### 2.12.6.5. Modifications to Snow Layer Logic[¶](#modifications-to-snow-layer-logic "Permalink to this headline")

A thickness difference \\(z\_{lsa} =s\_{\\min } -\\tilde{s}\_{\\min }\\) adjusts the minimum resolved snow layer thickness for lake columns as compared to non-lake columns. The value of \\(z\_{lsa}\\) is chosen to satisfy the CFL condition for the model timestep. By default, \\(\\tilde{s}\_{\\min }\\) = 1 cm and \\(s\_{\\min }\\) = 4 cm. See [Subin et al. (2012a; including Supporting Information)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#subinetal2012a) for further discussion.

The rules for combining and sub-dividing snow layers (section [2.8.7](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#snow-layer-combination-and-subdivision)) are adjusted for lakes to maintain minimum thicknesses of \\(s\_{\\min }\\) and to increase all target layer thicknesses by \\(z\_{lsa}\\). The rules for combining layers are modified by simply increasing layer thickness thresholds by \\(z\_{lsa}\\). The rules for dividing snow layers are contained in a separate subroutine that is modified for lakes, and is a function of the number of layers and the layer thicknesses. There are two types of operations: (a) subdividing layers in half, and (b) shifting some volume from higher layers to lower layers (without increasing the layer number). For subdivisions of type (a), the thickness thresholds triggering subdivision are increased by \\(2z\_{lsa}\\) for lakes. For shifts of type (b), the thickness thresholds triggering the shifts are increased by \\(z\_{lsa}\\). At the end of the modified subroutine, a snow ice and liquid balance check are performed.

In rare instances, resolved snow layers may be present over an unfrozen top lake body layer. In this case, the snow layers may be eliminated if enough heat is present in the top layer to melt the snow: see [Subin et al. (2012a, Supporting Information)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#subinetal2012a).
