Frequently asked questions in the context of LASAT

• (21) Can one apply PLURIS to line sources?

  PLURIS is designed for calculating the plume rise of a single stack. Applications deviating from this must be carefully checked with regard to physics and technical implementation in LASAT. In particular, LASAT uses the value Hq (default 10 m) as the formal exit height when calculating the dynamic plume rise, also for line sources defined via X1/Y1 etc. The source parameters used for the plume rise calculation can be checked in the log file lasat.log (call option --log-pluris in combination with -v3 or -v4).
  
  

• (20) How interprets IBJdis the values xmin and ymin in the header of DMNA files?

  IBJdis assumes that the values xmin/ymin refer, for area and volume means (entry in vldf is V), to the left/lower edge of the area i=1/j=1 and, for point values (entry in vldf is P), to the point i=0/j=0. this guarantees that the values of xmin/ymin can be maintained if an index subrange is taken. For example, if for a DMNA file with volume means and index range i=1..5 the data row i=1 is removed and the index range is changed accordingly to i=2..5 (parameters lowb, hghb), IBdis displays the areas still at the correct locations.
  
  

• (19) What must be considered when performing an AKS based calculation?
  
  
  • In emissions.def the parameter Emisfac must be defined as time dependent (Emisfac = ?).

    Take care that the parameter is defined in a continuation line or separated from the point (.) that starts the assignment line by at least one blank (this applies in general; without blank the parameter is ignored and the calculation gives a wrong result).

  • The files meteo.def and variable.def must be created with the program Lpraks from the provided dispersion class statistics (AKS).
  • Set parameter Interval to 1.00:00:00 in param.def and adjust parameters End and Average according to the AKS (see log output of Lpraks and further information in the reference book).
  • According to the draft TA Luft (2018/2019), wet deposition cannot be accounted for when applying an AKS.
  • Check the project with the LASAT tool DefCheck.
  
  
  
• (18) What are the main differences between the boundary layer versions 2.1 and2.6?
  
  
  • Version 2.1 applies a slightly different assignment of Monin-Obukhov lengths to Klug/Manier stability classes as compared to Version 2.6 (see the reference book).
  • Version 2.1 does not consider a turn of wind direction with height, in contrast to Version 2.6.
  • Version 2.1 yields for emission at larger heights larger more realistic plume widths as compared to Version 2.6.
  • In the displacement layer close to the ground, Version 2.1 applies a linear decrease of the turbulence profiles whereas Version 2.6 keeps them constant.
  • For small roughness lengths (approx. smaller 0.1m) the horizontal exchange coefficient in Version 2.1 shows a strong gradient close to the ground and requires for sources close to the ground (approx. below 10m) with a small spatial extent for the calculation of short time values a finer vertical resolution than provided by the default raster (0m, 3m etc.), which would yield in this case plumes that are too small.
  
  
• (17) What should be considered if spatially varying deposition velocities are applied?
  
  For some or all of the substances applied in a LASAT calculation, spatially  (and temporally) varying deposition velocities can be applied. For further information see the reference book. The following points should be considered:
  
  
  • If the calculation is carried out for several substances, a separate register can be applied for each of them and transferred into an input file for LASAT using the LASAT tool Lprgrd. The LASAT tool Lprgrd appends with each call the deposition velocities of the specified substance in form of a value matrix to file vdp*.dmna. See example DEMO-2D in the working book. For substances not encountered in file vdp*.dmna, the usual deposition velocity in file substances.def is applied, see Section 3.5.1 of the reference book.
    
    
  • If a data matrix is applied for the deposition velocities of a substance, the usual deposition velocity in file substances.def for this substance should be set to 0. The value in substances.def is applied if no value matrix for this substance is specified or found by the program. A value 0 thus helps cross-checking the results.
    
    
  • In file param.def, the parameters DepoBase and Depo must be set. Parameter Depo must be set to a value larger 0 (either explicitly or, if defined in form of a time series, within the time series). With Depo = 0 (this is the default) no deposition files are applied.
    
    
  • The register applied with the LASAT tool Lprgrd must cover at least the calculation area. If not, Lprgrd sets the value for grid cells not covered by the register to -1. This leads to an error message in the subsequent dispersion calculation. Beside this, the cell size and position of the applied register is not restricted and must not coincide with the settings of file grid.def.
    
  
  
  
• (16) How should calms be defined in a AKTerm?
  
  According to the specification of the German Weather Service DWD, calms (hours with no wind) are encoded in a AKTerm file by wind direction 0 (wind from north is defined by 360 deg). In addition, the wind speed should be set to 0: first for obvious plausibility, second to allow a consistent treatment of the wind direction redistribution according to paragraph 8.2 of TA Luft, Annex 3. Thus, a wind direction 0 should only occur together with wind speed 0 and vice versa. This is also the convention followed by the DWD. The programs Lprakt and Austal2000 detect calms at hand of wind speed 0.
  
  
  
• (15) What must be considered for a spatial average of odor hour frequencies?
  
  The existance of an odor hour is detected by internal comparison of the hourly mean concentration in a given grid cell  with the threshold value. For this purpose the concentration must be spatially resolved  with a sufficient accuracy. For hourly means, in general a finer spatial resolution is required than for long time means. This aspect can be of particular importance at small distances to near-ground point sources. As a rule of thumb for such a case, at least 5 grid meshes should be between the source and the point of interest. If the grid meshes are too large, the hourly concentration plume is not well resolved and the resulting odor hour frequency may be under- or overestimated, depending on the emission rate.
  
  Based on the results of a sufficiently resolved dispersion calculation, larger spatial averages can be derived. For example the auxiliary program A2KArea.jar can be used for this purpose. This distinctive feature of spatially averaging odor hour frequencies is due to the non-linear relationship between odor hour and concentration (yes/no decision: concentration above or below a threshold). For the spatial average of concentration in contrast, the values provided by the dispersion program on a coarse calculation grid can be directly used.
  
  
  
• (14) What meaning has a concentration time series in the context of a dispersion class statistics (AKS)?
  
  No direct one, because the concentration values contain the statistical weight and cannot be directly interpreted (see also FAQ 11). Therefore, for calculations with an AKS, flag MNT in param.def should not be used..
  

• (13) Why is Rate=8 (equivalent to AUSTAL2000 quality level 2) in some situations not sufficient to faithfully calculate the frequency of odor hours?
  
  A sufficiently large number of simulation particles is required to avoid systematic effects in the calculation of odor hour frequencies (see the discussion in the annex of the program manuals of AUSTAL2000 and LASAT). Usually (one or several sources near ground and close together) Rate=8 (AUSTAL2000 quality level 2) is sufficient for time series calculations. However, if the sources are far apart or if a source, which is dominant with respect to the emission, has almost no contribution to the total concentration near ground due to its construction height and/or plume rise (but nevertheless takes most of particles, which are assigned to the sources according to their emissions), a considerably larger rate may be required. This is also the case if the counting volume (averaging volume) is strongly reduced by a small horizontal or vertical mesh width. Once the systematic effect is avoided, the listed (absolute) statistical uncertainty is a faithful measure also for odor hours.
  
  
  
• (12) Why are some non AUSTAL2000 substances not evaluated by the LTool Lopxtr?
  
  Avoid using a minus sign ("-") in the name of the substance. It is interpreted by LTools as the marker for a subsequent diameter fraction according to TA Luft. Use an underscore ("_") instead.
  

• (11) What is the meaning of the total emission that is written out by LASAT (since 3.1) to lasat.log in a calculation for a dispersion class statistics (AKS)?
  
  The total emission has no direct meaning as such a calculation refers to a weighted average over stationary situations without time reference (an assignment of one hour to a single meteorological situation and of one year to the statistics in total is a matter of interpretation).
  
  In the calculation, each meteorological situation is formally run over one day (in a way that yields a result corresponding to a stationary situation). At the beginning of the day, the emission takes place over a time interval proportional to the statistical weight of the situation. The sum of emission times over all situations is one day. In addition, the constant emission rate q provided by the user is multiplied with the number of situations N, so that the average over all N days corresponds to the average over all stationary situations.
  
  The formal total emission is therefore q*24h*N. For comparison: The total emission in a true time series calculation over one year is (for constant q) is q*24h*365. The total emission in a calculation for a dispersion class statistics is thus by a factor N/365 heigher than the total emission of a corresponding time series calculation, where N is the number of situations that is provided in file param.def as parameter Average.
  
  Note that parameter Emisfac in file emissions.def must be declared as time-dependent. The time series is set by Lprtaks in file variable.def. It controls the turn on and off of the emission and the multiplication by N.
  
  
  
• (10)  Which precausions must be considered on applying  DEF files from the temporary AUSTAL directory WORK for a calculation with LASAT?
  
  This must be done with special care because  the DEF files do not necessarily obey the current LASAT conventions.
  
  
  
  
• (9)  Why are the values Ua and Ra specified in meteo.def for more than one anemometer height  (Ha = { ... }) not used?
  
  Specification of more than one anemometer height is not allowed with the TA Luft profile versions 2.6 and 5.x. Use the default version 2.1 instead. 
  

• (8)  What must be considered when applying externally generated wind fields?
  
  
  • The wind fields must be defined on the Arakawa-C net.
    
    
  • Before the actual dispersion calculation, any divergence in the provided wind field is internally removed by a simple adjustment of the z-components. To keep this modification at a minimum, the provided wind fields should be already sufficiently free of divergence.
    
    
  • For an application of the wind fields without scaling, file meteo.def must still contain an explicit value for Ua and the stability class (class Km or Obukhov length Lm) ) as these values are required by LASAT to determine the friction velocity and the (plain) turbulence profiles.
    The stability class (or Obukhov length) is also required internally to set the plume rise according to VDI 3782/3 or VDI 3784/2. If plume rise is calculated with PLURIS, PLURIS requires the friction velocity; it can be explicitly specified, otherwise it is calculated internally from the theoretical wind profile; in the latter case, specification of Km/Lm, Ua, Ha, Z0, D0 are required, even if all fields are otherwise specified externally.
    Without plume rise and with provision of all wind and turbulence fields, a formal stability class (or Obukhov length) is required to mark the time interval as valid.
    
    
  • The meteorological fields actually applied by LASAT can be inspected with the help of the program Lprprf. It writes out for a specified time to file prfa0li.dmna the model fields as they are applied in the actual disperrsion calculation. This is an important tool to check whether the fields have been created as required by LASAT. Before carring out the dispersion calculation, the files prfa0li.dmna should be deleted, otherwise Lasat takes the fields directly from these files and changes in meteo.def or the external wind fields would not be accounted for.
    
    
  • Buildings: Buildings are resolved on the calculation grid. A cell is interpreted as building cell if the vertical component Vs of the wind vector at the bottom of the cell is equal -99 m/s. If the wind field is modified (e.g. by adding topographic effects), care must be taken that this value is not changed. Below a building cell there may be no free cell (this would result either in an error message or in incorrect concentration values).  
    
    
  • Terrain profile and nested grids: LASAT adjusts the z-values specified in files srfa0li.dmna for the two boundary stripes of inner grids at hand of the values of the next coarser grid. This "tuning" guarantees a smooth transition between the grids. The resulting values are written to the wind fields generated by LASAT. If external fields are provided, they must contain these tuned z-values. In default mode (index Wind not set), LASAT checks the z-values of the wind field files with the internally created ones and aborts with an according error message if they do not agree.
    
    
  • Incorporation in a wind field library: If external wind fields are added to a standard wind field library in order to directly access them by a value larger 0 for index "Wind" (for example for modelling a time dependent drain of cold air), care must be taken that these fields are not applied for the usual superposition of base fields for situations with index "Wind" being equal 0. This can be avoided by setting the dispersion class index "akl" in the file header to for example 7. The field is then included in the list of base fields but never accessed as such, as for the superposition only indices 1 to 6 are addressed. The list of base fields and their class attribution can be printed to the screen and the log file lasat.log with the Lasat program call option -p. It should be carefully checked at hand of the messages in lasat.log for each interval of the dispersion calculation whether the fields are super-imposed or not super-imposed as desired.