Ships modify cloud microphysics by adding cloud condensation nuclei (CCN) toa developing or existing cloud. These create lines of larger reflectance incloud fields that are observed in satellite imagery. An algorithm has beendeveloped to automate the detection of ship tracks in Along Track ScanningRadiometer 2 (ATSR-2) imagery. The scheme has been integrated into the GlobalRetrieval of ATSR Cloud Parameters and Evaluation (GRAPE) processing chain.The algorithm firstly identifies intensity ridgelets in clouds which have thepotential to be part of a ship track. This identification is done bycomparing each pixel with its surrounding ones. If the intensity of threeadjacent pixels is greater than the intensity of their neighbours, then it isclassified as a ridgelet. These ridgelets are then connected together,according to a set of connectivity rules, to form tracks which are classed asship tracks if they are long enough. The algorithm has been applied to twoyears of ATSR-2 data. Ship tracks are most frequently seen off the west coastof California, and the Atlantic coast of both West Africa and South-WesternEurope. The global distribution of ship tracks shows strong seasonality,little inter-annual variability and a similar spatial pattern to thedistribution of ship emissions.
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