Trend analysis and change point detection of annual and seasonal horizontal visibility trends in Saudi Arabia

Horizontal visibility is of utmost importance in climate and air quality studies as it can provide key information related to changes in meteorological conditions, air quality, and their impact on human health. In this study, multiple trend analysis methods were applied to investigate the annual and seasonal horizontal visibility trends based on 1985 to 2018 at 23 locations in Saudi Arabia. Different forms of the Mann-Kendall (MK) test were conducted for the trend analysis, which includes the MK test with (without) autocorrelation corrections. In addition to that, Sen's slope estimator and Kendall's Tau were used to examine horizontal visibility trends and trend magnitudes. The sequential Mann-Kendall test was used to locate trend fluctuations over time and to detect abrupt changes in the horizontal visibility. It was found that 64.4% of the time series have significant decreasing trends (visibility decreasing with time). The average trend magnitude for Saudi Arabia was - 0.024 km/year over the evaluated period. Stations in the North-East quadrant (NE-Region) had a higher visibility degradation in spring and lower visibility degradation in summer. Stations in the Western area (W-Region) have higher visibility degradation in winter and lower visibility degradation in autumn. Northeastern part of the NE-Region showed the highest visibility deterioration. The sequential MK test detected increasing visibility trends in the early 1990s to the early 2000s, while during the mid-2000s to 2018, most of the data showed decreasing trends. The time series of annual horizontal variability in most of the stations located in the NE-Region showed more significant decreasing abrupt changes; however, few stations located in these two regions revealed insignificant increasing abrupt changes.