بررسی ویژگی‌های دید افقی در سواحل جنوبی دریای خزر با استفاده از ضریب خاموشی

Horizontal visibility is one of the most important optical characteristics of the atmosphere. Predicting horizontal visibility is of great importance in various aspects such as air pollution, air traffic, flight safety, road traffic, and maritime travel safety. The aim of the present study is to analyze the spatiotemporal variation of horizontal visibility in the southern coasts of the Caspian Sea. For this purpose, hourly data of horizontal visibility, present weather phenomenon, and relative humidity were used for a 70-year statistical period (1951-2020) in the study area. In this study, the extinction coefficient was calculated using the Koshmider formula. Finally, using the non-parametric Mann-Kendall test, the trend analysis of the frequency reduction of horizontal visibility phenomenon was conducted in the region. The results showed that among the studied stations, Babolsar, Ramsar, and Astara stations experienced an increasing trend, Nowshahr station experienced a decreasing trend, and Gonbad-e Kavus and Maraveh Tappeh stations had no trend in the extinction coefficient. Based on the Mann-Kendall output among the seasons, significant decreasing (increasing) trends were observed in the autumn and winter seasons, and no trend was observed in other time scales. The results of the analysis of the impact of weather parameters showed that among the factors affecting horizontal visibility, precipitation (39.75%) and dust (0.83%) had the highest and lowest percentage of impact, respectively, compared to other effective factors in the region. In general, it can be said that due to the forested nature and suitable vegetation cover in the region, the possibility of the occurrence of local dust storms is very low.
Introduction
Horizontal visibility is one of the most important weather parameters measured at all weather stations and is usually reported in kilometers or meters (Arami, 2015). Horizontal visibility is one of the most important optical characteristics of the atmosphere and predicting the level of horizontal visibility is important in various aspects such as air pollution, air traffic, flight safety, road traffic, and maritime safety (Sabetghadam et al., 2016; Jaberi et al., 2018). Atmospheric visibility is a common parameter in meteorology, and almost all weather stations have recorded it for a long time. Other researchers have also investigated horizontal visibility using various indices, such as: Howaizyng et al. (2007), Kork et al. (2008), Wang et al. (2010), Yu et al. (2010), Boudala et al. (2012), Qu et al. (2013), Masoumi et al. (2013), Li et al. (2015), Zhou et al. (2015), Guo et al. (2017), and Javadi et al. (2017). Many researchers have studied this topic both nationally and internationally. The aim of the present study is to obtain the necessary information and specialized and general use of meteorological and climatological data from weather stations in the southern coast of the Caspian Sea in relation to planning for natural disasters and preventing their hazards in the study area.
Methodology
The study area is the region between the southern coast of the Caspian Sea and the Alborz Mountains in Iran, which is located at 36/28° - 38/36° N and 48/52° - 56° E. This region includes the political boundaries of the provinces of Gilan, Mazandaran, and Golestan. In the present study, hourly data including horizontal visibility, present weather, and relative humidity obtained from the National Meteorological Organization were used to analyze the spatiotemporal variation of horizontal visibility over a 70-year period (1951-2020). Initially, the changes in horizontal visibility were investigated by calculating the extinction coefficient using the Koshmider formula. The annual and seasonal trends of the extinction coefficient were calculated by performing the non-parametric Mann-Kendall test. In this study, the frequency of each of the four categories of visibility-reducing phenomena (precipitation, fog and mist, haze, and dust) was calculated for each station. Additionally, correlation coefficients were used to determine the relationship between climatic parameters (temperature, precipitation, and wind speed) and horizontal visibility. The spatial distribution maps of the mean extinction coefficient (annual and seasonal) and horizontal visibility with climatic parameters were drawn and analyzed in the ARC GIS software.
The results of the variability analysis of the extinction coefficient among the selected stations showed that in Babolsar station, the extinction coefficient had an increasing trend throughout the study period, with a range of variability between 1.0 to 4.0. The variability showed an increasing trend at the beginning of the study period (1.0) and a decreasing trend at the end of the study period. Ramsar and Astara stations also showed an increasing trend in the extinction coefficient. Overall, the range of variability in Ramsar station was between a maximum increase of 4.0 to a maximum decrease of 16.0. Astara station experienced a variability range between a maximum decrease of 27.0 to a maximum increase of 57.0. In Nowshahr station, the extinction coefficient had a decreasing trend throughout the study period, with a variability range between a maximum increase of 42.0 to a maximum decrease of 19.0. The results showed no significant trend in the variability of the extinction coefficient in Gonbad-e Kavus and Marah Tepe stations. The spatial distribution analysis of the mean extinction coefficient in comparison to the annual extinction coefficient showed that the maximum extinction coefficient is concentrated in the northwestern and southern parts of the region. In contrast, the minimum extinction coefficient is significant in the central parts of the coastal strip. Among the studied stations, Astara (0.41) and Siah Bisheh (0.40) stations experienced the highest values of the extinction coefficient, while Ramsar and Babolsar stations (0.30) had the lowest values of the extinction coefficient. In the spring season, the spatial distribution of the extinction coefficient showed that the maximum value of it is concentrated in the northwestern part of the study area, while the minimum value is observable in the eastern half of the region. In the summer season, in addition to the northwestern part of the region, the spatial distribution of the extinction coefficient showed that the maximum values of it are observable in the northeastern and southern parts of the western half of the region, while the central part of the coastal strip indicates the minimum values of the extinction coefficient. In the autumn season, the maximum concentration of the extinction coefficient is in the southwestern part of the region, with a tendency towards the central part of the region. The relative increase in the extinction coefficient is also observable in the northwestern and northeastern parts of the region. The minimum values of the extinction coefficient are observed in the central part of the coastal strip. In the winter season, similar to the spring and summer seasons, the maximum extinction coefficient is concentrated in the northwestern part of the region. The relative increase in the extinction coefficient is also evident in the northeastern and southern-western parts of the region. The center of the minimum values of the extinction coefficient is also concentrated in the central part of the coastal strip. The output of the Z statistic (Kendall's tau) indicates that the trend of the extinction coefficient is significantly increasing and decreasing, as well as non-significant, for the annual, spring, and summer scales. However, in the autumn and winter seasons, it only shows significant increasing and decreasing conditions.Based on the results of the Q statistic, it can be stated that the highest slope of the increasing trend in the annual, spring, and autumn time scales has been experienced at the Babolsar and Gorgan stations. In the winter season, Babolsar, Gorgan, and Rasht stations had the highest slope of the increasing trend, while in the summer season, Babolsar station had the highest slope of the increasing trend. The maximum slope of the decreasing trend of the extinction coefficient in the annual, spring, and summer time scales has been experienced at the Sari station with a value of -0.005 kilometers per year. Among the most important factors affecting horizontal visibility reduction, precipitation (75.39%), fog and mist (36%), haze-smoke (9.91%), and dust (0.83%) had the highest frequency percentage. In terms of spatial distribution, the highest frequency of precipitation and fog and mist phenomena were observed in the northern coastal strip and western half of the region, respectively. Meanwhile, the highest frequency of haze-smoke was observable in the eastern part of the region. The spatial analysis of the extinction coefficient between relative humidity and horizontal visibility in the annual scale showed a negative correlation coefficient in the study area. In other words, it indicates an inverse relationship between relative humidity and horizontal visibility. Among the stations and in the monthly-annual scales, the Astara station in the northwestern part of the region had the highest negative correlation coefficient of -0.47. The spatial distribution of the correlation coefficient in both the annual and monthly scales indicates that the highest correlation coefficient is concentrated in the northwestern part of the region. The spatial distribution analysis of the correlation coefficient between climate parameters (precipitation, temperature, and wind speed) showed that there is both a direct and an inverse relationship between precipitation and horizontal visibility