== Canopy Parameters ==
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The canopy model embedded in PALM can be used to simulate the effect of vegetation canopies on a turbulent flow.\\ Thereby, the canopy is modeled as a porous viscous medium that removes momentum from the flow (Shaw & Schumann, 1992; Watanabe, 2004). The presentation [/wiki/doc/tut/job/canopy#Canopymodel "Canopy model"] provides detailed information on canopy-flow theory and the functionality of the canopy model. An example on how to model the flow across a simple canopy block can be found under exercise [/wiki/doc/tut/job/canopy#Exercise9:Canopyflow "Canopy flow"].\\\\

Starting at '''revision 1485''', all parts of the canopy-model-related PALM code are modularized in module [/browser/palm/trunk/SOURCE/plant_canopy_model.f90 plant_canopy_model_mod]. In this context, the newly created package {{{canopy_par}}} now contains all canopy-related input parameters. This means that the canopy model is now steered using the NAMELIST {{{canopy_par}}}, and no longer with the {{{inipar}}}-NAMELIST. Hence, in order to automatically enable the canopy model, NAMELIST {{{canopy_par}}} and the respective canopy parameters must be added to the parameter file ({{{_p3d}}}), subsequently to the NAMELIST {{{d3par}}}. The setting of parameter plant_canopy = ''.T.'' is no longer required, since this is automatically done in PALM if the NAMELIST {{{canopy_par}}} exists in the parameter file.
\\\\\\\\\\\\
'''NAMELIST group name: canopy_par''' \\\\

||='''Parameter Name'''  =||='''[../fortrantypes FORTRAN Type]'''  =||='''Default Value'''  =||='''Explanation'''  =||
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{{{#!td style="vertical-align:top"
[=#alpha_lad '''alpha_lad''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
9999999.9
}}}
{{{#!td
Dimensionless coefficient required for the construction of the leaf area density (LAD) profile, using following beta probability density function (following Markkanen et al., 2003, BLM 106, 437-459):\\\\
{{{
#!Latex
\[ f_{PDF}(\frac{z}{H},\alpha,\beta) = \frac{(\frac{z}{H})^{\alpha-1}\;(1-\frac{z}{H})^{\beta-1}}{\int_{0}^{1}\;(\frac{z}{H})^{\alpha-1}\;(1-\frac{z}{H})^{\beta-1}\;d(\frac{z}{H})}, \]
}}}
where ''z'' is the height above ground, ''H'' is the canopy height, and alpha ('''alpha_lad''') and beta ([#beta_lad beta_lad]) are the coefficients to be presribed. The actual leaf area density values follow from: 
{{{
#!Latex
\[ LAD(z,\alpha,\beta) = LAI * f_{PDF}(\frac{z}{H},\alpha,\beta) * H, \]
}}}
with the leaf area index LAI (LAI is the vertical integral over the LAD profile) being prescribed by canopy parameter [#lai_beta lai_beta].\\\\
[#lai_beta lai_beta] has to be set to a non-zero value in order to use the beta probability density function for the LAD-profile construction.
'''alpha_lad''' steers together with [#beta_lad beta_lad] the vertical distribution of leaf area within the canopy volume. Values for '''alpha_lad''' must be greater than zero. Increasing '''alpha_lad''' moves the leaf area more towards the canopy top. \\\\
'''Note:'''\\
The LAD profile can also be constructed by prescribing vertical gradients ([/wiki/doc/app/canpar#lad_vertical_gradient_level lad_vertical_gradient_level], [/wiki/doc/app/canpar#lad_vertical_gradient lad_vertical_gradient]) of the leaf area density, starting from the prescribed surface value [/wiki/doc/app/canpar#lad_surface lad_surface].
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#beta_lad '''beta_lad''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
9999999.9
}}}
{{{#!td
Dimensionless coefficient required for the construction of the leaf area density (LAD) profile, using a beta probability density function (see [#alpha_lad alpha_lad] for details).\\\\
'''beta_lad''' steers together with [#alpha_lad alpha_lad] the vertical distribution of leaf area within the canopy volume. Values for '''beta_lad''' must be greater than zero. Increasing '''beta_lad''' moves the leaf area more towards the canopy floor. \\\\
'''Note:'''\\
The LAD profile can also be constructed by prescribing vertical gradients ([/wiki/doc/app/canpar#lad_vertical_gradient_level lad_vertical_gradient_level], [/wiki/doc/app/canpar#lad_vertical_gradient lad_vertical_gradient]) of the leaf area density, starting from the prescribed surface value [/wiki/doc/app/canpar#lad_surface lad_surface].
}}}
|----------------
{{{#!td style="vertical-align:top;width: 150px"
[=#canopy_mode '''canopy_mode''']
}}}
{{{#!td style="vertical-align:top;width: 50px"
C*20
}}}
{{{#!td style="vertical-align:top;width: 75px"
'block'
}}}
{{{#!td
Canopy mode.\\\\
Besides using the default value, that will create a horizontally homogeneous plant canopy that extends over the total horizontal extension of the model domain, the user may add code to the user interface (see [#3.5.1 3.5.1]) subroutine {{{user_init_plant_canopy}}} to allow further canopy modes.\\\\
The simulation of a plant canopy requires the setting of a non-zero [#canopy_drag_coeff canopy_drag_coeff].
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#cthf '''cthf''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Average heat flux that is prescribed at the top of the plant canopy.\\\\
The user can prescribe a heat flux at the top of the plant canopy.
It is assumed that solar radiation penetrates the canopy and warms the foliage which, in turn, warms the air in contact with it.
Based on '''cthf''', the heat fluxes inside the canopy down to the canopy floor ([#surface_heatflux surface_heatflux]) are determined from a decaying exponential function that is dependent on the cumulative leaf_area_index (Shaw and Schumann, 1992, BLM 61, 47-64).
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#canopy_drag_coeff '''canopy_drag_coeff''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Drag coefficient used in the {{{plant_canopy_model}}}.\\\\
This parameter has to be greater than zero for the simulation of a plant canopy.
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#lad_surface '''lad_surface''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Surface value of the leaf area density (in m^2^/m^3^).\\\\
This parameter assigns the value of the leaf area density (LAD) at the surface (k=0). Starting from this value, the LAD profile is constructed with [#lad_vertical_gradient lad_vertical_gradient] and [#lad_vertical_gradient_level lad_vertical_gradient_level].\\\\
'''Note:'''\\
The LAD profile can also be constructed using a beta probability density function by prescribing values for parameters [#alpha_lad alpha_lad], [#beta_lad beta_lad] and [#lai_beta lai_beta].
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#lad_vertical_gradient '''lad_vertical_gradient''']
}}}
{{{#!td style="vertical-align:top"
R(10)
}}}
{{{#!td style="vertical-align:top"
10 * 0.0
}}}
{{{#!td
Gradient(s) of the leaf area density (in m^2^/m^4^).\\\\
This leaf area density gradient holds starting from the height level defined by [#lad_vertical_gradient_level lad_vertical_gradient_level] (precisely: for all uv levels k where zu(k) > lad_vertical_gradient_level, lad(k) is set: lad(k) = lad(k-1) + dzu(k) * '''lad_vertical_gradient''') up to the level defined by [#pch_index pch_index]. Above that level, lad(k) will automatically be set to 0.0. A total of 10 different gradients for 11 height intervals (10 intervals if lad_vertical_gradient_level(1) = 0.0) can be assigned. The leaf area density at the surface is assigned via [#lad_surface lad_surface].  
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#lad_vertical_gradient_level '''lad_vertical_gradient_level''']
}}}
{{{#!td style="vertical-align:top"
R(10)
}}}
{{{#!td style="vertical-align:top"
10 * 0.0
}}}
{{{#!td
Height level from which on the gradient of the leaf area density defined by [#lad_vertical_gradient lad_vertical_gradient] is effective (in m).\\\\
The height levels have to be assigned in ascending order. The default values result in a leaf area density that is constant with height up to the top of the plant canopy layer defined by [#pch_index pch_index]. For the piecewise linear construction of an LAD profile see [#lad_vertical_gradient lad_vertical_gradient].
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#lai_beta '''lai_beta''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Leaf area index used in the {{{plant_canopy_model}}} to construct the vertical profile of the leaf area density (lad) with a beta function (see [#alpha_lad alpha_lad] for details).\\\\
'''lai_beta''' has to be set to a non-zero value, and parameters [/wiki/doc/app/canpar#alpha_lad alpha_lad] and [/wiki/doc/app/canpar#beta_lad beta_lad] have to be given.
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#leaf_surface_conc '''leaf_surface_conc''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Concentration of a passive scalar at the surface of a leaf (in kg/m^3^).\\\\
This parameter is only of importance in cases where [#passive_scalar passive_scalar] = ''.T.''. The value of the concentration of a passive scalar at the surface of a leaf is required for the parametrization of the sources and sinks of scalar concentration due to the canopy.
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#pch_index '''pch_index''']
}}}
{{{#!td style="vertical-align:top"
I
}}}
{{{#!td style="vertical-align:top"
0
}}}
{{{#!td
Grid point index (w-grid) of the upper boundary of the plant canopy layer.\\\\
Above '''pch_index''' the leaf area density (LAD) is automatically set to zero. Up to '''pch_index''' a leaf area density profile can be prescribed in two possible ways:\\\\
1) Creating a piecewise linear LAD-profile by prescribing the parameters [#lad_surface lad_surface], [#lad_vertical_gradient lad_vertical_gradient] and [#lad_vertical_gradient_level lad_vertical_gradient_level].\\\\
2) Employing a beta probability density function for the vertical leaf area distribution, prescribing coefficients [#alpha_lad alpha_lad], [#beta_lad beta_lad] and [#lai_beta lai_beta] (see e.g. Markkanen et al., 2003, BLM 106, 437-459). 
}}}
|----------------
{{{#!td style="vertical-align:top"
[=#leaf_scalar_exch_coeff '''leaf_scalar_exch_coeff''']
}}}
{{{#!td style="vertical-align:top"
R
}}}
{{{#!td style="vertical-align:top"
0.0
}}}
{{{#!td
Scalar exchange coefficient for a leaf (dimensionless).\\\\
This parameter is only of importance in cases where [#passive_scalar passive_scalar] = ''.T.''. The value of the scalar exchange coefficient is required for the parametrization of the sources and sinks of scalar concentration due to the canopy.
}}}
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