Stochastic Rain Generator Based on Disaggregation of Precipitation into Convective and Stratiform for Climate Change Studies

The outputs from regional climate models (RCMs) have to be further downscaled. This is usually done via a bias correction method.  This study presents a novel approach to statistical downscaling of outputs from RCMs. The novelty lies primarily in distinguishing the convective and stratiform precipitation by rain generator operating in 6-hour time step. For this purpose, the technique based on determination of threshold rainfall intensity is used, built on the observation that the convective precipitation amounts follow exponential distribution.

The resulting rain generator operates in the following steps: disaggregation of 6-hour cumulative precipitation into convective and stratiform types, fitting of the first order 3-state discrete time Markov chain to the data, and simulation of long time series of precipitation. Then the mixture of log-normal and Generalized Pareto distribution is fitted to stratiform events and the Generalized extreme value distribution to convective events.

The impact of climate change on precipitation is represented by change factors that are identified for precipitation occurrence (by comparing the transition matrices for the future and control period) and for precipitation amounts (by comparing the scale and location parameters of distributions fitted for the future and control period). The observational data are then altered with the obtained change factors.

From evaluation of observational data it stems that the average volume of a convective event is higher for the western region than the eastern region of the Czech Republic. Additionally, statistically significant trends in the number and volume of convective events were identified for the western region. The analysis of the RCM simulations shows that even though the overall precipitation is projected to be lower in future, the proportion of convective events (versus stratiform ones) would be higher. In a future climate, the number of convective events is projected to be lower while the mean volume of a convective event to be larger.