Canopy Parameters

---UNDER CONSTRUCTION---

The canopy model embedded in PALM can be used to simulate the flow across vegetation canopies.
Thereby, the canopy is modeled as a porous viscous medium that removes momentum from the flow (according to Shaw & Schumann, 1992; Watanabe, 2004). The presentation "Canopy model" provides detailed information on canopy-flow theory and the functionality of the canopy model. An example how to model the flow across a simple canopy block can be found under Exercise "Canopy flow".

Since revision 13XX all components of the canopy model are modularized into module plant_canopy_model_mod. In this context, the steering parameters for the canopy model are, since revision 13XX, part of the new package canopy_par.

The package and the application of the canopy model is activated by adding the NAMELIST canopy_par to your parameter file, subsequently to the NAMELIST d3par.




NAMELIST group name: canopy_par

Parameter Name	FORTRAN Type	Default Value	Explanation
canopy_mode	C*20	'block'	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) subroutine user_init_plant_canopy to allow further canopy modes. The setting of canopy_mode becomes only active, if plant_canopy has been set .T. and a non-zero drag_coefficient has been defined.
cthf	R	0.0	Average heat flux that is prescribed at the top of the plant canopy. If plant_canopy is set .T., 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. Note: Instead of using the value prescribed by surface_heatflux, the near surface heat flux is determined from an exponential function that is dependent on the cumulative leaf_area_index (Shaw and Schumann (1992, Boundary Layer Meteorol., 61, 47-64)).
drag_coefficient	R	0.0	Drag coefficient used in the plant_canopy_model. This parameter has to be non-zero, if the parameter plant_canopy is set .T..
lad_surface	R	0.0	Surface value of the leaf area density (in m2/m3). This parameter assigns the value of the leaf area density lad at the surface (k=0). Starting from this value, the leaf area density profile is constructed with lad_vertical_gradient and lad_vertical_gradient_level.
lad_vertical_gradient	R(10)	10 * 0.0	Gradient(s) of the leaf area density (in m2/m4). This leaf area density gradient holds starting from the height level defined by 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. 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_vertical_gradient_level	R(10)	10 * 0.0	Height level from which on the gradient of the leaf area density defined by 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. For the piecewise construction of temperature profiles see lad_vertical_gradient.
leaf_surface_concentration	R	0.0	Concentration of a passive scalar at the surface of a leaf (in K m/s). This parameter is only of importance in cases in that both, plant_canopy and passive_scalar, are set .T.. The value of the concentration of a passive scalar at the surface of a leaf is required for the parametrisation of the sources and sinks of scalar concentration due to the canopy.
pch_index	I	0	Grid point index (scalar) of the upper boundary of the plant canopy layer. Above pch_index the arrays of leaf area density and drag_coeffient are automatically set to zero in case of plant_canopy = .T.. Up to pch_index a leaf area density profile can be prescribed by using the parameters lad_surface, lad_vertical_gradient and lad_vertical_gradient_level.
plant_canopy	L	.F.	Switch for the plant canopy model. If plant_canopy is set .T., the plant canopy model of Watanabe (2004, BLM 112, 307-341) is used. The impact of a plant canopy on a turbulent flow is considered by an additional drag term in the momentum equations and an additional sink term in the prognostic equation for the subgrid-scale TKE. These additional terms depend on the leaf drag coefficient (see drag_coefficient), and the leaf area density (see lad_surface, lad_vertical_gradient, lad_vertical_gradient_level). The top boundary of the plant canopy is determined by the parameter pch_index. For all heights equal or larger than zw(k=pch_index), the leaf area density is 0 (i.e. there is no canopy at these heights!). By default, a horizontally homogeneous plant canopy is prescribed, if plant_canopy is set .T.. However, the user can define other types of plant canopies (see canopy_mode). If plant_canopy and passive_scalar are set .T., the canopy acts as an additional scalar source or sink, respectively. The source/sink strength depends on the scalar concentration at the leaf surface, which generally does not vary with time in PALM, and which can be specified with parameter leaf_surface_concentration. Additional heating by the plant canopy is taken into account, if the default value of parameter cthf is altered in the parameter file. In that case, the value of surface_heatflux specified in the parameter file is not used in the model. Instead, the near-surface heat flux is derived from an exponential function that depends on the cumulative leaf area index. plant_canopy = .T. is only allowed with a non-zero drag_coefficient.
scalar_exchange_coefficient	R	0.0	Scalar exchange coefficient for a leaf (dimensionless). This parameter is only of importance in cases in that both, plant_canopy and passive_scalar, are set .T.. The value of the scalar exchange coefficient is required for the parametrisation of the sources and sinks of scalar concentration due to the canopy.