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When I get backward trajectories starting at a rural East Coast site in NJ, which starting height(s) do you suggest is (are) appropriate? The terrain of the site is flat and near sea level. I am also wondering how long of a trajectory duration is recommended.

The answers to your questions will depend on the application your are performing. If you are looking for where air near the surface came from you would start the trajectory at 10 meters. If you are looking for where air contributing to acid rain came from, then you would choose a height near or above the cloud base. Generally, we tend to choose 2 or 3 heights near the height we are interested in to get a better idea of the wind shear, if any, aloft.

As to the length of the trajectory, the shorter the time, the better the accuracy. However, you can feel fairly confident in the trajectory position out to 48 hours. Longer term trajectories are best used in a statistical sense such as "trajectory clustering" to determine transport regimes. We did a sensitivity study on trajectories by varying the spatial and temporal resolution of the data and found using the NGM (which was available at the time; the Eta/EDAS is better now) that the horizontal deviations are less than 200 km at 24 hours and between 200 and 700km at 96 hours. (Journal of Applied Meteorology, October 1990, Rolph and Draxler, pp1043-1054).

Glenn Rolph
NOAA ARL