Weather regimes and analogues downscaling of seasonal precipitation for the 21st century; A case study over Israel

Careful planning of the use of water resources is critical in the semi-arid eastern Mediterranean region. The relevant areas are characterized by complex terrain and coastlines, and exhibit large spatial variability in seasonal precipitation. Global climate models provide only partial information on local‐scale phenomenon, such as precipitation, primarily due to their coarse resolution. In this study, statistical downscaling algorithms, based on both synoptic scale past weather regimes and  analogues and their associated observed precipitation at rain gauges, are operated for eighteen Israeli rain gauges in four hydrological basins with an altitude ranging between ‐200 and ~1000 m ASL. In order to project seasonal precipitation over Israel and its hydrologic basins, the algorithms are applied to six Coupled Model Inter‐comparison Project Phase 5 (CMIP5) models for the end of the 21st century, according to the RCP4.5 and RCP8.5 scenarios. The downscaled models are able to capture quite well the seasonal precipitation distribution. All models display a significant reduction of seasonal precipitation for the 21st century of up to ~50% with variations depending on the scenario, algorithm and hydrological basin. The reduction is less acute when applying the weather regimes algorithm as it relies on past  daily mean precipitation values per regime, while the analogues downscaling algorithm relies on the daily precipitation of the individual past analogues and therefore better captures the tails of the distribution. Moreover, the analogues downscaling algorithm projects a significant increase of outliers in the right tale of the distribution i.e. increase in extreme precipitation events. The reduction in seasonal precipitaton is due to both decrease in the frequency of the synoptic systems responsible for precipitation as well as reduction in the daily precipitation amounts at the stations. While the percentage of reduction is quite similar among stations (same reduction in the precipitating synoptic systems that affect the whole area), the reduced amounts are different as they are characterized by different seasonal precipitation amounts. In some cases reductions in precipitation can lead to transition of some areas to semi-arid and arid climates. The statistical downscaling methods applied in this study can be easily transferred to other regions where long‐term datasets of observed precipitation are available.