The Otaki Precipitation Estimation by Radar (OPERA) programme was designedto investigate the processes that lead to enhancement of rainfall over theTararua ranges of New Zealand. These ranges rise to 1500 m above the coastalplain and enhancement of rainfall by windflow over these hills leads toannual hill-top rainfall of over four times that upwind. The OPERAexperimental campaigns aimed to characterise the enhancement processes byanalysing data collected from a transect of high-resolution rain gaugesand a locally deployed, high-resolution radar, supported by scanning radarand satellite observations. Measurements made during these experimentsshowed that orographic enhancement led to hill-top accumulations often twicethat upwind, and up to as much as a factor of seven in one case. The datasuggest that the most frequent occurring enhancement mechanism was triggeredconvection. This mechanism leads to an increase in rainfall over the hillsof around a factor of two, primarily through an increase inthe duration of rain. Seeder/feeder-type enhancement occurs less frequentlybut leads to larger enhancements.