Atmospheric conditions influencing the spillover of rainfall to lee of the Southern Alps, New Zealand

Abstract

Atmospheric conditions influencing precipitation distribution over a large lee-side catchment in the Southern Alps of New Zealand are investigated. A significant proportion of rainfall received in the upper Waimakariri catchment is generated by its spillover from the windward side of the alps during conditions of westerly flow. A transect of rain gauges was installed to examine the distribution of rainfall across the mountains during northwesterly events, and an index was devised to quantify the magnitude of each spillover event, allowing comparison with other events that occurred within the study period. The spillover index combines a measure of the total quantity of rainfall reaching the ground with its distance east of the main divide. Correlation and regression techniques were used to assess the role of various processes in determining the observed spillover distributions during 42 events. Several atmospheric parameters were considered, including measures of air-mass temperature, humidity, instability, windspeed and direction, and frontal intensity. It was found that spillover of rainfall into the upper Waimakariri catchment is largely determined by lower level wind speeds, latent instability and frontal intensity. Existing knowledge of atmospheric processes is used to explain how these factors influence spillover activity. © 1998 Royal Meteorological Society.