Analysis of Distribution of Temporal-Spatial Trend of Horizontal Visibility on the Southern Coasts of the Caspian Sea using Ridit Analysis

Analysis of Distribution of Temporal-Spatial Trend of Horizontal Visibility on the Southern Coasts of the Caspian Sea using Ridit Analysis
 ABSTRACT
The present study aims to investigate the temporal-spatial trend of horizontal visibility in the southern coasts of the Caspian Sea. For this purpose, horizontal visibility and climatic parameters (relative humidity, temperature, precipitation, and wind speed) were obtained in 13 selected meteorological stations from 1951-2010. After the data screening, the temporal changes of the perspective were examined. Also, Ridit tests were applied to examine the trend of horizontal visibility, and a correlation coefficient was used to identify the relationship between the climatic parameters effective in reducing horizontal visibility. The results of the research showed that the average annual dispersion of horizontal visibility in the studied stations experienced relatively different periods; the first period from 1951 to 1976 period of horizontal increase, the second period from 1977 to 1985 period of noticeable decline, the third period from 1986 to 2002 fixed period (no fundamental change) and the fourth period from 2003 to 2020 relative increase. Among the stations, the highest negative correlation coefficient was experienced by Astara station, with a value of -0.47, and the lowest was experienced by Sari station, with a value of -0.07. In the correlation between horizontal visibility and precipitation in the studied stations, the maximum significant negative correlation was observed in the Manjil station (with a value of -0.15) and the least in the Maratape station (with a value of -0.012). The correlation between horizontal visibility and temperature is direct and positive in most stations, and the maximum positive correlation is assigned to the Astara station with a value of 0.38. It also showed a negative correlation between wind speed and horizontal visibility. The inverse correlation's maximum and minimum range fluctuates between -0.28 in Sari and -0.08 in Babolsar
Extended abstract
Introduction
Horizontal visibility is one of the indicators of air quality. Atmospheric visibility refers to the maximum horizontal distance at which a person with naked eyes can detect objects on the ground. The range of visibility in a clear atmosphere is between 145 and 225 kilometers; in a normal atmosphere, it is between 10 to 100 kilometers, and in polluted areas, it is less. Horizontal visibility depends on the level and concentration of air pollutants, and its relationship with temperature and weather conditions is of particular interest in the studied region. This research aims to obtain the necessary information and specialized and public use of meteorological and climatological station data in northern Iran concerning natural disaster planning and prevention of resulting hazards in the studied area.
 
Methodology
This research was conducted on the southern coasts of the Caspian Sea, which includes three provinces. All three provinces are located downstream from the Caspian Sea and are highly humid. First, filtering was done so that the data related to the hours of the day 06, 09, 12, and 15 Greenwich synopses were used, and also, the synopses with missing horizontal visibility and precipitation phenomena and relative humidity above 90% were screened. Then, to analyze the data, the annual average changes in horizontal visibility were checked by calculating the regional average and drawing a graph. Ridit analysis was used to analyze the behavior of horizontal visibility during the study period in selected stations. Finally, using the correlation coefficient, the relationship between the climatic parameters of the horizontal wind in the region was examined. ArcGIS software prepared horizontal visibility spatial distribution maps with climatic parameters.
 
Results and discussion
From 1951 to 1976, the first period had a mostly higher average horizontal visibility, ranging from 17 to 5.21 kilometers. The second period, covering the years 1977 to 1985, experienced a significant decrease in horizontal visibility, with its average decreasing to 10 kilometers. The third period, from 1986 to 2002, was stable, with only minor fluctuations in the average horizontal visibility (1 kilometer). The fourth period, from 2003 to 2020, was a period of relative improvement in the average horizontal visibility, with a relatively high variability. Among the studied stations, Babolsar and Ramsar stations experienced an increasing trend compared to the regional average. The average horizontal visibility trend analysis showed a decrease from early winter to early spring, with its value reaching 8.11 kilometers in March. From mid-spring to late summer, horizontal visibility increased, with the highest average in June at 8.12 kilometers. In the autumn, a relative increase in horizontal visibility was observed, with the highest average reaching 13 kilometers in March. Almost all studied stations in the region followed a similar pattern, except for Babolsar and Qara Khial stations. In the spring season, the eastern part of the region has higher horizontal visibility compared to the western part. The maximum horizontal visibility is observed in the coastal strip of the Caspian Sea in the summer. This season, the southernmost part of the region, which is close to the Alborz Mountains, has lower horizontal visibility. As we approach the Alborz Mountains in the autumn, horizontal visibility decreases due to increased precipitation and fog. Horizontal visibility increases in the western and eastern parts of the region, respectively. In the winter season, the lowest horizontal visibility is concentrated in the southernmost part of the region. The results of the RIDIT analysis for selected stations showed that the Babolsar station had the best horizontal visibility conditions at the beginning of the study period and the worst conditions at the end of the period, showing a decreasing trend in horizontal visibility overall. The horizontal visibility at Ramsar station showed a decreasing trend from the beginning to the end of the study period, with RIDIT values ranging from 0.6 to 4.0. The Astara station had a decreasing trend in horizontal visibility, with RIDIT values ranging from 5.0 to 4.0, which is very close to the reference distribution conditions. The Nowshahr station showed an increasing trend in horizontal visibility, with a relatively small difference in RIDIT values ranging from 4.0 to 6.0, close to the reference distribution line. The Gonbad Kavus station showed no trend in horizontal visibility, with very little difference in RIDIT values. The Maraveh Tappeh station had a very small difference in RIDIT values throughout the study period. The spatial distribution of RIDIT analysis for annual scales showed that Maraveh Tappeh, Gonbad Kavus in the eastern region, and Nowshahr near the coastal strip had no trend in horizontal visibility conditions. The Babolsar and Ramsar stations near the coastal strip showed an increasing trend in horizontal visibility. The Astara station in the northwest of the region had a decreasing trend. In the spring and summer, the Nowshahr and Ramsar stations near the Caspian Sea experienced a decreasing trend, while Babolsar station had an increasing trend. In the autumn, the Nowshahr and Astara stations had a decreasing trend, while the Ramsar station had an increasing trend. In the winter, a decreasing trend was observed in the Babolsar, Nowshahr, and Astara stations, while an increasing trend was observed in the Ramsar station near the coastal line. The correlation between relative humidity and horizontal visibility showed a negative correlation coefficient in the study area. In the annual scale, the Astara, Bandar Anzali, Nowshahr, Gonbad Kavus, and Siah Bisheh stations showed a relatively strong inverse relationship between relative humidity and horizontal visibility, with a range of values from -23.0 to -47.0. On the monthly scale, the same stations showed a range of values from -25.0 to -47.0. The correlation between horizontal visibility and precipitation in the area was positive and negative. A significant correlation between horizontal visibility and precipitation was found in 27% of the area. The correlation between horizontal visibility and temperature was direct and significant in 72% of the area. The correlation between horizontal visibility and wind speed was direct and inverse, significant in 54% of the area.
 
Conclusion
The results indicate that the annual mean of horizontal visibility in the study stations shows four relatively different periods. The most significant number of stations with a decreasing trend compared to other seasons was observed in winter. The spatial distribution of horizontal visibility on an annual scale shows that horizontal visibility has been higher in the eastern region compared to the western region. The results of the correlation analysis between horizontal visibility and climatic parameters showed that the correlation coefficient between horizontal visibility and precipitation was both direct and inverse, and 27% of the area had a significant correlation.
                                      
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.