Ice layer between freshwater and cold brine ($a(t)=2A\sqrt{Dt},b(t)=2B\sqrt{\kappa t}$)

In a planar no-flow model with pure ice, a stable cold brine below freshwater can support an ice layer whose upper phase boundary freezes freshwater while its lower boundary melts into diluted brine. Heat conduction and salt diffusion obey different similarity scales. Local liquidus equilibrium, salt conservation and two Stefan conditions determine both boundary positions. The heat lost into the brine supports net ice growth even while some basal ice melts.

For $S=L/(c_p\Delta T)\gg 1$, $ϵ=\sqrt{D/\kappa }\ll S^{-1}$ and $R=m{C}_0/\Delta T>1$, the leading thickness is the displayed expression. The basal position is $2A\sqrt{Dt}$, with $A$ determined byOnly when $ϵSA\ll 1$ can the basal translation be omitted from the upper position. Then $A{e}^{A^2}\simeq \sqrt{\pi }RS/4$, exposing the logarithmic refinement to the scale ordering.