عنوان مقاله [English]
In recent years, by intensified desertification in Iraq, Saudi Arabia and Syria, dust storms have affected different aspects of people's lives over the West and southwest of Iran. Atmospheric aerosols affect public health, air quality, energy balance of the Earth and the hydrological cycle. Thus, knowledge of the spatio-temporal dust storm's distribution characteristics is extremely important to quantify these effects. By increasing in dust storms in the West and southwest of Iran much discussion about the various causes of this phenomenon has been increasing.
Materials and methods
Aerosol optical depth (AOD) is an important remote sensing parameter that serves as a columnar proxy variable representing aerosol abundance. In this study, monthly mean AOD from Moderate Resolution Imaging Spectroradiometer (MODIS) are used to investigate the spatio-temporal distribution of dust storms in these affected areas for the period between 2000 and 2015. Monthly AOD data from the MOD08 Level 3 product (http://ladsweb.nascom.nasa.gov/data) at a resolution of 1° × 1° are used. This study focuses on AOD at 550 nm over land, as this is close to the peak of the solar spectrum and is, therefore, associated with major radiative effects. Synoptic data are examined to identify how well geopotential height, temperature, and wind field at various levels (i.e., sea level, /500/250 hPa isobaric levels) are related to AOD and to dust activity. Meteorological data from the ECMWF archived operational initialized analyses at a resolution of 2.5° × 2.5° (Bou Karam et al. 2010; http://apps.ecmwf.int/datasets/data/interim-full-moda/levtype=sfc) is used.
MODIS monthly mean AOD validation results using the Pearson correlation test to determine the correlation between monthly mean AOD and monthly visibility data indicated high efficiency of AOD products in the dust storm intensity study. To distinguish between the Rub’ al Khali and the influence of factors such as dams in other areas, the study domain is divided in two sections consisting of desert areas in (i) Saudi Arabia and (ii) Iraq-Syria.
AOD plot of the provinces of the west and southwestern of Iran showed provinces of Khuzestan, Ilam, Kermanshah and Kohgiluyeh in order have experienced the highest dust storms from 2000 to 2015.
As the dust storms in all months of the year are not active, to evaluate the spatial distribution of mean AOD per year, the average for the months in which the dust storms are active evaluated (named as annual AOD). Annual and monthly average plots showed that the trend of the amount of AOD in many cases in the deserts of Iraq and Syria desert was similar to deserts of Saudi Arabia.
Considering the wide range of climatic conditions affecting the occurrence of dusty storms, the best way to find out the effect of circulation on the formation of dusty storms is to compare the synoptic state of a special month in two active and inactive periods. To achieve this goal, the monthly anomaly maps (monthly average of 30 years (1981-2010)) were extracted at surface level, 250 and 500 Hp. Two months as representative the active and passive dust storm period was selected to study atmospheric circulation associated with dust. To investigate the impact of synoptic systems on factors influencing dust in the study region, precipitation, temperature a case study analysis of July 2009 and July 2014 was conducted.
Also analysis the map of the average annual distribution of dust in each region shows that the trend of AOD value in many cases in the deserts of the Saudi Arabian desert was similar to Iraq and Syria desert, but AOD value in which years the dust in deserts of Iraq and Syria were still down is high in East and South East Arabian Peninsula.
In this study, changes in the behavior of dust storms in the west and southwest of Iran were investigated using quantitative monthly and annual AOD data from 2000 to 2015. The result of the Pearson correlation test with the aim of determining the correlation between the monthly average visibility and mean monthly AOD and indicates the high ability of the AOD monthly data to study dust storms.
Trends of AOD show that from 2006 to 2012 there was increasing trend in February, March, April, May, June and July with a significant slope. The gradient of the monthly AOD slope of Kermanshah province was similar to Iraq and Syrian deserts and also Khuzestan province comply Saudi desert or desert of Iraq-Syria in different cases. These differences can only be caused by differences in atmospheric circulation in different dates.
By comparing the synoptic maps of active and passive anomaly, it seems at sea level pressure existence of Pakistan's low pressure center and increasing pressure on the Mediterranean Sea exacerbated atmospheric turbulence and wind speed in the Middle East play an important role on the intensification of dust storm activities. At the middle and upper levels of the atmosphere, the sovereignty of the subtropical high pressure over the Arabian Peninsula, south of Iraq, the Persian Gulf and the southern part of Iran, also increasing the activity of the Mediterranean waves, increasing the mean wind speed at the upper levels of the atmosphere, as well as the flow of jet stream core has led to an intensification of dust storms in July 2009. That's why limited dust storms have been created in central and eastern regions of the Arabian Peninsula.