Making Trajectories with data composites

[Bernat Codina]

2009/11/17 Jason Snyder <jmssnyder at ucdavis.edu>:

> I have a *.nc data set that is a composite of wind data based on certain
> phases of the Madden Julian Oscillation and I was interesting in looking at
> the backwards trajectories of air parcels coming from Lake Tahoe in such a
> scenario. Â I have been trying to work with the script traj.gs, but GrADS
> said that TDEF is undefined. Â How do I fix this and make a trajectory?

Data needs to be arranged in time steps, otherwise there is no way to
calculate trajectories. Therefore, a TDEF field has to be defined in
your ctl file. Once you have fixed this, set the variable time to the
instant at which you want to compute the backwards trajectory (e.g.
SET T 27), run "traj.gs lon_ini lat_ini" and you'll get the path of
the parcel that was at point (lon_ini, lat_ini) at time t=27, defined
as a set of points from t=27 to t=1.

Bernat Codina
Dept. d'Astronomia i Meteorologia
Universitat de Barcelona