Multiple Trajectories

Multiple Trajectory Starting Locations

After choosing the meteorological data set, the first menu (below) gives the user the option of computing trajectories from 1, 2 or 3 different starting locations. Choosing more than one can be useful if more than one trajectory is needed to be plotted on the same map. The only limitation is that the starting heights (up to 3) must be the same for each location.

Trajectory Restart Option

Normally trajectories are started only at the initial time at the locations and heights specified by the user.  However, by entering a value (hours) in the Start a new trajectory every field, trajectories can be started at regular time intervals from the same location. At this time, a maximum of 24 trajectories for a total of 48 hours duration are allowed on HYSPLIT-WEB due to the resources needed to compute these trajectories. To demonstrate this feature see Example 4 Powerpoint (Ex4_hysplit.ppt)] or, change the restart interval to 3 and leave all the other parameters the same as in the original 700 hPa isobaric trajectory, i.e., NAM 40 km source location: 46N, 68W(Maine) Start a new trajectory every: 3 hours starting height: 2500 m vertical motion method: isobaric duration: 84 hrs vertical plot height unit: Pressure Use current GFS forecast if trajectory runs off current grid

Trajectory Matrix Option

Although HYSPLIT-WEB can be used to configure only up to 3 starting locations, the model can support an unlimited number of locations. HYSPLIT-WEB and PC HYSPLIT have a shortcut method to configure a regular matrix of locations by defining three points, representing the lower left, upper right, and location increment. HYSPLIT-WEB does have a limitation of 125 starting locations which represents about a 11x11 lat/lon grid with a 1 degree horizontal spacing.

Trajectory Matrix Example

After choosing a Matrix run and clicking Next, a menu for entering the 3 matrix starting locations will be presented (right).  Other than entering 3 locations the rest of the configuration is similar to a normal trajectory computation except that the locations defined by the matrix grid will be calculated and written to the CONTROL file before executing the run. Rerun the 700 hPa isobaric case again but change the Total run time to 24 hours and choose the 3 source locations as shown to the right (42.0,-72.0), (50.0,-64.0) and (44.0,-70.0). This configuration will produce 25 starting locations separated by 2 degrees of latitude (~222 km). Also, set the restart interval back to 0 if it is not 0.

NAM 40 km Matrix starting location 1: 42.0N, 72.0W starting location 2: 50.0N, 64.0W starting location 3: 44.0N, 70.0W starting height: 2500 m vertical motion method: isobaric duration: 24 hrs vertical plot height unit: Pressure Use current GFS forecast if trajectory runs off current grid

The resulting graphic (right) shows the matrix of 24-h duration isobaric trajectories. Again there is very little differences noted between the trajectories indicating small temporal and spatial differences in the meteorology in this region. Note that we are using spatial and temporal offsets to get at trajectory error or in this case sensitivity to the meteorological data.

Trajectory Ensemble Option

Frequently it is necessary to attribute a pollutant measurement to a specific source location.  One approach is to compute a backward trajectory to determine the air’s origin.  Although it is not uncommon to see sources identified by a single trajectory, the uncertainties inherent in a single-trajectory can preempt its utility.  One way to reduce those uncertainties would be to compute multiple trajectories, in height, time, and space.  HYSPLIT can be configured to automatically run trajectories in an ensemble fashion from the chosen starting location. Each member of the trajectory ensemble is calculated by offsetting the meteorological data by a fixed grid factor (HYSPLIT-WEB assumes one meteorological grid point in the horizontal and 0.01 sigma units in the vertical). This results in 27 members for all-possible offsets in X,Y, and Z. Note: the starting height should be greater than 250 m for optimal configuration of the ensemble.

Trajectory Ensemble Example

For this example choose: NAM 40 km Ensemble starting location: 40.0N, 85.0W Backward starting date/time: December 23 at 0000 UTC starting height: 750 MAGL

The resulting plot (right) shows that 3 different paths are possible depending on the selected location in X, Y, and Z. This method gives the user a sense of the sensitivity of the chosen starting location within the meteorological grid as well as an "envelope" of the uncertainty about the single trajectory result.