Introduction

RAINFALL CELL SIZE FROM RIT CURVE ANALYSIS

RAINFALL CELL SIZE FROM RIT CURVE ANALYSIS

Technical Report No. 136
RAINFALL CELL SIZE FROM RIT CURVE ANALYSIS

Charles M. Fullerton, S.K. Wilson, Arne Austring

June 1980

ABSTRACT
A small and closely-spaced network of three to seven rapid response (4-s) rainfall intensity gages has been used to measure intense, short duration rain showers on the windward (eastern) coast of Hawai’i island. During a one year, intensive monitoring period, 54 events were recorded with rainfall intensities greater than 95 mm/hr on at least one gage. Four of these events had peak rainfall rates greater than 250 mm/hr. The 54 events were analyzed in an attempt to derive some properties characteristic of individual high intensity rainfall-producing “cells”. A total of 75 cells, involving 397 separate gage measurements, here extracted from the 54 events by applying an 11-point running mean, averaging technique. The cell data were studied by sorting the rainfall intensity-time plots into curves of symmetrical shape (35.5%), and those exhibiting a positive (43%) or a negative (21.5%) skew. Normalized mean curves then were constructed for each of these three types. Ten cases appeared to involve multiple, overlapping cells, including two cases of long duration thunderstorm rainfall. These ten events were excluded from the results listed below. The average of the 65 single cell cases produced about 5 mm of rain in about 5.5 min, with an average maximum rainfall rate of about 110 mm/hr. The ground rainfall pattern moved across the network with an average speed of about 6.5 m/s. If the cell is assumed to be circular across a plane parallel with the ground, these results imply a mean cell diameter of about 2 km and an average cross-sectional cell area of about 3.2 kM2.