Investigation of trend analysis of the number of dust stormy days and aerosol concentration derived from satellite in Khuzestan province by using non-parametric Mann-Kendall test

Dust storms are one of important natural disasters that affect vast areas in the world and makes damage to people lives and causes many financial problems. Recently, change in precipitation and temperature pattern happened in the Persian Gulf countries due to climate change. Every year, dust storms causes many problems in different regions of Iran especially in Khuzestan province and this phenomenon frequently disrupt social, economic and official mechanism of the province. The areas are affected by Syria and Saudi Arabia dust particles. So analysis and simulation of dust storms will be helpful to investigate the potential sources and transport mechanism of dust particles in this area.In this research, 11 synoptic weather stations in Khuzestan province were investigated in 25years duration. Also results were investigated by AOD and NDVI indexes in all areas of the province. Ahvaz, Bostan, Abadan stations had the most dust storms frequency and Behbahan and Ramhormoz had the least. The average dusty days was 37.3 days per year and 2008,2009 and 2011 years had the most number of dusty days. The maximum amount of AOD and minimum amount of NDVI were in 2008 and 2009 that is in good agreement with number of dusty days in these years.

In this study, the trend of monthly and yearly dusty day frequency investigated in some stations in Khuzestan province in 25years duration (1993 to 2017). Then The Mann-Kendall Test is used for all of them. For this purpose, dust codes (06 and 07) separated in Khuzestan weather stations in 25years duration. Furthermore, reported visibility must be 5000m or less in dusty days. In the next steps, the MODIS Terra AOD was used to investigate Aerosol optical depth in the atmosphere. AOD is available daily and at a spatial resolution of 1 degree. Finally, the normalized difference vegetation index (NDVI) was investigated in the study area. The index investigates vegetation coverage changes in the study area and it used red and near-infrared channels.

In this case study, dust storms were investigated climatically in 25years duration (1993 to 2017). Monthly average number for dusty days showed that the maximum temperature and the minimum precipitation happened in June and July in Khuzestan province. Also the maximum frequency of dusty days happened in that two months and the minimum of dust frequency happened in November.The maximum amount of monthly AOD was in June from 2000 to 2018 in Khuzestan province that it is in good agreement with maximum average number of dusty days. Ahvaz, Bostan, Abadan weather stations had the maximum frequency of dusty days, but Behbahan and ramhormoz stations had the minimum frequency of dusty days.The seasonal average number of dusty days showed that the maximum frequency of dusty days was in spring and summer and the minimum frequency was in autumn. Also the maximum number of dusty days happened in 2008, 2009 and 2011. The maximum amount of AOD was in 2008 and 2009 in 19years duration (2000 to 2018). The mean annual NDVI index was the least amount in 2008 and 2009 that it is in good agreement with AOD and dusty days in the province.Totally, annual trend coefficient of dusty days was 1.39 in Khuzestan province which shows and increasing trend. Investigation of Mann Kendall Test in dusty days of Khuzestan weather stations confirms 1 to 10 percent confidence levels of the observation trend at 7 stations, but it was meaningless in 4 weather stations.  Seasonal analysis showed that the trend of number of dusty days was significant at 5% confidence level in the spring. This trend was not significant in summer but it was significant at 10% confidence level in autumn and 1% in winter.