Details and instructions on the trajectory tool page

Page will plot trajectories that are calculated using various reanalysis datasets. A 3D calculation is used, using analyzed pressure level winds from 10000mb to 100mb.
Datasets | Dates | Starting/Ending location | Plot Options | Output | Page Creation | Feedback | Caveats

If you have problems, please email with ALL the options you chose. You will get a plot as well as the data used to create the final plot (in netCDF), a post script image and information on the dataset.

  • Datasets
    Various global reanalyses are provided. Note that different reanalyses dataset have different levels. We use all the levels up to 100mb.
  • Enter Dates
  • Enter a starting and ending date. If the 2nd date is greater than the first, you will get a backward trajectory calculation. All dates in the trajectory date range need to be in your selected dataset and they need to be proper dates. Hours are 0,6,12, and 18 (Z). Only enter numbers.
  • Starting point
    These values with be used as a starting point (or an ending point for backward trajectories). It is possible to enter a location that is below the surface. The reanalyses datasets used on this page are interpolated below the ground.
    Latitude range is -90 to 90.
    You can enter longitudes as degrees east (0 to 360) or degrees west which is entered as negative longitudes to -180 (e.g. -100 is 100W or 260E).
    Pressure level of initial point, in millibars. You can select multiple levels. If you choose a single level, the plot returned will show the level of the trajectory using colors unless you choose the "box" option where multiple trajectories are drawn from/to points near your entered point. In that case, the selected trajectory is black and those around are all blue. If you choose multiple levels, each level will have it's own color which is shown in the key..
  • Plot Options
    Markers drawn ever 6 hours
    Optionally, markers will be drawn every 6 hours to indicate where the trajectory is at each model time step.
    Select Region
    You can select from a set of regions to plot or you can choose a custom plot. For custom plots, choose S to N and use negative values for the first longitude if going over 0E.
    Select ProjectionYou can also select polar stereographic or cylindrical equidistant map projections. In some cases, choosing one over the other may not make sense or may make a 'bad' plot.
    Draw trajectories from box around selected point
    Optionally, you can choose to calculate trajectories for a box of 4 points around your selected start/end point.
    Select grid spacingSelect 05,1.0,1.5, and 2.0 degrees for the spacing between the selected point and those around it.
  • Output
    Google Earth
    If you have the Google Earth module installed on your browser, you can view the trajectories in the Google Earth browser API. The level of the trajectory is in meters and not millibars. Pressure was converted to meters using a standard atmosphere. If you don't have the plug-in, you can download the kmz file as per below. Note in Google Earth and the KMZ files, single level trajectories are drawn as one color. This may be changed in the future.
    You will get a plot with the trajectories drawn on it (as per the level instructions above)
    Postscript Plot
    You can download a higher resolution postscript plot.
    KMZ file
    The data is stored in a kmz file which can be downloaded and used in an Google Earth desktop application.The level of the trajectory is in meters and not millibars. Pressure was converted to meters using a standard atmosphere. Note in Google Earth and the KMZ files, single level trajectories are drawn as one color. This may be changed in the future.
  • How this page was created
    The main interface for this page is an HTML form. The data that are input into this form are processed by a Perl script. The script reads the inputs, tests for bad inputs, runs the trajectory code and then is plotted using NCL. NetCDF, KML and postscript are also produced. The netCDF file, image and other files are kept in a directory in which the files are periodically deleted.
  • Caveats
    This page uses code that calculates trajectories from 3D output from reanalysis models. While the code itself is well tested, there are a number of factors that will impact the trajectories calculated and may be considered sources of "error".
    1. Model Resolution (vertical and horizontal: Higher spatial resolution models will likely model climate and weather processes better. In addition, they will have better orography.
    2. Time output resolution of reanalyses. Most of the reanalysis models output every 6 hours. A higher temporal resolution will likely model some processes better.
    3. Reanalyses values are not "measured" and have their own sources of error. Reanalysis models have measured data as part of the inputs. The observations do not exist everywhere and even when they do, may not match each other. Reanalysis tries to account for this and similar issues but ultimately, there will be areas where reanalyses may not match the actual atmospheric values.
    4. Through time, small errors will grow non-linearly. Users should be very cautious when looking at trajectories that are more than a 2-3 days long.
    5. This model doesn't know about a surface. Trajectories can hit the surface and go through it in this model. The boundary layer in not explicitly modeled. The model uses variables output to pressure level data and not sigma level. Sigma layer explicitly accounts for the surface.
    6. Models that produce ensembles of values rather than single analysis are problematic for this tool in that a trajectory calculation really ought to be done using each individual ensemble member and not the ensemble mean. The average of the trajectories calculated from individual values is not the same as the trajectory calculated from the ensemble mean of the winds.
  • Feedback
    Please let me know if find the page useful. Likewise, let me know if you think any instructions are unclear, if you think there are bugs or if you have any suggestions for improvements of any type. I would particularly like to know if you use these pages for teaching purposes and if so, how. I can be reached by email at or by phone at (303) 497-6263.
WRIT is supported in part by NOAA/ESRL Physical Sciences Laboratory, the NOAA Climate Program Office, and the US Department of Energy's Office of Science (BER). WRIT contributes to the Atmospheric Circulation Reconstructions over the Earth (ACRE) Initiative.