How do mountains enhance rainfall? This paper investigates the processes behind orographic rainfall enhancement, focusing on the Tararua ranges of New Zealand. The OPERA (Otaki Precipitation Estimation by Radar) programme was designed to investigate the processes that lead to enhancement of rainfall over the Tararua ranges of New Zealand. The OPERA experimental campaigns used high-resolution rain gauges, radar, and satellite observations to characterize these processes. Measurements revealed that orographic enhancement often doubled rainfall accumulations on hilltops compared to upwind areas. In extreme cases, enhancements reached a factor of seven. The data suggest that the most frequent occurring enhancement mechanism was triggered convection. This mechanism leads to an increase in rainfall over the hills of around a factor of two, primarily through an increase in the duration of rain. Analysis of the data suggests that triggered convection is the most common mechanism, increasing rainfall duration. Seeder/feeder-type enhancement, though less frequent, results in even greater rainfall increases. This research provides valuable insights into the complex dynamics of orographic precipitation. The data suggest that the most frequent occurring enhancement mechanism was triggered convection.
Published in Meteorological Applications, this paper addresses a key area of interest for the journal: understanding and characterizing meteorological phenomena. By providing a detailed analysis of orographic rainfall, the study contributes to the knowledge of precipitation processes and their regional variations. The paper has practical implications for water resource management and weather forecasting in mountainous regions.