High-resolution studies of rainfall on Norfolk Island: Part 1: The spatial variability of rainfall

The spatial variability of rainfall on Norfolk Island, which measures 8 km by 5 km, is examined for the years 1991–93 at time scales from one hour to one year using a raingauge network of density 0.44 km −2 sampling over 15 s intervals. Sites showed max:min ratios of up to 1.2:1 and 2.5:1 for annual and monthly accumulations, respectively, and a peak difference in daily accumulations of 80 mm. Events lasting one hour or less which generated no rain at one site provided an average maximum of 8 mm among the remaining sites (peak 28 mm). In the summers, integration over 16 days was needed before rain fell everywhere. Variability was assessed from distributions of numbers of sites with no rain when the island mean rainfall was non-zero. Data at individual sites greatly underestimated the frequency of island rainfall and varied in their ability to represent island-mean accumulations and correlate with island-mean time series. The distribution of rainfall showed some dependence on wind direction. Windward (leeward) sites tended to show deficits (excesses) of rain. The 15 s peak intensity was 511 mm h −1 and many coherent, localised high-intensity events of extremely short duration were identified. Low accumulations at sites often correlated with a shorter duration of rainfall, and intensities during rain were similar to those at sites with high accumulations.