Barberpoll,
FYI there is another form of radar on smaller aircraft which can provide similar WS alerting over & above the reactive from GPWS. This is Turbulence Detection - it is a normal WXR with a very stable transmitter and a more accurate frequency gate provided by an airspeed input into the radar. This gives a theoretical center frequency - turbulence will be shown up by doppler-shifted signals both above or below the center frequency as the precip or dust moves towards or away from the aircraft.
As far as I can remember this does not have a height limit - the crew can just select TURB mode at any altitude.