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8.7.3.4. Non-spectral Light¶

To have non-spectral light, disable the radtrans package.

There are three alternatives for incoming PAR at the surface: if PARfile is set, it is read in from files (it can also be set to a constant, PARconst); if darwin_useQsw=T, it is computed from model shortwave radiation,

\[I(0) = -f_{\op{PAR}} c_{\op{PAR}} Q_{SW} \;;\]

otherwise, it is computed as a function of latitude using solar declination, see darwin_insol.F. Note that this only works for lat-lon grids.

Incoming PAR is reduced by the fraction of the ocean surface covered by ice, unless model shortwave radiation, which has seaice already taken into account, is used,

\[I(0) = (1 - f_{\op{ice}}) I_{\op{in}} \;.\]

The ice cover fraction, \(f_{\op{ice}}\), is taken from the seaice package if compiled, and read in from icefile otherwise.

PAR is attenuated by water and chlorophyll,

\[I(r) = I(0) \op{e}^{-\int_0^r k_{\op{w}} + k_{\op{Chl}} \op{Chl} \,\mathrm{d}r}\]

If DARWIN_AVPAR is defined, the resulting function is averaged over the thickness of a grid layer. When no chlorophyll tracer is used (#undef DARWIN_ALLOW_CHLQUOTA), it is computed from total phytoplankton carbon biomass. With DARWIN_ALLOW_GEIDER, it is taken from the computation at the previous time step, without, a fixed Chlorophyll-to-C ratio, R_ChlC, is used.

Table 8.31 Non-spectral light parameters¶
Param	Symbol	Default	Units	Description
parfrac	\(f_{\op{PAR}}\)	0.4		fraction of Qsw that is PAR
parconv	\(c_{\op{PAR}}\)	1/0.2174	µEin/s/W	conversion from W/m2 to uEin/m2/s
katten_w	\(k_{\op{w}}\)	0.04	1/m	attenuation coefficient of water
katten_Chl	\(k_{\op{Chl}}\)	0.04	m²/mg Chl	attenuation coefficient of chlorophyll
aphy_chl_ave		0.02	m²/mg Chl	Chl-specific absorption coefficient for photosynthesis