### How do I estimate the absolute (in km) and relative (%) errors when using the Hysplit4 trajectory model? Where does the error mostly come from (e.g., truncation error, interpolation error or wind field error)?

The error is caused by a numerical and a physical component. Overall, from the literature, one can estimate the total error to be anywhere from 15 to 30% of the travel distance. The physical component of the error is related to how well the numerical fields estimate the true flow field. We have no way of knowing this without independent verification data. The numerical component of the error we can estimate. This error is composed of the integration error (part of which is due to truncation) and an error due to the data resolution – trying to represent a continuous function, the atmospheric flow field, with gridded data points of limited resolution in space and time. The integration error can be estimated by computing a backward trajectory from the forward trajectory endpoint – the error is then 1/2 the distance of the final endpoint and starting point. The resolution error can be estimated by starting several trajectories about the initial point (offset in the horizontal and vertical). The divergence of these trajectories will give an estimate of the uncertainty due to divergence in the flow field. You can use an initial offset that is comparable to your estimated integration error. One component of the resolution error that is difficult to estimate relates to the size and speed of movement of various flow features through the grid. There should be sufficient number of sampling points (in space and time) to avoid aliasing errors.

Roland Draxler