Radiation Parameters

PALM offers radiation simulation using a radiation_scheme chosen by the user from these three integrated radiation models:

• Constant radiation
• Simple clear sky
• RRTMG

In addition to these radiation models, a Radiative Transfer Model (RTM) is implemented in PALM to describe the radiative transfer processes within the urban canopy layer, including multiple reflections between buildings and other surfaces located in the domain and interactions with resolved plant canopy.

Current RTM version 3.0 involves the following processes

• Calculation of the incoming/outgoing shortwave radiation components on each surface element in the domain. Direct radiation from sun, diffusion radiation from sky, reflected radiation from surfaces and emitted radiation from surfaces and trees are processed separately.
• Estimation of longwave thermal emission/absorption for all surfaces.
• Reflections of shortwave and longwave radiation by all surfaces (finitely iterated).
• Absorption of shortwave radiation by resolved plant canopy (trees) which are treated as semitransparent medium.
• Absorption and emission of longwave radiation by resolved plant canopy (trees) which are treated as semitransparent medium.
• Plant canopy LW and SW radiative exchange (used as an input for calculation of plant canopy transpiration and latent heat).

All parts of the radiation code (and the interface of the radiation code) are modularized in module radiation_model_mod. In this context, a new Fortran NAMELIST radiation_parameters was added, containing all radiation-related steering parameters. The radiation module is automatically activated if a radiation_parameters is set in the parameter file (_p3d).

When using the RRTMG radiation model, it is required to compile the radiation code first as a library. But before this, the RRTMG libraries need to be installed. Moreover, the use of netCDF is mandatory.

NAMELIST group name: radiation_parameters

Parameter Name	FORTRAN Type	Default Value	Explanation
albedo	R	0.2	Surface albedo (value of 0-1).
albedo_lw_dif	R	depending on albedo_type	Surface albedo for longwave diffuse radiation for a solar angle of 60°.
albedo_lw_dir	R	depending on albedo_type	Surface albedo for longwave direct radiation for a solar angle of 60°.
albedo_sw_dif	R	depending on albedo_type	Surface albedo for shortwave diffuse radiation for a solar angle of 60°.
albedo_sw_dir	R	depending on albedo_type	Surface albedo for shortwave direct radiation for a solar angle of 60°.
albedo_type	I	5	This parameter controls the choice of the surface albedos for direct/diffuse/broadband radiation according to the following list of predefined land surfaces. For radiation_scheme = 'rrtmg' the shortwave and longwave albedo values are used and the actual albedos are then calculated based on the current solar angle after Briegleb et al. (1986) and Briegleb (1992). The displayed values are given for a solar zenith angle of 60° and will vary in the course of the day according to Briegleb et al. (1986) and Briegleb (1992). Setting one or more of the parameters albedo_lw_dif, albedo_lw_dir, albedo_sw_dif, albedo_sw_dir will overwrite the respective values set by albedo_type. By default, albedo_lw_dif = albedo_lw_dir and albedo_sw_dif = albedo_sw_dir. Moreover, constant_albedo = .T. will keep the albedos constant during the simulation. For radiation_scheme = 'clear-sky' the broadband albedo is used and does not vary in the course of the simulation. albedo_type Description broadband longwave shortwave Notes 0 user defined - - - 1 ocean 0.06 0.06 0.06 2 mixed farming, tall grassland 0.19 0.28 0.09 3 tall/medium grassland 0.23 0.33 0.11 4 evergreen shrubland 0.23 0.33 0.11 5 short grassland/meadow/shrubland 0.25 0.34 0.14 6 evergreen needleleaf forest 0.14 0.22 0.06 7 mixed deciduous forest 0.17 0.27 0.06 8 deciduous forest 0.19 0.31 0.06 9 tropical evergreen broadleaved forest 0.14 0.22 0.06 10 medium/tall grassland/woodland 0.18 0.28 0.06 11 desert, sandy 0.43 0.51 0.35 12 desert, rocky 0.32 0.40 0.24 13 tundra 0.19 0.27 0.10 14 land ice 0.77 0.65 0.90 land ice is treated differently than sea ice 15 sea ice 0.77 0.65 0.90 16 snow 0.82 0.70 0.95 17 bare soil 0.08 0.08 0.08 18 asphalt/concrete mix 0.17 0.17 0.17 dummy values 19 asphalt (asphalt concrete) 0.17 0.17 0.17 dummy values 20 concrete (Portland concrete) 0.30 0.30 0.30 dummy values 21 sett 0.17 0.17 0.17 dummy values 22 paving stones 0.17 0.17 0.17 dummy values 23 cobblestone 0.17 0.17 0.17 dummy values 24 metal 0.17 0.17 0.17 dummy values 25 wood 0.17 0.17 0.17 dummy values 26 gravel 0.17 0.17 0.17 dummy values 27 fine gravel 0.17 0.17 0.17 dummy values 28 pebblestone 0.17 0.17 0.17 dummy values 29 woodchips 0.17 0.17 0.17 dummy values 30 tartan (sports) 0.17 0.17 0.17 dummy values 31 artificial turf (sports) 0.17 0.17 0.17 dummy values 32 clay (sports) 0.17 0.17 0.17 dummy values 33 building (dummy) 0.17 0.17 0.17 dummy values 34 building wall reflective 0.60 0.60 0.60 35 building wall (very) bright) 0.30 0.30 0.30 36 building wall standard 0.07 0.07 0.07 37 building window double layer glazing 0.12 0.12 0.12 38 building window tripple layer glazing 0.17 0.18 0.18 39 building window reflective 0.48 0.48 0.48 40 building roof reflective 0.60 0.60 0.60 41 building roof (very) bright 0.30 0.30 0.30 42 building roof standard 0.07 0.07 0.07
bufsize_alltoall	I	0	Parameter sets max number of items which will be sent in MPI_AlltoAll calls at once (0=infinite). Some implementations of MPI protocol or their underlying hardware have limits of size of sending/receiving buffers. If user experience problems in MPI_AlltoAll transfer, this switch can help limit the large transfer to a number of smaller exchanges. The default setting 0 means no limit and it will suit in most situations. The setting of this parameter to too small value can lead to performance degradation.
constant_albedo	L	.F.	Parameter to fix the surface albedos. When constant_albedo = .T., the surface albedos are kept constant during the entire simulation If set to .F., the actual albedos are calculated after Briegleb et al. (1986) and Briegleb (1992).
dt_radiation	R	0.0	Time step of the radiation model (in s).
emissivity	R	0.95	Surface emissivity (0-1) for user-defined grid points. Please note, by default emissivity is determined by the prescribed vegetation-, pavement-, water-, or building type at the given surface element. Setting this parameter has only an effect if vegetation-, pavement-, water-, or building type is set to zero, i.e. a user-defined value. At the moment, only a single emissivity value can be used for each band in the atmospheric window.
lw_radiation	L	.T.	Parameter to switch off the calculation of longwave radiation. When using RRTMG, longwave radiation calls can be switched off by setting lw_radiation = .F..
max_raytracing_dist	R	-999.0	Maximum distance for raytracing (in meters). Parameter is not used in case of rad_angular_discretization = .T. It is used to set the maximum distance allowed to consider the radiative exchange between two surfaces. This limits the number of surfaces view factors (VF) and hence save much memory by neglecting the very small VF resulting from the mutually visible far surfaces. At the end of VF calculations, the values will be scaled so that energy is conserved. If not set, the model will assume a value equal to double the urban surface layer height. Please note that the calculated FV needs to be higher than the threshold value set in #min_irrf_value in order to be considered.
min_irrf_value	R	1e-6	Minimum potential irradiance factor value for raytracing. Parameter is not used in case of rad_angular_discretization = .T. It is used to set the threshold to consider any view factor (VF) calculated between two surfaces. This limits the number of VF and hence save much memory by neglecting the very small VF resulting from the low mutual visiblity. At the end of VF calculations, the values will be scaled so that energy is conserved. Please note that this parameter will neglect any VF smaller than #min_irrf_value even though the coressponing surfaces are within the #max_raytracing_dist.
mrt_geom	I	1	Method for MRT direction weights simulating a sphere or a human body: 0 - sphere (no weighting), 1 - original weighting function simulating human body, 2 - human body shape simulated as ellipsoid.
mrt_geom_params	R(2)	0.12, 0.88	Parameters of selected weighting method. For mrt_geom = 1 and mrt_geom = 2 it represents width and height of the human body.
mrt_include_sw	L	.T.	Parameter to include SW radiation into the mean radiant temperature (MRT) calculation. If you want to include #rad_mrt_sw in the output, then mrt_include_sw should set to TRUE.
mrt_nlevels	I	0	Number of vertical boxes above surface for which to calculate mean radiant temperature (MRT). Note that this value is used also for calculation of MRT and PET values in the biometeorology module (bio_mrt and bio_pet output variables).
mrt_skip_roof	L	.T.	Parameter to skip calculating mean radiant temperature (MRT) above the roof surfaces.
net_radiation	R	0.0	Net radiation at the surface in W/m². This parameter is only used in case of radiation scheme= 'constant'.
nrefsteps	I	3	Number of reflection steps to be performed inside RTM for the reflected short- and long-wave radiation between mutually visible surfaces. The default value nrefsteps = 3 should be sufficient for typical urban area settings. The residual radiation after the nrefsteps reflections can checked in output variables usm_rad_ressw and usm_rad_reslw and the number of reflections should be adjusted accordingly.
plant_lw_interact	L	.T.	The parameter steers if plant canopy interacts with LW radiation. The value .T. enables absorbtion and emission of LW radiation by resolved plant canopy, if value is set .F. the plant canopy is transparent for LW radiation.
rad_angular_discretization	L	.T.	Parameter to enable using of the angular discretization for calculation of view factors for reflected radiation. Diffuse radiation from sky is always calculated using this discretization regardless of setting of this parameter. The recommended setting is to use angular discretization for regular simulations as it typically gives smaller discretization errors for near surfaces and it scales better for large domains. Setting rad_angular_discretization = .TRUE. requires raytrace_mpi_rma = .TRUE..
radiation_interactions_on	L	.T.	The setting of this parameter to .FALSE. forces not to activate RTM even if vertical urban/land surfaces or trees exist in the domain. In this case, all surfaces will receive radiation fluxes directly from the choosen radiation model (e.g. rrtmg or clear sky). Warning: radiation_interactions_on = .T. is not allowed, in case the bulk cloud model (BCM) is used together with the urban- and/or land-surface model (USM and/or LSM) and the radiation model.
radiation_only	L	.F.	The setting of this parameter to .TRUE. forces to activate the radiation model without urban/land surfaces being defined. In such radiation-only simulations the radiative transfer equation is only solved for the atmosphere and no energy balance is solved at the surface. The surface parameter albedo and emissivity are either loaded by the namelist (albedo_type, albedo, emissivity or read by a static driver file. Warning: radiation_only = .T. is not allowed in combination with the urban- and/or land-surface model (USM and/or LSM) or radiation_interactions_on = .T.. Furhtermore, it is only implemented for radiation_scheme = 'rrtmg'.
radiation_scheme	C*10	'clear-sky'	Radiation scheme to be used. The user can choose between the following schemes: 'constant' A constant net radiation at the surface is prescribed (see #net_radiation). 'clear-sky' A simple clear sky model is used to calculate the radiation fluxes at the surface (shortwave incoming, shortwave outgoing, longwave incoming, longwave outgoing) and consequently the net radiation at the surface. This scheme requires setting albedo, origin_date_time, longitude, and latitude. 'external' External radiative input of short- and longwave downwelling radiation is used, which can be taken from observations or mesoscale models. Further information concerning external radiation input via the dynamic input file can be found here. Note, in case of a nested run each child model will read the radiation input either from its own dynamic input file, or, in case a dynamic input file is not available for each of the child models, it will be read from the dynamic input file for the root model. External radiation input can be provided as level-of-detail = 1 (radiation depends only on time and is horizontally homogeneous) or as level-of-detail = 2 (radiation depends on time, y, x and can be horizontally heterogeneous). Please note, at the moment level-of-detail = 2 is not possible in combination with buildings or complex orography where surface reflections occur. 'rrtmg' The ​RRTMG model is used to calculate the radiative heating rates for each model column. This scheme requires setting origin_date_time, longitude, latitude. The following parameters can be set optionally: albedo_type (and/or albedo_lw_dif, albedo_lw_dir, albedo_sw_dif, albedo_sw_dir). constant_albedo can be used to fix the albedo during the simulation. It is also possible to use the switches lw_radiation and sw_radiation. Please note that the use of RRTMG requires to compile PALM with -D__rrtmg preprocessor directive (in .palm.config.<configuration_identifier> file). But before this, the RRTMG libraries need to be installed.
raytrace_discrete_azims	I	80	Number of horizontal discrete directions (azimuths) for angular discretization used to calculate the sky view factors, surface-surface view factor, and direct solar visibility for all surfaces.
raytrace_discrete_elevs	I	40	Number of vertical descrete elevations for angular discretization used to calculate the sky view factors, surface-surface view factor, and direct solar visibility for all surfaces.
raytrace_mpi_rma	L	.T.	Parameter to enable the one-sided MPI communication to access LAD array and grid surfaces for raytracing. This parameter is only for debugging purposes and it should not be switched off in real simuations. The setting .F. is not compatible with rad_angular_discretization = .TRUE.. Attention: some machine related settings may be necessary to avoid system degredation when using MPI-RMA communication. See for example http://palm.muk.uni-hannover.de/trac/wiki/doc/app/machine/hlrnIII
skip_time_do_radiation	R	0.0	Time after which the radiation model is switched on. This parameter can be used to allow the LES model to develop turbulence before it is affected by radiation.
surface_reflections	L	.T.	Parameter to switch off the surface-surface reflections in RTM. This parameter is intended only for special purposes (e.g. debugging, testing of sensitivities) and it should not be switched off for real simulations.
svfnorm_report_thresh	R	1e21	Thresholds for reporting of normalization factors in calculation of surface view factors to report. It is not used for angular discretization (rad_angular_discretization = .TRUE.).
sw_radiation	L	.T.	Parameter to switch off the calculation of shortwave radiation. When using RRTMG, shortwave radiation calls can be switched off by setting sw_radiation = .F.. Note that shortwave radiation is automatically switched off during nighttime.
trace_fluxes_above	R	-1.0	This parameter enables tracing of values of radiative fluxes in RTM. In case of exceedance of this threshold, RTM prints debug message with maximal value of corresponding flux with additional useful information (location of maximum, corresponding albedo and emissivity,...). The value 0.0 means to print maxima in all timesteps for all particular radiative fluxes while value -1.0 (default) disables this check completly.
unscheduled_radiation_calls	L	.F.	Parameter to allow additional calls of the radiation code. Usually the radiation is called each #dt_radiation, however in case of fast changes in the skin temperature, it is recommended to update the radiative fluxes independently from the prescribed radiation call frequency to prevent oscillations. The value of the temperature threshold used is 0.2 °K.