Frazil Ice Formation

Frazil Ice Formation

Frazil ice forms in the model when the in situ temperature drops below the local freezing point, taking into account the in situ salinity and pressure. Starting at the bottom and working up through the water column, if the water is below freezing, set it to freezing and add the heat required to the heat deficit. If the water above is warmer than freezing, use that heat to take away the heat deficit and to cool the water. If you get all the way to the surface with a heat deficit, that quantity is passed to the ice model as a heat flux it will need to provide to the ocean.

The local freezing point code is provided by the equation of state being used by MOM6. See Freezing Temperature of Sea Water for the MOM6 options.

The salinity is adjusted only at the surface when frazil ice is formed. This happens when the ice model creates ice with the heat deficit, taking salt out of the surface waters. We inherit this behavior from older versions of MOM, but the effect of not adjusting the in situ salinity is thought to be small.

Note that versions simply whisking all the heat deficit to the surface without checking for warm water above tended to produce rapidly-melting ice floes in warm waters. This was deemed unphysical and was corrected.

A similar process that we are also omitting is the formation of salt crystals when the salinity becomes too high. The salt crystals should form and sink, leaving a layer on the bed that will be diluted when the salinity drops again. This process can be seen in a lake in Death Valley.