AWT Weather Routing/Routeing: Applied Weather Technology

Satellite Observed Winds Vs. Ship Observed Winds

By John Caito

We are frequently queried regarding the accuracy of satellite wind observations. Satellite observed wind speeds have been found to be very accurate, especially in the range of Beaufort Force 4 to 5. But how does this compare to that of shipboard wind observations?

Measuring wind velocity from a ship is not a straight forward process. There are two methods of wind observations from a vessel. Observations made by anemometer would appear to be the most accurate possible. However, factors such as the height of the anemometer above the deck and the shape of the vessel’s superstructure affect the accuracy of these observations. Wind speeds vary with height, and the shape of the vessel’s superstructure distorts the flow of air reaching the anemometer, affecting the observed wind speed. Both factors can be corrected, but not easily, and not before these observations appear on synoptic weather charts frequently used for weather verification.

The other method of observing winds from a vessel estimates wind speed by matching sea state photographs to the Beaufort scale. This is a subjective process and therefore subject to human bias.

We often see shipboard weather observations referred to as having been made by the ‘man on the spot,’ implying that such observations are the most accurate possible. This is an assumption not supported by research, which has found that ship wind observations are frequently inconsistent, even when the reporting vessels are in close proximity.

This inconsistency is noted in the research paper, “A Comparison of Ship and Scatterometer Derived Wind Speed Data in Open Ocean and Coastal Areas,” by E.C. Kent, P.K. Taylor, and P.G. Challengor. The report states:

“The scatterometer (such as QuikSCAT) data are of higher quality than the ship data and can be used to quality control the ship wind speeds by identifying ships which report biased or poor quality wind speeds.”

This is not to say that satellite observed ocean winds should replace ship observations. However, the inconsistencies of ship weather observations make it difficult to assess which ship observations are accurate and which are not. And vessels participating in the WMO’s Voluntary Observing Ships (VOS) Scheme are few and far between. It is not unusual to find no ship weather reports for hundreds of kilometers on a synoptic weather chart. This situation is unlikely to improve soon, as VOS participants have been in decline for the last twenty years. QuikSCAT provides approximately 400,000 wind observations per day. By comparison, there are approximately 4,000 VOS participants at present.

Maritime endeavors have benefited significantly from the direct measurement of ocean winds by satellite. There are three satellites providing this data at present: QuikSCAT, a NASA mission, Jason 1, a NASA and CNES (the French space agency) mission, and Jason 2, a NASA, CNES, NOAA (National Oceanic and Atmospheric Administration), and EUMETSAT (European Meteorological Satellite Organisation) mission. Together, these satellites are returning an unprecedented amount of high quality data on the marine environment.