES project has been quantified at a wind speed of 10 m/s for relative wind directions of 0 (bow-on), 10, 20, 30, 50, 70, 90 and 110 degrees off the bow. The anemometers used in this study were located in the bows of the ship. Temperature sensors were located on the port side of the monkey island. For bow-on flows the anemometers in the bows of the ship experienced relatively small flow distortion. At these sites the flow was decelerated by about 1% of the free stream wind speed. Over the full range of relative wind directions the flow to the R3 sonic is generally accelerated with the largest wind speed biases at flows directly over the beam. The vertical displacement of the airflow increases from around 1 to 2 m for flows directly over the bow, to around 5m for flows over the ships beam as the blockage of the airflow by the ship becomes greater.The airflow distortion at the temperature sensor locations above the monkey island was typically greater than the well-exposed foremast locations. These locations experienced wind speed biases from 6% increase for an airflow directly over the bow, to large decelerations of 55 % when the instruments were in the large recirculation region for flows directly over the starboard side
Going with the flow: state of the art marine meteorological measurements on the new NERC research vessel
Full text linkAirflow distortion at instrument sites on the RV Knorr
Full text linkThis report describes an investigation of the air flow around the RV Knorr using the Computational Fluid Dynamics software package "VECTIS" to simulate a flow of air directly over the bows of the ship. Section 2 gives a brief description of the model. This work was undertaken at the request of researchers from the Bedford Institute of Oceanography (BIO), the National Oceanic and Atmospheric Administration (NOAA) and Kiel University who had used the ship to carry meteorological instrumentation on a purpose-built mast (the "lattice tower") during an experiment in the Labrador Sea in the winter of 1997. One advantage of the VECTIS software is the ability to vary the computational mesh density; this allows regions of particular interest to be modelled using a much higher resolution than that employed elsewhere in the model. In the case of the Knorr, the highest mesh density was located around the instrument sites on the lattice tower. In Section 3 the distortion of the air flow to these instrument sites is examined, and the vertical displacement of the flow and the percentage wind speed error is calculated for each site. Section 4 describes the flow at the separate IMET instrument sites on the ship’s foremast and bowmast. The mesh resolution at the IMET sites is not as great as that around the lattice tower since data for the IMET sites were requested towards the end of the study. The results are summarised and discussed in Section 5.University of Kiel (Germany)Bedford Institute of Oceanography (Nova Scotia)NOAANaval Postgraduate Schoo
Air Flow Distortion Over Merchant Ships
Full text linkAnemometers on voluntary observing ships (VOS) are usually sited above the bridge in a region where the effects of flow distortion may be large. Until recently it was unclear whether measurements from such anemometers would be biased high or low, and the magnitude of any such bias was not known. This report describes the progress made in determining the effects of flow distortion and hence in predicting the possible bias in such anemometer measurements of the wind speed
The airflow distortion at instruments sites on the RRS "James Cook"
Full text linkWind speed and air-sea flux measurements made from instrumentation on ships are affected by the airflow distortion created by the presence of the ship. The airflow can be eitheraccelerated or decelerated depending on the shape of the ship and the location of the anemometer. The computational fluid dynamics (CFD) package VECTIS was used to examinethe extent of the flow distortion at potential anemometer locations on the foremast platform of the RRS "James Cook". This technique has been previously used to study the airflow over many research ships, but this is believed to be the first time it has been applied to a research ship in the design/build stage.CFD modelling of the airflow over the ship showed that the foremast platform of the RRS "James Cook" is a good location to locate instrumentation and make high quality air-sea flux measurements. The wind speed is decelerated by about 2 % of the freestream wind speed for bow-on flows at well-exposed anemometer sites on the foremast platform. For relative wind directions up to ±30° of the bow the airflow is accelerated by up to 5 %.The ship’s anemometers are located on the main mast and are relatively close to the ship’s large satellite communication radome. For winds within 15° of the bow the wind speeds at these anemometer sites are accelerated by up to about 7 %. For wind directions at ±30° the satellite radome has a significant effect on the flow and the wind speeds will be severely biased, with the magnitude of the bias varying rapidly with wind direction and the angle of pitch of the ship. It is strongly recommended that these anemometers be moved higher up and further away from the mast
The effect of ship shape and anemometer location on wind speed measurements obtained from ships
No full textWind speed measurements obtained from ship-mounted anemometers are biased by the distortion of the airflow around the ship's hull and superstructure. These wind speed measurements are used both in numerical weather prediction and in climate studies and need to be known as accurately as possible. This paper presents results from threedimensional CFD studies of the mean airflow over various research ships and a generic tanker/bulk. It will be shown that the bias in the wind speed measurements is highly dependent upon anemometer position and ship shape. CFD results are compared to in situ wind speed measurementsmade from a number of anemometers above the bridge of the RRS Charles Darwin
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