17.4 Polar Coordinate Concentration Grid




If you are not continuing from the previous section, first retrieve the previously saved sage5_control.txt and the namelist file sage5_setup.txt. In this section we will re-run the simulation using a polar coordinate concentration grid rather than the default latitude-longitude rectangular grid. The HYSPLIT polar grid is defined by downwind distance and a clockwise angle from north. The conversion between polar and rectangular grids is defined using sines and cosines. However, because the conversion assumes a flat plane for the polar grid, it should only be applied to distances near the source where the latitude-longitude intersections can be assumed orthogonal within an acceptable range of error.

  1. Opening the GUI to the Concentration Grid menu and change the latitude grid spacing to 1.0 which represents the angular distance between cells and change to longitude spacing to 0.1 which represents the downwind spacing in kilometers. The span of the grid will always be 360.0, a full circle, with a maximum downwind distance of 5 kilometers. Also give the output file another name such as pdump and then save to exit.

  2. Now open the Advanced / Concentration / Menu #8 Grid Packing Method and select the polar grid radio-button. This sets cpack=3 which causes HYSPLIT to interpret the previously set grid parameters in polar rather than rectangular units. Then save to exit and run the model. The standard output shows that the model has been configured for a polar concentration grid.

  3. Next open the Display / Concentration / Polar Grids menu and change the map labels to every 0.1 degrees. The resulting plume display shows the concentration color scale at the bottom. The number at the bottom right, 1.0E-04, is the contour multiplier required to obtain the final units. Note this program does not yet provide the option to convert g/kg to ppt.

  4. Because this is a relatively new feature, most HYSPLIT post-processing programs provide limited support of polar grids. In addition to the special plotting program, only two other options are available: the simple listing of the concentration file (conread) and the convert to ASCII utility (con2asc).

This example demonstrated how a standard HYSPLIT simulation could be applied to very short range dispersion simulations but using a polar coordinate grid which is a more natural option for distances near the source where the typical plume structure will have a similar shape to the polar grid. This option provides for greater concentration grid resolution near the source and less resolution at greater distances.