Sea-ice pressure ridge

A pressure ridge is a pile of fractured sea ice formed by horizontal convergence. Ice displaced upward forms a sail; ice displaced downward forms a keel. Newly formed ridges contain angular blocks and pores, and freezing, melting and further deformation alter their strength and shape.

A sonar ridge-identification rule can miss a shallow peak near a deeper peak because the intervening valley fails the required separation criterion. This apparent shadowing suppresses short observed spacings even without an acoustic blind zone. Finite beam width and genuine geometric occlusion can impose additional limitations. The resulting minimum resolvable spacing need not be a universal physical exclusion distance.

Here $n(H)dH$ is the expected number of ridge peaks per track length with sea-ice draft in $[H,H+dH]$. If total line density is $\mu$ and mean peak draft is $h_m>h_0$, integration gives $b=1/(h_m-h_0)$ and $B=\mu b{e}^{b{h}_0}$. The normalized peak draft has a shifted exponential distribution. This is a model over a specified draft range, not a universal description of every ridge.

For nonoverlapping triangular sea-ice pressure ridges with a common along-track side slope $\tan \delta$, a ridge whose peak exceeds $h$ contributes $2\cot \delta dh$ of track length in a draft interval $dh$. Summing over peaks proves the identity. Above the cutoff of an exponential ridge-draft model, $g(h)=A{e}^{-bh}$ with $A=2B/(b\tan \delta )$. This draft occupation is not the normalized peak probability density function; level ice and shallower parts supply the remaining probability.

Relative tangential motion across a fracture in sea ice can break and pile blocks along the fracture. Local convergence at rough contacts makes ridging possible even when the large-scale deformation is mainly shear.