Why are there small differences in my trajectories when I run more than one at a time versus running just one trajectory?

To save computer time in processing a large number of trajectories on the web, the trajectory computation is done using a sub-grid that moves along with the trajectory. There is no point in processing all of the meteorological data, especially the global data (360×181), if you are interested in only a few points. However, the diagnostic vertical motion options (all but the model vertical motion) require a computation of the slope (horizontal and vertical) of the field in question. The slope is computed using a centered difference in the sub-grid interior and a simple difference along the edges. When the trajectory endpoint approaches the edge of the sub-grid, the sub-grid is re-centered and then the data fields are recomputed. So when you add additional trajectories the sub-grid calculation (size and location) may not be the same, so each trajectory of the multiple trajectory calculation may approach the edge at a different time than in a previous calculation with a different number of trajectories, which means that for one trajectory at a particular point the vertical motion may have been computed with a simple difference or a centered difference, resulting in a slightly different trajectory. This is not an issue if you choose the default vertical motion option. On the PC, the simple solution to this problem is to increase the size of the subgrid in the namelist configuration file (SETUP.CFG).

Roland Draxler