When transmitted radio energy is circular polarised, any photon transmitted that is reflected off a water droplet is always phase shifted 180 degrees, therefore with a quiet simple bit of signal processing that reject signals 180 degrees out of phase with transmitted signal results in most of the rain clutter being rejected. It does however result in a loss of overall range performance. Hence the use of normal polarisation when rain clutter is not a problem. Modern PAR like RPAR still has a rain mode which results a reduction of range, so I would be surprised if it doesn't have a switchable polariser of some description within the antenna system. (All the software does is replace a shed load of hard-wired circuits within the old style signal processing system which processes the received signal after the analog receiver system has amplified it and done things like phase detection.)