Virtual measurements

Since -r3707 a virtual-measurement module is available in PALM. The module enables an easy setup of virtual measurements in the model domain emulating 'real-world' observations. Virtual measurement locations are placed within the model domain according to the UTM coordinates of the 'real-world' observations. This way, not only point and profile measurements can be emulated in the model, but also complex trajectories such as spiral-shaped flight measurements or bicycle/pedestrian measurements. With masked data output this would be hardly possible. Please note, to correctly setup the virtual measurement locations the origin UTM coordinates of the lower-left edge of the model domain (origin_x, origin_y) within the static input file should be as accurate as possible since these are used to place the virtual site. Note, only sites that are located within the model domain will be setup, for sites outside your domain no NetCDF will be created.

For different measurement sites the variables which should be sampled can be prescribed individually for each site. Variables will be sampled at each model time step at all given coordinates and adjacent grid points. Virtually sampled data is output using parallel NetCDF.

In order to setup the measurement locations, a setup file is required. This file contains all information about the measurement coordinates, the measured variables, the type of the measurement (timeseries-, profile-, or trajectory measurements), as well as further attributes such as site names, etc.. For a description of required attributes and variables, as well as an example input file, please see here. Since revision -r4400 PALM comes with a pre-processor script palm_cvd​, which is a python3 script to setup the input file according to your prescribed observation locations.

palm_cvd has two options. First, the custom-mode, where you can manually setup a variable number of measurements points (in UTM coordinates) and specify the respective output variables yourself. However, so far only point- or tower measurements are possible, complex trajectories cannot be described this way.
In the second mode, palm_cvd already reads existing NetCDF files from observation campaigns and extract the required information. However, this requires that the input NetCDF files are already formatted according to the [UC]2 data standard (see ​here). More information of how to setup a measurement input file can be found here.

Please note, all sampled variables are defined at the grid center, i.e. velocity components and fluxes are interpolated online. The following list gives an overview about the quantities that can be sampled (naming convention follows the [UC]2 standard):

Quantity name	Meaning	Unit
theta	potential temperature	K
thetas	scaling parameter for temperature in surface layer	K
ta	air temperature	degree C
hus	specific humidity	kg/kg
haa	absolute atmospheric humidity	kg/m3
hur	relative humidity	-
pwv	water vapor partial pressure	hPa
ssdu	duration of sunshine	s
t_lw	water temperature	degree C
mcpm1	PM1 concentration	kg/m3
mcpm10	PM10 concentration	kg/m3
mcpm2p5	PM2.5 concentration	kg/m3
mcno	NO concentration	ppm
mcno2	NO2 concentration	ppm
tro3	O3 concentration	ppm
ps	surface pressure	hPa
lwp	liquid water path	kg/m2
hfdg	ground heatflux into soil	W/m2
hfls	surface latent heat flux	W/m2
hfss	surface sensible heat flux	W/m2
rsds	surface radiation flux (shortwave down)	W/m2
rsus	surface radiation flux (shortwave up)	W/m2
rsd	radiation flux (shortwave down)	W/m2
rsddif	radiation flux (diffuse shortwave down)	W/m2
rsu	radiation flux (shortwave up)	W/m2
rnds	surface net radiation flux	W/m2
rld	radiation flux (longwave down)	W/m2
rlu	radiation flux (longwave up)	W/m2
rlus	surface radiation flux (longwave up)	W/m2
t_soil	soil temperature	degree C
m_soil	soil moisture	m3/m3
uv	total horizontal momentum-flux	m2/s2
vw	total vertical momentum flux of v-component	m2/s2
uw	total vertical momentum flux of u-component	m2/s2
us	friction velocity	m/s
u	u-component	m/s
v	v-component	m/s
w	w-component	m/s
wspeed	horizontal wind speed	m/s
wdir	wind direction	degree
utheta	total flux of potential temperature in x-direction	Km/s
vtheta	total flux of potential temperature in y-direction	Km/s
wtheta	total flux of potential temperature in z-direction	Km/s
uqv	total flux of humidity in x-direction	kg m/(kg s)
vqv	total flux of humidity in y-direction	kg m/(kg s)
wqv	total flux of humidity in z-direction	kg m/(kg s)

NAMELIST group name: virtual_measurement_parameters

Parameter Name	FORTRAN Type	Default Value	Explanation
dt_virtual_measurement	R	0.01	Temporal interval at which virtual measurements shall be output (in s). Please note, this parameter must not be zero.
off_pr	I	1	Number of surrounding grid points (in horizontal direction) around the original measurement location where variables shall be sampled (for profile measurements). Please note, the number must not be larger than the number of ghost points minus one.
off_pr_z	I	0	Number of surrounding grid points (in vertical direction) around the original measurement location where variables shall be sampled (for profile measurements).
off_tr	I	1	Number of surrounding grid points (in horizontal direction) around the original measurement location where variables shall be sampled (for trajectory measurements). Please note, the number must not be larger than the number of ghost points minus one.
off_tr_z	I	1	Number of surrounding grid points (in vertical direction) around the original measurement location where variables shall be sampled (for trajectory measurements).
off_ts	I	1	Number of surrounding grid points (in horizontal direction) around the original measurement location where variables shall be sampled (for stationary timeseries measurements). Please note, the number must not be larger than the number of ghost points minus one.
off_ts_z	I	50	Number of surrounding grid points (in vertical direction) around the original measurement location where variables shall be sampled (for stationary timeseries measurements).
switch_off_module	L	.F.
vm_time_start	R	0.0	Start time of the virtual measurements. Please note, vm_time_start refers to the simulation start and sampling will thus start at origin_data_time + vm_time_start . Time information given in the measurement file is ignored at the moment.