ARCTIC SEA-ICE LEADS FROM ADVANCED VERY HIGH-RESOLUTION RADIOMETER IMAGES

A large number of advanced very high resolution radiometer (AVHRR) ima ges from throughout 1989 are analyzed to determine lead characteristic s. The units of analysis are square 200-km cells, and there are 270 su ch cells in the data set. Clouds are masked manually. Leads are determ ined from images of the potential open water delta, a scaled version o f the surface temperature or albedo that weights thin ice by its therm al or brightness impact. The lead fraction is determined as the mean d elta; the monthly mean lead fraction ranges from 0.02 in winter to 0.0 6 in summer in the central Arctic and is near 0.08 in the winter in th e peripheral seas. A method of accounting for lead width sampling erro rs due to the finite sample areas is introduced. In the central Arctic the observed mean lead width for a threshold of delta = 0.1 ranges fr om 2 or 3 km (near the resolution of the instrument) in the winter to 6 lan in the summer. In the peripheral seas it is about 5 km in the wi nter. Width distributions are often more heavily weighted in the tail than exponential distributions and are well approximated by a power la w. The along-track, number density power law N = aw(-b) has a mean exp onent of b = 1.60 (standard deviation 0.18) and shows some seasonal va riability. Mean flee widths in the central Arctic are 40 to 50 lan in the winter, dropping to about 10 km in the summer. For floes the power law has a mean exponent of 0.93 and exhibits a clearer annual cycle. Lead orientation is determined with a method based on the direction of maximum extent.