The Accuracy of Voluntary Observing Ships' Meteorological Observations-Results of the VSOP-NA

Abstract

For the Voluntary Observing Ships Special Observing Project for the North Atlantic (VSOP-NA), the layout, meteorological instrumentation, and observing practices of 45 voluntary observing ships (VOS) operating in the North Atlantic were cataloged. Over a two-year period these ships provided extra information with each observation, and the effect of different observing practices has been quantified by using analysis fields from an atmospheric forecast model as a comparison standard. Biases of order several tenths of a degree Celsius were detected in sea surface temperature data from engine intake thermometers, in dewpoint temperatures from screens (and to a lesser extent, psychrometers), and in air temperatures due to solar heating. Wind speeds from anemometers were high compared to visual winds by about 2 kt for winds up to about 25 kt. The VSOP-NA data do not, however, indicate which is the more accurate. Correction for anemometer height and use of the WMO Commission for Marine Meteorology version of the Beaufort scale reduced this difference significantly. The result of these corrections on mean heat flux estimates was only a few watts per square meter but much greater changes resulted for particular areas and seasons. The project identified observing methods that are to be preferred for future use on the VOS, and demonstrated that the combined use of VOS data and a forecasting model allowed the detection of biases both in the observations and in the model analyses.