Model of rainfall-rate distribution for radio system design

Owing to the interest raised by the precipitation effects in the design of satellite and terrestrial microwave radiolinks, it is necessary to have a simple and valuable mathematical model which can represent, with good accuracy, the whole rain-rate distribution, from lowest rain-rate values to the tail of this distribution. Previously the author suggested a two-parameter model which adequately represents the rainfall intensity cumulative distributions for several hydrometeorological zones of the world. This model has log-normal asymptotic behaviour for low rainfall rates, and gamma asymptotic behaviour for the high ones. The present paper deals with a further improvement by expressing both parameters as a function of the rainfall rate exceeded during 0.01% of the time, and recommended by the CCIR to evaluate the availability of microwave radio links.