Compound intensity/duration/frequency-relationships of extreme precipitation for two seasons and two storm types

Intensity/duration/frequency (IDF)-relationships of extreme precipitation are widely used for design of stormwater facilities. As the properties of extreme precipitation may be very different for different storm types and different seasons, IDF-relationships which permit decomposition into different components are established. The different components refer to different storm types or to different seasons, depending on the type of decomposition. The present study is based on the long raingauge record of 10-min precipitation depths at Uccle/Ukkel (Belgium) for the period 1967–1993. Storms of two different types (type a: airmass thunderstorms, and type b: cyclonic/frontal storms) are separated based on their distribution of peak-over-threshold intensity. This is done for each duration (aggregation-level) in the range 10 min–15 days, using a two-component exponential distribution. The exponential decay parameters and the mixing parameter for the two components are estimated as functions of the aggregation-level. Those functions show scaling properties for the parameters. Different mixtures of storms of types a and b are also estimated for summer and winter conditions. It brings new elements in our current understanding of what determines the IDF curves and their scaling properties.