The characterisation of orographic rainfall

The Otaki Precipitation Estimation by Radar (OPERA) programme was designed to investigate the processes that lead to enhancement of rainfall over the Tararua ranges of New Zealand. These ranges rise to 1500 m above the coasta l plain and enhancement of rainfall by windflow over these hills leads to a nnual hill-top rainfall of over four times that upwind. The OPERA experimen tal campaigns aimed to characterise the enhancement processes by analysing data collected from a transect of high-resolution min gauges and a locally deployed high-resolution radar, supported by scanning radar and satellite o bservations. Measurements made during these experiments showed that orograp hic enhancement led to hill-top accumulations often twice that upwind, and up to as much as a factor of seven in one case. The data suggest that the m ost frequent occurring enhancement mechanism was triggered convection. This mechanism leads to an increase in rainfall over the hills of around a fact or of two, primarily through an increase in the duration of rain. Seeder/fe eder-type enhancement occurs less frequently but leads to larger enhancemen ts.