Canopy Radiation Model
Subroutine Radiat (curadia.f)
This subroutine constitutes Cupid's canopy radiation model. It
returns:
View definitions of variables used in this subroutine:
Calculate the diffuse radiation for VIS and NIR:
In these first 2 steps, we are dealing with VIS and NIR only. To see the
thermal calculations, click here.
- For low sun zenith angles (night) and/or low direct beam radiation (due
to cloud cover or slope interception), zero the beam factors and
source terms. [60]
- Calculate EXPDIR - the direct-beam non-interception factor: [100]
- EXPDIR = EXP[-CLUMP*XINT(IHR)*DF*PATH(IHR)]
- where XINT represents the geometrical weighting factor based on
leaf-normal to sun angle.
- Calculate TBEAM, the fraction of direct beam going into each layer.
- TBEAM(J) = TBEAM(J+1)*EXPDIR
- Calculate the up (SUP) and downward (SDN) diffuse radiation sources in
each layer, expressed as a fraction of the total radiation above the
canopy.
- A certain amount of direct beam rad is converted to diffuse
in each layer - these are considered "sources" for VIS and NIR.
- For SUP(1), the diffuse
radiation originating on the soil surface, the soil reflectance,
RSOIL, is assumed as the source term.
- In a convergence loop the relative upward and downward diffuse
fluxes U and D are approximated. [130-170]
- First, ADUM, the upward to
downward flux ratios are calculated bottom to top, setting the
ADUM(1) to the soil reflectance.
- Then,
the diffuse fluxes in both directions can be evaluated for
each layer in a first approximation according to equation
(xxx) and adding it to the diffuse source terms.
- In iteration
DOWN and UP are recalculated through all layers, taking
into account the transmittance TLAYER and reflectance RLAYER
for a layer and the source terms.
- Iteration is interrupted if,
for all layers, convergence between DOWN and D, and UP and U
is less than 0.0001.
Calculate the radiation flux densities in VIS and NIR for all leaf
angle classes:
- Calculate SUNLIT - the LAI of the layer that is sunlit. [180]
- Calculate FRAREA(I,J) - the fraction of sunlit leaf area in
layer J that is contained in angle class I. [190]
- To calculate the flux densities incident into an angle class,
first the direct beam flux density on a surface perpendicular
to the incident beam is calculated. This maximum direct
radiation flux, BEAM, is calculated from the radiation flux
onto the horizontal plane and amplified by 1/cos(ZENANG).
- Divide BEAM by cos(SLOPE), because FBEAM and RADTOP have already
been modified by this amount to take slope into account.
- Convert the ratios D,U and TBEAM into actual fluxes by multiplying
by radiation level above the canopy.
- Determine DSTRAD(K,I,J) - the total radiation at wavelength K into
angle class I in layer J.
- DSTRAD is composed of:
- diffuse radiation, including that arising from multiple scatterings
within the layer, and
- direct beam radiation, which is BEAM decremented by the cosine
of the leaf angle CT (only leaves perpendicular to the beam receive
the full beam flux).
VIS and NIR calculations continue here.
The thermal calculations are somewhat different than those for VIS
and NIR in that the leaves and the soil themselves are emitters ("sources"),
and they will emit both day and night. Also ignore thermal reflectance
(and transmittance???) because it is small in comparison with foliage
emission.
- If it is night:
- set the layer sources to thermal emission corresponding to
the current average leaf temperature in the layer x (1-EXPDIF) (???). [255,265]
- set DSTRAD = the source not multiplied by the interception
factor.
- If it is day:
- Here, emission is summed over all angle classes because they
will have different temps, depending on the relative sun angle.
For each class, the emission is multiplied by FRAREA - I guess
because the emission from sunlit leaves so strongly dominates
that of shaded leaves (???).
- Find U and D for thermal:
- Boundary conditions:
- The downward thermal flux of the topmost layer is an input
variable.
- The thermal emission from the soil is calculated on
the basis of the soil surface temperature (TSFC).
- The layer
transmittance is set to the non-interception factor, while the
layer reflectivity is calculated from the interception factor
and the leaf emissivity according to equation (XXX).
- In a
first approximation D is calculated as the sum of transmitted
downward flux from above and the downward radiation source, U
as the sum of transmitted upward radiation and the upward
radiation source.
- In two iterations the values for U and D are
further approximated by adding a third term for reflected
opposite radiation.
- CLAI, the cumulative leaf area index for a layer is calculated
by adding the delta LAI's for each layer. Hence, CLAI refers
to the bottom of each layer.
- Some important radiation parameters:
- RNLAM is the net radiation flux through one layer for each
wavelength band, with down being positive.
- RNET is the total
net flux on a layer of all wavelengths.
- RNDIV is calculated
from the difference of RNET from consecutive layers, hence
it is the total radiation lost to a layer.
- The thermal diffuse net radiation at layer midpoints for each
angle class are calculated from the averages of D and U for
consecutive layers, taking into account the emissivity of a
layer, plus diffuse radiation of VIS and NIR that are absorbed
by the leaf and minus two times the thermal flux emitted by
the leaves of that angle class. Hence, the thermal flux into
the leaf angle class in a layer is positive.
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