Gunther Haase, SMHI: 
Orographic enhancement:
One mechanism leading to an increase in rainfall over mountainous terrain (orographic enhancement) is the forced ascent of moist air, initiating condensation and release of potential instability (Hill 1983). Another is the interaction between an existing upper cloud layer and the cloud formed during ascent, the seeder-feeder mechanism proposed by Bergeron (1965). Due to the condensation process, drop size distribution changes significantly and thus reflectivity factor Z, rainfall rate R and the Z-R-relation itself. Since the enhancement often occurs at low levels, radar rainfall values from aloft underestimate the surface rainfall totals. 
Orographic enhancement is related to certain weather situations as the warm sector of extra-tropical cyclones (Kitchen et al. 1994; Neimann et al. 2001; White et al. 2001). Therefore, correction algorithms have to consider the specific meteorological fields (e.g., wind, moisture) as well as the orography itself. Kitchen et al. (1994) proposed a real-time correction of weather radar data for orographic growth in widespread precipitation. 
References:
Bergeron, T., 1965: On the low-level redistribution of atmospheric water caused by orography. Int. Conf. on Cloud Physics, Tokyo, 96-100. 
Hill, F. F., 1983: The use of average annual rainfall to derive estimates of orographic enhancement of frontal rain over England and Wales for different wind directions. J. Climat., 3, 113-129. 
Kitchen, M., R. Brown, and A. G. Davies, 1994: Real-time correction of weather radar data for the effects of bright band, range and orographic growth in widespread precipitation. Q. J. R. Meteorol. Soc., 120, 1231-1254. 
Neimann, P. J., F. M. Ralph, A. B. White, D. E. Kingsmill, and P. O. G. Persson, 2001: Using radar wind profilers to document orographiv precipitation enhancement during the CALJET field experiment. 30th Conf. on Radar Meteorology, AMS, 509-511. 
White, A. B., J. R. Jordan, F. M. Ralph, P. J. Neimann, D. J. Gottas, D. E. Kingsmill, and P. O. G. Persson, 2001: S-band radar observations of coastal orographic rain. 30th Conf. on Radar Meteorology, AMS, 512-514.