There's an interesting piece about this at <A HREF="http://www.wco.com/~shp/waveforatc.html" TARGET="_blank">http://www.wco.com/~shp/waveforatc.html</A> which goes into details of how ATC can spot that an a/c is in downdraft/rotor and what can be done to help.
Quite a frightening read.
R