The zero degree of freedom non-central chi squared distribution for ensemble postprocessing

Abstract: In this note the use of the zero degree non-central chi squared distribution as predictive distribution for ensemble postprocessing is investigated. It has a point mass at zero by definition, and is thus particularly suited for postprocessing weather variables naturally exhibiting large numbers of zeros, such as precipitation, solar radiation or lightnings. Due to the properties of the distribution no additional truncation or censoring is required to obtain a positive probability at zero. The presented study investigates its performance compared to that of the censored generalized extreme value distribution and the censored and shifted gamma distribution for postprocessing 24h accumulated precipitation using an EMOS (ensemble model output statistics) approach with a rolling training period. The obtained results support the conclusion that it serves well as a predictive distribution in postprocessing precipitation and thus may also be considered in future analyses of other weather variables having substantial zero observations.