ON THE DERIVATION OF THE AREAL REDUCTION FACTOR OF STORMS

A stochastic derivation of the areal reduction factor (ARF) of the sto rm intensity is presented: it is based on the analysis of the crossing properties of the rainfall process aggregated in space and time, As a working hypothesis, the number of crossings of high rainfall intensit y levels is assumed to be Poisson-distributed and a hyperbolic tail of the probability of exceedances of rainfall intensity has been adopted . These hypotheses are supported by the analysis of radar maps during an intense storm event which occurred in Northern Italy. The reduction factor derived from this analysis shows a power-law decay with respec t to the area of integration and the duration of the storm. The areal reduction results as a function of the storm duration and of its frequ ency. A weak, but significant decrease of the areal reduction factor w ith respect to the return period is shown by the functions derived, an d this result is consistent with that of some recent studies on this t opic. The results derived, although preliminary, may find useful appli cations for the definition of the design storm in urban catchments of a size greater than some square kilometres and with duration of some h ours.