Radiation Parameters

PALM offers radiation simulation using a radiation model 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. The model treats direct and diffuse radiation separately.

Current RTM (Version 2.0) involves the following processes

• Calculation of the incoming/outgoing shortwave radiation components on each surface element in the domain
• Estimation of longwave thermal emission/absorption for all surfaces
• Reflections of shortwave and longwave radiation by all surfaces (finitely iterated)
• Partial absorption of shortwave radiation by vegetation (trees)

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. 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_type	I	5	This parameter controls the choice of the surface albedos for direct/diffuse/broadband albedo for a solar angle of 80° 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 (1992). Setting on 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 shortwave longwave broadband Notes 0 user defined - - - 1 ocean 0.06 0.06 0.06 2 mixed farming, tall grassland 0.09 0.28 0.19 3 tall/medium grassland 0.11 0.33 0.23 4 evergreen shrubland 0.11 0.33 0.23 5 short grassland/meadow/shrubland 0.14 0.34 0.25 6 evergreen needleleaf forest 0.06 0.22 0.14 7 mixed deciduous forest 0.06 0.27 0.17 8 deciduous forest 0.06 0.31 0.19 9 tropical evergreen broadleaved forest 0.06 0.22 0.14 10 medium/tall grassland/woodland 0.06 0.28 0.18 11 desert, sandy 0.35 0.51 0.43 12 desert, rocky 0.24 0.40 0.32 13 tundra 0.10 0.27 0.19 14 land ice 0.90 0.65 0.77 *1 15 sea ice 0.90 0.65 0.77 16 snow 0.95 0.70 0.82 17 bare soil 0.08 0.08 0.08 18 asphalt/concrete mix 0.17 0.17 0.17 *2 19 asphalt (asphalt concrete) 0.17 0.17 0.17 *2 20 concrete (Portland concrete) 0.30 0.30 0.30 *2 21 sett 0.17 0.17 0.17 *2 22 paving stones 0.17 0.17 0.17 *2 23 cobblestone 0.17 0.17 0.17 *2 24 metal 0.17 0.17 0.17 *2 25 wood 0.17 0.17 0.17 *2 26 gravel 0.17 0.17 0.17 *2 27 fine gravel 0.17 0.17 0.17 *2 28 pebblestone 0.17 0.17 0.17 *2 29 woodchips 0.17 0.17 0.17 *2 30 tartan (sports) 0.17 0.17 0.17 *2 31 artificial turf (sports) 0.17 0.17 0.17 *2 32 clay (sports) 0.17 0.17 0.17 *2 33 building (dummy) 0.17 0.17 0.17 *2 (*1) land ice is treated differently than sea ice (*2) preliminary/dummy values
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°.
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 longwave radiation. When using RRTMG, longwave radiation calls can be switched off by setting lw_radiation = .F..
net_radiation	R	0.0	Net radiation at the surface in W/m². This parameter is only used in case of radiation scheme= 'constant'.
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, day_of_year_init, longitude, latitude, and time_utc_init. 'rrtmg' The ​RRTMG model is used to calculate the radiative heating rates for each model column. This scheme requires setting day_of_year_init, longitude, latitude, time_utc_init. 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 __rrtmg preprocessor directive. By the same token it is not possible to switch the radiation scheme to 'clear-sky' or 'constant' for such a compiled PALM executable.
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.
sw_radiation	L	.T.	Parameter to switch 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.
unscheduled_radiation_calls	L	.T.	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.
max_raytracing_dist	R	-999.0	Maximum distance for raytracing (in meters). 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. 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.
nrefsteps	I	0	Number of reflection steps to be performed to accound for the refelcted short- and longwave between musually visible surfaces. Although the effect of reflection depends on the surface characteristics (albedo, emissivity, etc.), however for typical urban areas it is recommended to set nrefsteps = 3 because the effect of reflection after 3 reflections is negligible.
mrt_factors	L	.F.	Parameter to generate extra view factors for specific surfaces to calculate MRT during the post processing. Please note: if set to TRUE, then extra input file (MRT_TARGETS) is needed to set the ID of the surface as well as its coordinates. The model will write out unformated file contains all the mutually visible surfaces for each requested surface along with its type, coordinates, view factor, and transparency.
rma_lad_raytrace	L	.F.	Parameter to use One-sided communication (MPI-RMA) to access LAD array for raytracing. For large domain it is recommended to use MPI-RMA to access LAD array for raytracing instead of allocating the global array for each processor. This saves memory and allow for simulations of large domains. Please note: For Intel compiler, if you observe performance degradation with an MPI application that utilizes the RMA functionality (i.e. rma_lad_raytrace = .TRUE.), you are recommended to set I_MPI_SCALABLE_OPTIMIZATION = 0 to get a performance gain.
surface_reflections	L	.T.	Parameter to consider surface-surface reflections. It is not recommended to set surface_reflections = .F. because When it switched to off, only the effect of building and tree shadows will be considered and all reflection processes will be disabled. However fewer no surface-surface VFs are caclulated and less memory requirement is expected.
svfnorm_report_thresh	R	1e21	thresholds of surface view factors normalization values to report. This array contains 30 values of the histogram steps of logged VF normalization values to show the VF distribution.
radiation_interactions_on	L	.T.	Parameter to force RTM activiation regardless to vertical urban/land surface and trees. RTM is usually set according to the existance of vertical urban/land surface or trees. If set to FALSE, no RTM is used even when there are vertical urban/land surface or trees in the domain. This simply means that all surfaces will receive radiation fluxes directly from the radiation model without considering other radiation processes such as reflections.