Hi Song,
You should be able to achieve this using a Body forcing.
To do this, add the following Forcing section to your xml file
<FORCING>
<FORCE TYPE="Body">
<BODYFORCE> BodyForce </BODYFORCE>
<FIELDFORCETIMEFCN> s1 </FIELDFORCETIMEFCN>
</FORCE>
</FORCING>
and define the BodyForce function
<FUNCTION NAME="BodyForce">
<E VAR="rho" VALUE="s0_rho" />
<E VAR="rhou" VALUE="s0_rhou" />
<E VAR="E" VALUE="s0_E" />
</FUNCTION>
where you should replace s1 and s0 by the expressions you need for your model.
I think you can obtain a discontinuous s1 by using logical expressions (e.g. s1 = (t>1)*2 + 1).
However, since the symbols < and > can cause problems in the xml format, they must
be replaced by < and >
Cheers,
Douglas