Analysis of Time Groups Governing the Temporal-Spatial Changes of Iran's Annual Rainfall

ABSTRACT
Precipitation is one of the elements and complex climatic processes in time and space and is particularly important due to its vital role. In the present study, the analysis of the 60-year rainfall time groups of Iran was done using trend analysis and coefficients. In this regard, ECMWF-ERA5 data with a spatial resolution of 0.25*0.25 was used from 1963 to 2022. First, the country's 60-year rainfall average and change coefficient were investigated, and then the rainfall trend was checked by fitting a linear regression model using a parametric method. Finally, by using spectral analysis, coefficients related to annual precipitation were extracted. According to the research results, the scope of the significant negative trend of precipitation during the statistical period was wider than that of the significant positive trend. About 69 percent of the country has a negative annual trend with a decrease of -0.6 mm of precipitation per year, and 30 percent of the country has positive precipitation and an increase of 0.2 mm of precipitation per year. The negative trend of precipitation generally corresponds to the northwest, north and east areas of the study area, and the significant positive trend corresponds to the high Zagros. The output of the spectrum analysis, which was not evident in other studies, shows the weak role of significant isomers in the northern coastal areas of the country. Also, the southern half of the country is mainly influenced by 2-7 year cycles and the western regions are generally dominated by long-term 7-20 year cycles. Therefore, the patterns related to these cycles can be attributed to local factors, macro-scale atmospheric systems, neighbors and sometimes the combination of all factors that caused the diversity of cycles in one place
Extended Abstract
Introduction
Today, several methods are used to investigate temporal and spatial changes in precipitation and obtain its obvious and hidden relationships. This research reveals the obvious and hidden relationships by analysing the trend of obvious changes and the analysis of harmonics to investigate the hidden and obvious relationships between Iran's rainfall over 60 years. In this regard, ECMWF-ERA5 data with a spatial resolution of 0.25*0.25 was used from 1963 to 2022. Then, by fitting the linear regression model with the parametric method, the precipitation trend was checked. Finally, using spectral analysis, coefficients related to annual precipitation were extracted. The investigation of the rainfall trend showed that the country's rainfall during the studied period has been decreasing in most areas, and about 11% of the decreasing changes have statistical justification. Only 0.2 areas of the country have had a significant positive trend in the areas corresponding to the high Zagros. The results of the spectral analysis of co-factors show the weak role of significant co-factors in the northern coastal strip and the country's central regions. However, the south, southeast, east, and southwest of the study area are mainly under the influence of 2-7 year cycles. Precipitation patterns related to these co-factors can be attributed to the influence of local factors and processes, nearby water zones, and atmospheric circulation elements and are generally dominated by the connection patterns from Doro, especially Anso.
 
Methodology
In the current research, using ECMWF precipitation network data version ERA5 with a spatial resolution of 0.25*0.25 degrees of arc and a daily time resolution, it was tried to determine the trend of annual changes and precipitation cycles during the statistical period of 1963-2022, as a manifestation and index of climate changes and also as one of the essential research fields about Iran's rainfall should be exposed to attention. Thus, by applying the methods of analysis of trends and cycles, precipitation events in Iran were studied. In order to analyze the findings, firstly, the average and coefficient of annual changes in precipitation were analyzed, and then the role of spatial factors and their impact on precipitation were analyzed using correlation relationships. In the next step, by fitting the linear regression model to the parametric method, the annual precipitation trend was investigated. Finally, the characteristics of Iran's precipitation were investigated using analogs.
 
Results and Discussion
In the analysis of the average and coefficient of changes of precipitation during the statistical period, the highest average annual precipitation is related to the edge of the Caspian Sea, the heights of Zagros and parts of the northwest of the country, and the highest amount of the coefficient of variation corresponds to the south and southeast regions of the country. Also, the lowest coefficient of variation is related to the northern and northwestern regions of the country. The average maximum rainfall in the northern coastal strip and the parts related to the northwest of the country is related to the west and southwest of the country, which corresponds to the areas with the highest atmospheric precipitation; the investigation of the behavior of precipitation in the long term (trend) indicated that about 69% of the country has a negative annual trend and is associated with a decrease of -0.6 mm of precipitation per year, and approximately 30% of the country's area experiences a 0.2 mm per year increase in positive rainfall. The results of the analysis of cycles, which were not evident in other studies, indicated the existence of 30 coefficients for the 60 years of the studied statistical period, and each pixel of the precipitation map was identified and displayed in separate maps.
The first covariant was analyzed in comparison with the annual precipitation trend because the return period is equal to the length of the statistical period and corresponds to the trend. Based on the other results obtained, the homogenizers were divided into five different groups. The related maps were grouped and analyzed based on cycles, the impact of atmospheric systems, topographical conditions, the impact and role of water zones and geographical location. The second group of consonants decreases from the south to the north of the country and from the west to the east of the areas associated with these consonants. This homogenizer indicates the existence of ten cycles of about 2 to 30 years.  This group likely has a long-term cycle (30 years) that includes short-term cycles (2-9). According to the geographical location of these fluctuations, they may be affected by the country's coastal strips. Therefore, these rainfall cycles can be attributed to these regions' heavy and heavy rains.
The spatial distribution of the significant homogenizers of the third group includes the regions corresponding to the eastern half of the country, which decreases from the northeast to the southwest. With this description, sometimes heterogeneous nuclei disrupt this uniformity. Based on the 12-year cycle, the significance of precipitation fluctuations in the covered areas can be attributed to sunspots. According to the 12-year cycle, the significance of precipitation fluctuations in the covered areas can be attributed to sunspots. According to Lashkari (2000), the cycles belonging to homosaz 16, 20 and 21 can be attributed to the expansion of the high-pressure center in Siberia and its tongues and the location of the pressure centers in Sudan, which leads to the activation of low-pressure systems spread over Iran.
The spatial distribution of precipitation fluctuations for the fourth group coefficients indicates the presence of the largest range in the western and southwestern regions. Although sometimes parts of the country's east, northeast, and southeast are affected by the significant fluctuations of these coefficients in a scattered manner, generally, the role of these fluctuations in these regions is more than that of other regions.
 
Conclusion
Finally, the maps related to different groups show the weak role of the significant homogenizers in the northern coastal strip and the country's central regions. However, the study area's south, southeast, east, and southwest areas are mainly affected by 2-7 year cycles. Precipitation patterns related to these co-factors can be attributed to the influence of local factors and processes, nearby water zones, and atmospheric circulation elements (simultaneous systems) and are generally dominated by the connection patterns from Doro, especially Anso (according to the return period of significant co-factors). Also, the western areas are dominated by long-term cycles of 7-20 years, so according to the existing cycles, the changes in these areas can be attributed to the impact of precipitation events and solar flares.
 
Funding
 There is no funding support.
 
Authors’ Contribution
 All of the authors approved thecontent of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
Here, the authors would like to express their gratitude to the developers of the MATLAB program as well as the European Center for Medium-Range Weather Forecasts (ECMWF) version (ERA5).