A simple rule-of-thumb approach to estimate probability and return-values of future heavy 24-hr rainfall.

Abstract:

We make use of an equation for the probability of heavy 24 h precipitation amounts Pr(X > x) in the downscaling of future rainfall scenarios, based on a function of the wet-day frequency and the wet-day mean precipitation. The expression has been evaluated against 9817 daily rain gauge records world-wide and is the basis for mathematical expressions for different rainfall statistics in terms of the wet-day frequency and the wet-day mean precipitation. We show how versatile the framework is in terms of how expressions for probabilities, mean, variance, and return-values are easily derived. In the past, the results indicate that there has been a general increase in the probability of precipitation exceeding 50 mm/day, and that the main cause for this increase has been a boost in the intensity of the rain, but there have also been some cases where it has been due to more rainy days. This analysis is extended to the downscaling of large multi-model ensembles through state-of-the-art empirical-statistical downscaling to provide local projections for a future climate. The approach based on this rain equation provides crude and approximate "rule-of-thumb" results that are generally robust and can be employed even for time series of limited length.