KLIWAS 
Seite 15 
shown by Castelao (2006) and by Belkin and O’Reilly (2009). However, gradient 
algorithms are not able to recognise and discard false fronts caused by noise. On the 
other hand, histogram algorithms are able to detect weak fronts in the presence of 
high background noise as shown by Cayula and Comillon (1992, 1995, 1996), Diehl 
et al. (2002) and, Ullman and Cornillon (1999, 2000, 2001). In the present study, a 
new approach was developed which combines and modifies the gradient algorithm of 
Canny (1986) and the histogram algorithm of Cayula and Cornillon (1992) in order to 
improve the front detection ability. It was also found that a proper pre-processing of 
the data is important in order to have a good detection quality for fronts. The pre 
processing includes mainly cloud filtering and noise reduction. We call this new 
approach GRADHIST. The flowchart in Fig. 2 outlines the overall processing chain 
of GRADHIST which includes three steps: pre-processing, the application of the 
modified gradient and histogram algorithms and the matching module. 
“ 7-' 
Input data 
Fig. 2: Flowchart of front detection algorithm 
KLIWAS 
Climatology 
of North Sea 
Fronts