### 2.12.5.3. Boundary Conditions[¶](#boundary-conditions "Permalink to this headline")

The top boundary condition, imposed at the top modeled layer \\(i=j\_{top}\\), where \\(j\_{top} =-n\_{sno} +1\\), is the downwards surface flux _G_ defined by the energy flux residual during the surface temperature solution (section [2.12.5.3](#boundary-conditions-lake)). The bottom boundary condition, imposed at \\(i=N\_{levlak} +N\_{levgrnd}\\), is zero flux. The 2-m windspeed \\(u\_{2}\\) (m s\-1) is used in the calculation of eddy diffusivity:

(2.12.33)[¶](#equation-12-28 "Permalink to this equation")\\\[u\_{2} =\\frac{u\_{\*} }{k} \\ln \\left(\\frac{2}{z\_{0m} } \\right)\\ge 0.1.\\\]

where \\(u\_{\*}\\) is the friction velocity calculated in section [2.12.5.3](#boundary-conditions-lake) and _k_ is the von Karman constant ([Table 2.2.7](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html#table-physical-constants)).

