Treatment dispersion errors

[iporras]

Hello,

What treatment does the dispersion model on errors? It gives some kind of relative error or can easily obtain?

Thanks,

[ariel.stein]

Part of the model uncertainty can be assessed by running the ensembles that HYSPLIT has incorporated in the Graphical User Interface (GUI).
Go to Concentration/Special Runs/Ensemble and choose one of the 3 options.

Here is more information about the HYSPLIT ensemble capabilities:

The use of dispersion model ensembles -- with the objective of improving plume simulations and assessing their uncertainty -- has been an increasingly attractive approach to study atmospheric transport (e.g. Potempski et al 2008; Lee et al 2009; Solazzo et al, 2013; Stein et al, 2015). The HYSPLIT system has a built-in capability to produce three different simulation ensembles. This ensemble approach has been applied to case studies using different sets of initial conditions and internal model physical parameters (Draxler, 2001; Stein et al, 2007; Chen et al, 2012). These built-in ensembles are not meant to be comprehensive and only account for some of the components of the concentration uncertainty, such as those arising from differences in initial conditions and model parameterizations. The first, called the “Meteorological Grid” ensemble, is created by slightly offsetting the meteorological data to test the sensitivity of the advection calculation to the gradients in the meteorological data fields. The rationale for the shifting is to assess the effect that a limited spatial and temporal resolution meteorological data field -- an approximation of the true flow field which is continuous in space and time -- has on the output concentration (Draxler, 2001). The second, called the “Turbulence” ensemble, represents the uncertainty in the concentration calculation arising from the model’s characterization of the random motions created by atmospheric turbulence (Stein et al, 2007). This ensemble is generated by varying the initial seed of the random number generator used to simulate the dispersive component of the motion of each particle. The model already estimates this turbulence when computing particle dispersal. However, normally, a sufficiently large number of particles would be released to ensure that each simulation gives similar results. In the Turbulence ensemble approach, the number of particles released is reduced and multiple simulations are run, each with a different random number seed. The third, the “Physics” ensemble, is built by varying key physical model parameters and model options such as the Lagrangian representation of the particles/puffs, Lagrangian timescales, vertical and horizontal dispersion parameterizations, etc.

References can be found at “NOAA’s HYSPLIT atmospheric transport and dispersion modeling system” Bulletin of the American Meteorological Society. A.F. Stein, R.R. Draxler, G.D. Rolph, B.J.B. Stunder, M.D. Cohen, and F. Ngan. 96, 2059–2077, 2015. doi: http://dx.doi.org/10.1175/BAMS-D-14-00110.1

Particular details about each ensemble can be found below:


Ensemble-Meteorology
The ensemble form of the model, an independent executable, is similar to the trajectory version of
the ensemble. The meteorological grid is offset in either X, Y, and Z for each member of the
ensemble. The model automatically starts each member on a single processor in a multi-processor
environment or cycles through the simulations on one processor. The calculation offset for each
member of the ensemble is determined by the grid factor as defined in the Advanced Concentration
Configuration Tab. The default offset is one meteorological grid point in the horizontal and
0.01 sigma units in the vertical. The result is 27 ensemble members for all offsets. The normal
Setup Menu tab is used to configure the CONTROL file. Note that if fewer than 27 processors are
available, the ensemble configuration menu permits starting the calculation at any ensemble member
number within the valid range. Because the ensemble calculation offsets the starting point, it is
suggested that for ground-level sources, the starting point height should be at least 0.01 sigma
(about 250 m) above ground. The model simulation will result in 27 concentration output files named
according the file name setting in the control file "{cdump}.{001 to 027}" with a suffix equivalent
to the ensemble member number. On a single processor system, the calculation may take some time to
cycle through all the memberes. The menu will be locked until the simulation has completed. A
message file window will open after termination. Computational progress may be monitored by noting
the generation of new concentration output and message files with the ensemble number suffix in the
/working directory. The concentration output from each member can be displayed through the
concentration display menu tab. However, to display the probabilities associated with the multiple
simulations, it is necessary to pre-process the data through the Display Ensemble menu tab. Using
the default configurations for the sample simulation, the illustration below represents the 90th
percentile concentrations aggregating all four output time periods. For instance the blue contour
in this 90th percentile plot represents the region in which only 10% of the ensemble members have
air concentrations greater than 10-15. If the meteorological ensemble is run through a script or
batch file instead of the GUI, the executable, with a command line parameter of the member number,
must be run once for each of the 27 members.

Ensemble-Turbulence

Another ensemble variation is the turbulence option, which also creates 27 ensemble members, but
due to variations in turbulence rather than variations due to gradients in the gridded
meteorological input data. The variance ensemble
should only be run in the 3D particle mode and with fewer particles, in proportion to the number of
ensemble members to the number of particles required for a single simulation. For instance, if
27,000 particles are required to obtain a smooth plume representation, then each member should be
run for 1000 particles. Normally the same random number seed is used when computing the turbulent
component of the particle motion. However, in the variance ensemble, the seed is different for each
member, resulting in each member representing one realization of the ensemble.

The variance ensemble can also be used to determine the number of particles required for a
simulation, by progressively increasing the particle number until the variance decrease with
increasing particle number is no longer significant.
Because the number of particles required for a simulation increases with increasing distance from
the source, a typical downwind receptor location needs to be selected and then by using the box
plot display option, the concentration variability (max-min range) can be estimated and then
determining when the decrease in range is no longer cost effective (in terms of computational time)
with increasing particle number.

Ensemble-Physics

The physics ensemble is created by running a script that varies in turn the value of one namelist
parameters from its default value. These are the parameters defined in the file SETUP.CFG and
normally set in the Advanced
Concentration Configuration menu tab. If the namelist parameters have not been defined through the
menu, then the default values are assigned. In this first iteration, the GUI menu permits no
deviation from the values assigned by the script. The entry box is for information purposes only to
show the progress of the computation.

A summary of the current 20 ensemble variations is also written to the file ensemble.txt showing
the name of the concentration output file and member variation. Check the advanced menu help files
for more information on each variable.

cdump.001 : initd = 0 cdump.002 : initd = 3 cdump.003 : initd = 4 cdump.004 : kpuff = 1 cdump.005 :
kmixd = 1 cdump.006 : kmixd = 2 cdump.007 : kmixd = 1500 cdump.008 : kmix0 = 50 cdump.009 : kzmix =
1
cdump.010 : kzmix = 2 ; tvmix =0.5 cdump.011 : kzmix = 2 ; tvmix =2.0 cdump.012 : kdef = 1
cdump.013 : kbls = 2 cdump.014 : kblt = 1 cdump.015 : kblt = 3 cdump.016 : tker = 0.10 cdump.017 :
vscales = 5.0 cdump.018 : vscaleu = 1000.0