OSI SAF central
Observing sea ice in the Skagerrak with new OSI SAF product
Situation February 2010Following a particularly cold winter, sea ice has formed in the outer parts of the Oslo fjord and is present in quite an unexpected quantity in the Skagerrak Strait, between Norway, Sweden and Denmark.
The following 3 figures are satellite images acquired by the NOAA Advanced Very High Resolution Radiometer (AVHRR) instrument on Monday, 1st February 2010 around 12:00 UTC.
The first two images (a and b) are false color images using the AVHRR channels 1 to 4. The first one (1+2+4) is dominated by the visible channels of the instrument that allow for good discrimination between sea ice and open water. The second one (2+3+4) is dominated by the thermic contributions in the scene. Thermic channels are especially good at discriminating water in clouds from cloud-free areas.
Using a combination of all the AVHRR channels, the OSI SAF science team designed an algorithm for computing the probability of sea ice and snow, displayed in the third image (c). Low probability is drawn in blue while high probability of sea ice is displayed with red color. High probability for sea ice is clearly visible in the Skagerrak and along the coast of Norway and Sweden, while consistent low probability is observed in open sea regions and where cloud was detected.
By using adapted coefficients, the algorithm is also able to detect snow over land.
Image (d) is the final product in which each satellite pixel have been assigned to a class, namely ice/snow (white), cloudy (grey) and open water (blue). The sea ice covered area in the Skagerrak is clearly visible, as well as all the cloudy pixels covering most of the scene. Cloudy conditions are also observed over sea ice.
Upcoming OSI SAF product
The upcoming Regional Sea Ice Edge product of the OSI SAF is based on this algorithm.