Projection of Iran’s precipitation in 21st Century using downscaling of selected CMIP6 Models by CMHyd

Geographical location of Iran in arid and semi-arid regions has strongly affected its food security, water resources and weather- and climate-related extreme events due to climate change. Global warming, on average, increases precipitation on Earth by increasing evaporation from the oceanic surface that enters the atmosphere, but the response of the West Asian region to global warming is generally a decrease in precipitation. Some studies have confirmed a decrease in the average precipitatiomn of Iran with an increase of precipitation in the south and southeast of the country. They also confirmed that the largest decrease in the precipitation of Iran occurs in the Zagros region.
To this end, in this study, an overview of possible changes in precipitation trends in 43 stations of Iran is presented. To achieve the goals of the paper, the data of four Earth System Models from CMIP6 models, including MIROC6, FGOALS_g3, BCC-CSM2-MR and ACCESS-ESM1-5 were used. The precipitation output of the selected models has been statistically downscaled using CMHyd software for 43 synoptic stations over Iran. Observational period of 1985-2014 and the next three 25-year periods as the near future 2020-2026, the mid future 2075-2051 and the far future 2100-2076 were considered as study periods. Future changes in precipitation under three Shared Socio-economic Pathways (SSP) scenarios of SSP1-2.6, SSP2-4.5 and SSP5-8.5 were estimated. During the downscaling process, the Classius-Clapiron (CC) rate was applied to increase the trend of heavy rainfall. The air holding capacity is controlled by the Clausius-Clapiron relation, which is the water vapour holding capacity of the air at 7% /oC, the so-called Clausius–Clapeyron (CC) rate (bellow equation).
Since the maximum temperature increase in the country was considered 10 degrees Celsius based on the worst possible scenario (SSP5-8.5), so all severe precipitation events were projected that were above the threshold of 130% compared to the normal period that were reduced to 130% of its normal amount.
The results showed that future rainfall changes were not significant in about 78% of the stations. The increase and decrease of rainfall were significant in 19% and 3% of the stations, respectively. The largest increase in precipitation will occur in the south-southeast and the largest decrease will occur in the central Zagros area. The average rainfall of the country will increase by 0.4% annually (with a uncertainty range of 14%). On a seasonal scale, precipitation changes in spring, summer, autumn and winter were estimated to be + 15.2, -11, -6 and +3.5, respectively. Although a 15.2% increase in precipitation is projected for spring, the range of uncertainty with 81.9% indicates a lack of confidence in future precipitation estimates of spring season. The maximum uncertainty is related to the spring rains, which shows that the rains of this season are becoming more and more distrustful. Under warming conditions, more spring rainfall increases will occur. In summer, in the near future, summer rains tend to increase and then decrease with more level of Global Warming. After spring, most uncertainty is related to summer precipitation. The autumn precipitation tends to be less than normal with lower amount of uncertainty, although its decrease is not significant. The small range of the autumn rainfall uncertainty chart indicates the agreement of different model-scenarios in projection of autumn rainfall. Winter rainfall projection does not have significant uncertainty and almost all model-scenarios agree on the relatively low trend of rainfall increase. In this season, under higher amount of Global Warming, the precipitation increases and the amplitude of uncertainty also increases. Among the next three periods, the lowest and highest range of uncertainty in the annual changes of precipitation in the first and last decade of the present century will occur with the range of changes of 14 and 29.1%, respectively. The lowest and highest amplitude of uncertainty on the seasonal scale with 3.6 and 81.9 are estimated in winter (near future) and spring (far future), respectively. This situation indicates that in the future most precipitation fluctuations will occur in the spring.