Measure of rainfall time structure using the dimensionless n-index

Abstract

The n-index is defined as the exponent of the power law relating the maximum average intensity over given periods of time (e.g. minutes, hours or days) and these periods of averaging. The n-index (between 0 and 1) reports on temporal variability of rainfall behaviour within an event. This study analyses the n-index around the world and its uncertainty according to several criteria of calculation and daily/sub-daily data. The results show that precipitation generally has a nested time structure, i.e. each maximum accumulation of shorter duration is included within a longer one, following a fractal behaviour. The obtained n-index is generally independent of the intensity and the event duration, but is sensitive to the temporal resolution of data. The distribution of the n-index around the world shows that areas with lower n are the coldest regions and zones with marked orography. The highest n-index values are found near the warmest seas.