Browsing by Author "Hovestadt, Hadassa"

Surface winds are crucial information for seafarers, however, in Finland, marine weather station measurements have never had their observations verified in order to define surface winds. The wind measurement instruments of marine weather stations are located on top of the lighthouses at a height of 14-55 meters. The instrument is typically located on the helicopter platform or close to it, and is therefore cannot usually be higher than one meter above it. The wind measured at this height often differs from the surface wind, both in speed and direction. However, for seafarers, the surface wind information is often more relevant than the wind observation from the much higher lighthouse roof. This study investigated how well the marine lighthouse observations represents the sea surface wind at 10 meters. Between the years of 2012 to 2017, the wind conditions from four different marine lighthouses were studied: Kemi I lighthouse, Raahe Nahkiainen, Kristiinankaupunki lighthouse and Porvoo Kalbådagrund. The wind benchmark used was the ASCAT wind product, developed by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and the Ocean and Sea Ice Satellite Application Facility (OSI SAF), which determines wind at 10 meters above the sea surface in a neutral atmosphere. The difference between the lighthouse observations and the benchmark were examined by calculating the mean absolute error (MAE), the root mean squared error (RMSE), and the mean error (ME). In addition the season, wind speed, and wind direction were evaluated for their effects on the results. Similar research for wind observations by marine weather stations have never previously been carried out in Finland. This studied shows that the wind speed measured from the lighthouse has a moderate positive bias compared to the 10 meter surface wind. In the autumn this bias is reduced compared to spring, this is likely due to the average seasonal variation in atmospheric hydrostatic stability. The lighthouse observations are more representative of surface winds when the wind speed is moderate to strong (as defined on the Beaufort scale), then when the wind speeds are weak or gale. With weak wind speeds the lighthouse is underestimating the surface wind, but when the wind speed increases to moderate or more, the lighthouse observation overestimates the surface wind. Lighthouse observations have a wind direction, which is counter clockwise to the surface wind, however, there is high uncertainty in this result. For further studies, more lighthouses are needed, and a suitable benchmark for observations close to the coast should be used. The ASCAT wind product is a good tool for defining surface water on the open sea, but at present scatterometers cannot determine the wind adjacent to the coast line. For further studies; mast or buoy measurements of the wind would be instrumental for assessing lighthouse observations.