HIGH-RESOLUTION STUDIES OF RAINFALL ON NORFOLK ISLAND PART I - 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 on e hour to one year using a raingauge network of density 0.44 km(-2) sa mpling 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 p eak difference in daily accumulations of 80 mm. Events lasting one hou r or less which generated no rain at one site provided an average maxi mum of 8 mm among the remaining sites (peak 28 mm). In the summers, in tegration over 16 days was needed before rain fell everywhere. Variabi lity was assessed from distributions of numbers of sites with no rain when the island mean rainfall was non-zero. Data at individual sites g reatly underestimated the frequency of island rainfall and varied in t heir ability to represent island-mean accumulations and correlate with island-mean time series. The distribution of rainfall showed some dep endence on wind direction. Windward (leeward) sites tended to show def icits (excesses) of rain. The 15 s peak intensity was 511 mm h(-1) and many coherent, localised high-intensity events of extremely short dur ation were identified. Low accumulations at sites often correlated wit h a shorter duration of rainfall, and intensities during rain were sim ilar to those at sites with high accumulations. (C) 1998 Elsevier Scie nce B.V. All rights reserved.