Hi Claudia,
I am afraid that I am not aware of an easy way to do this in PRECIS itself.
The best option that I can think of is to install Iris
http://scitools.org.uk/iris/docs/latest/whatsnew/1.9.htmlWhich includes this function:
http://scitools.org.uk/iris/docs/latest/iris/iris/analysis/cartography.html?highlight=rotate#iris.analysis.cartography.rotate_winds.
In fact you may find that a lot of the analysis you plan to do with grads, can also be done with Iris.
Note that currently Iris can not read the little endian PP files that PRECIS produces. Before reeding in to Iris you will need to type
Code:
bigend -32 [PRECIS_file.pp] [Iris_readable.pp]
There are plans to change this.
All the best,
Simon