## 2.9.5. Canopy scaling[¶](#canopy-scaling "Permalink to this headline")
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When LUNA is on, the \\(V\_{c\\max 25}\\) for sun leaves is scaled to the shaded leaves \\(J\_{\\max 25}\\), \\(T\_{p25}\\), \\(k\_{p25}\\), and \\(R\_{d25}\\) scale similarly.

(2.9.17)[¶](#equation-9-17 "Permalink to this equation")\\\[\\begin{split}\\begin{array}{rcl} {V\_{c\\max 25 sha}} & {=} & {V\_{c\\max 25 sha} \\frac{i\_{v,sha}}{i\_{v,sun}}} \\\\ {J\_{\\max 25 sha}} & {=} & {J\_{\\max 25 sun} \\frac{i\_{v,sha}}{i\_{v,sun}}} \\\\ {T\_{p sha}} & {=} & {T\_{p sun} \\frac{i\_{v,sha}}{i\_{v,sun}}} \\end{array}\\end{split}\\\]

Where \\(i\_{v,sun}\\) and \\(i\_{v,sha}\\) are the leaf-to-canopy scaling coefficients of the twostream radiation model, calculated as

(2.9.18)[¶](#equation-9-18 "Permalink to this equation")\\\[\\begin{split}i\_{v,sun} = \\frac{(1 - e^{-(k\_{n,ext}+k\_{b,ext})\*lai\_e)} / (k\_{n,ext}+k\_{b,ext})}{f\_{sun}\*lai\_e}\\\\ i\_{v,sha} = \\frac{(1 - e^{-(k\_{n,ext}+k\_{b,ext})\*lai\_e)} / (k\_{n,ext}+k\_{b,ext})}{(1 - f\_{sun})\*lai\_e}\\end{split}\\\]

k\_{n,ext} is the extinction coefficient for N through the canopy (0.3). k\_{b,ext} is the direct beam extinction coefficient calculated in the surface albedo routine, and \\(f\_{sun}\\) is the fraction of sunlit leaves, both derived from Chapter [2.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#rst-surface-albedos).

When LUNA is off, scaling defaults to the mechanism used in CLM4.5.

