Degradation of the scatterometer wind data due to the presence of ships

Using scatterometer winds for research and application in regional seas like the Mediterranean, permanent signatures on the mean fields have been noticed, with difficulty ascribable to geophysical phenomena. These signatures are characterized by high winds and located along the main ship routes: when possible, SAR images have been used to evidence that the anomalous winds inside the scatterometer swaths are very probably due to ships. This led to consider these wind data as artefacts and to implement statistical methods to suppress them. To filter out the scatterometer pixels contaminated by the presence of ships is tricky, as it implies the knowledge of the spatial-local characteristics of the wind (of course not know in advance) as well as of the strength of the contamination in terms of the wind speed and spatial extent. The wind speed seems to be increased by the presence of ships, but care must be given in the detection to avoid to filter small scale geophysical phenomena so well detected by scatterometer winds. Several methods of spatial outliers detection have been attempted to mitigate the contamination, all variants of robust analysis (Rousseeuw, 1984, Verboven et al., 2005, Chen et al. 2008), applied to each scatterometer swath inside the area of interest. Results will be shown for both QuikSCAT and MetOp a/b monthly mean fields over the Mediterranean Sea. They show that contamination occurs both on the ship routes linking the Gibraltar to Suez and close to coasts. Suppression of the scatterometer winds contaminated by ships is very important in any climatological context and when scatterometer data are used in coastal meteorology.