The study of a rare frontal dust storm with snow and rain fall: Model results and ground measurements

Journal Pre-proof The study of a rare frontal dust storm with snow and rain fall: Model results and ground measurements Sara Karami, Nasim Hossein Hamzeh, Khan Alam, Abbas Ranjbar PII:

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Journal of Atmospheric and Solar-Terrestrial Physics

Received Date: 7 May 2019 Revised Date:

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Please cite this article as: Karami, S., Hamzeh, N.H., Alam, K., Ranjbar, A., The study of a rare frontal dust storm with snow and rain fall: Model results and ground measurements, Journal of Atmospheric and Solar-Terrestrial Physics, https://doi.org/10.1016/j.jastp.2019.105149. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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The study of a rare frontal dust storm with snow and rain fall: Model results and ground measurements

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Sara Karami1, Nasim Hossein Hamzeh1 , Khan Alam2, Abbas Ranjbar1

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*Corresponding author. [email protected]

Department of Atmospheric Science & Meteorological Research Center (ASMERC), Tehran, Iran Department of Physics, University of Peshawar, Peshawar 25120, KPK, Pakistan

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Abstract:

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Dust plays an important role in the modification of microphysical and optical properties of

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clouds. The presence of dust at an elevated level significantly increases snow mass and rain

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concentration. A rare phenomenon of rain and snowfall with dust was occurred simultaneously in

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the eastern Iraq, Syria, and west and south west of Iran on 19 and 20 January 2018. In the

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meantime, while the dust impact was dominating over a large area within Syria, Iraq, Saudi-

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Arabia, Kuwait and Iran, it consequently caused extreme temperature drop, rain and snowfall

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problems for the residents living in the Middle East. Likewise, a visibility drop together with

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rainfall was also observed in the same region. The visibility was reduced to less than 500 m over

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most of the region. Further, an image showing dust mass taken from the satellite with higher

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amount of AOD (≥1) indicated the severe dust activity. It is worth to mention that a dynamic low

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pressure system covering the surface, and also the cold and warm fronts and the relevant

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occluded front caused such a phenomenal problem. The dust storm on 19th January was mainly

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prefrontal typed and transported from Horolazim Lagoon, Ad Dahna and An Nafud deserts,

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under the impact of the south easterly winds. The cold front was displaced to the eastward with a

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consequent dust storm on 20th January, originated from the vaster parts in Iraqian and Syrian

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deserts and Ad Dahna and An Nafud deserts. The back trajectory analysis of HYSPLIT MODEL

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shows the different sources of dust in the study region during 19-20th January. On the whole, it

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can be concluded that the dust concentration and atmospheric quantities, including the wind, the

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precipitation, and the cold and warm fronts having a permissible effect on simulation processes,

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are well simulated by the WRF Chem model. The comparison of WRF-Chem and NAAPS

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outputs revealed that the dust pattern and the procedure of vertical transfer of particles of each

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model are very identical, but the amounts of dust in the Model NAAPS are underestimated.

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Keywords: Dust; Snow & rainfall; WRF-Chem model; NAAPS model; Frontal system

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1. Introduction

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Recently, dust phenomenon has been set forth as a fundamental problem in many countries

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worldwide. Dust events impacts financial damages and human disasters on a large scale (Wilhite,

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2000). These events cause air pollution with their associated health risks, e.g., respiratory and

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heart diseases (Hyun et al., 2011). Among several impacts, caused by dust phenomena flight

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cancellations, accidents due to low visibility are prominent. Dust activity reduces the horizontal

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vision, and consequently causes severe incidents and accidents, besides that increases death toll

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(Cao, 2009). Most of the world’s dust phenomena occur in the different areas including the

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Sahara desert, Middle East, Central Australia, Mongolia, and certain parts of America continents

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(Yang, 2015). These regions are called dust belt, based on the dust dispersions worldwide. They

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are geographically located in the northern hemisphere, extended from North Africa to China.

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However, the dust rate outside this belt is low (Prospero, 2002). The Sahara desert located in

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Africa, is the largest source of dust particles in the world. Sahara desert is adding 700 Megatons

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of dust into the atmosphere annually (Schlesinger et al., 2006). Several studies have been carried

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out to investigate the origins of dust particle in the atmosphere (Chen et al., 2018). It is found

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that ~ 81% of dust phenomena originated from natural dust sources and the remaining 19% of

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the dust is caused by human activities. In addition, 80% of PM10 concentration exists in the

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atmosphere, containing natural dust in the Middle-East (Aydin, 2012). In contrast, 10% to 20%

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dust is emitted from the fossil fuels (Kocak, 2009).

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The Middle-East is located in dust belt and this region has many important dust sources,

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including, Rub-Al-Khali, Ad Dahna, An Nafud, Lut and Kavir deserts (Namdari, 2018). Some

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different meteorological factors cause dust particle emission and transportation. For example,

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Shamal wind causes the transportation of large amounts of dust, particularly in the summer

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season in the Middle-East (Yu et al., 2016) and Sistan region, summer winds caused the visibility

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drop below 100 m (Alizadeh et al., 2014). One of the main elements of the dust formation in

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winter season is the atmospheric fronts. Extreme winds cause dust particles originating from the

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surface due to cold and warm fronts and lack of sufficient humidity in the region. Seven 2

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insensitive dust storms in the Arabian Peninsula were studied from 2014 to 2017 by Beegum et

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al. (2018), and concluded that 5 of them were relevant to Synoptic scale and the two concerning

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Haboob and mesoscale.

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In the past two decades, various researchers have presented models and simulation approaches

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for dust storms forecasting (Zakey et al., 2006; Colarco et al., 2010). Also, being concerned

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about this matter, certain International Institutes are established to actively involved in studying

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the dust phenomenon across the world, (such as SDS-WAS: https://sds-was.aemet.es/). Ngan et

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al. (2018) studied about the time and the location of the dust particles during October 2013 using

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integrated HYSPLIT/WRF Model. They showed that any boundary layer scheme of WRF cannot

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estimate wind gust changes. Besides, comparison to the simulation between online and offline,

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HYSPLIT shows that the online simulation has a much better function and estimates of more

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densities closed to the dust source. Xu (2018) has also compared the output of aerosol optical

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depth (AOD) of the Model-CHIMERE-DUST applied to the north Africa, with the satellite and

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AERONET data. For this purpose, he used ECMWF and CFSR data, which are downscaled by

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WRF Model. Both simulations of PM10 and AOD provide similar results leading to have high

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correlations with satellite data (Xu, 2018). Also, Song et al. (2017) studied the sensitivity of the

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WRF–Chem model to different emission schemes and the earth’s surface at the dust simulation

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in the spring time in the eastern Asia.

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The purpose of this study is to analyze a rare event of dust and their association with snow and

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rainfall in the Middle East using a variety of data sets and modeling approaches. In section 2, a

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detailed methodology is discussed, including the study area, satellite data, and lagrangian &

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numerical modeling. The results and discussion are thoroughly described in section 3. Finally a

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summary and the conclusions are discussed in section 4.

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2. Methodology

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In this rare dust storm, a mass of dense dust covered eastern Syria, central and South-East

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regions of Iraq, and West and South-West of Iran and caused an intense reduction in visibility on

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19 and 20 January 2018. Also a severe temperature drop, with rain and snow falls was observed

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simultaneously with this phenomenon in the region, due to which, in certain cities, while snow

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covered the earth’s surface and the rain wetted it. The existing sever dust storm in the 3

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atmosphere caused a visual reduction as well as respiration difficulties for the local inhabitants.

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In this study visibility, precipitation and satellite images were studied and atmospheric factors

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affecting the formation of the dust phenomenon were investigated, using different synoptic maps.

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Then, the dust storm is simulated by WRF–Chem Model. In order to evaluate the outputs, PM10

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concentrations are used at some stations in the west and southwest of Iran. Finally, the output of

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the two NAAPS and WRF–Chem models is compared in the simulation of this phenomenon.

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The dust cover and varying precipitation in the study region are assessed by analyzing visibility

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and precipitation data collected from the synoptic stations. The rate of snow, rain, and the

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varying Aerosol Optical Depth (AOD) concentration are studied through remote sensing satellite

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data. In addition, the synoptic analysis is performed on the data executed by Global Forecast

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Model (GFS). Similarly, to investigate about the sources of a dust storm and the tracks of the

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dust particles, Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is

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used. The combined emitted scheme of WRF–Chem Model with Air Force Weather Agency

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(AFWA) is used to simulate the dust phenomenon (Grell et al., 2005; Jones et al., 2012). For the

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quantitative evaluation of the model results, the data of PM10 concentrations of the two air

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pollution monitoring stations in Ahwaz and Khoramshahr cities located in southwest Iran are

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compared with the model outputs. At the end, the output of the Navy Aerosol Analysis and

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Prediction System (NAAPS) model (Hogan and Rosmond, 1991), is compared with the output of

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the WRF-Chem model and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations

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(CALIPSO) satellite data.

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2.1 Study area and meteorological situation

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The study area, including Syria, Iraq, Kuwait, Saudi Arabia and Iran is shown in the Figure 1.

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These locations are the hot spots of dust aerosols, which strongly affects air quality, visibility

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and hydrological cycle (Cao, 2015). Wind causes dust emission (or transportation) in the study

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areas because of the existence of extensive dust throughout the year, mostly when soil moisture

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is low due to low rainfall. In the summer, Shamal wind activates the dust sources in this region,

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especially in Iraq. In the winter, intense winds with atmospheric fronts make dust emission,

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particularly during low humidity in the mid-levels of the atmosphere (Hamidi, 2013).

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Figure 1. Map of the study area.

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2.2 Instrumentations and Data

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In this study, multiple sensor products are used to investigate the rare dust storm. The satellite

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products from MODIS and CALIPSO are used in order to analyze aerosol, snow and rain rate. A

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brief description of satellite sensors is discussed in the following sub-sections.

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2.2.1 Moderate Resolution Imaging Spectroradiometer (MODIS)

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The Moderate Resolution Imaging Spectroradiometer (MODIS) is a space born sensor mounted

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on both Terra and Aqua spacecrafts. The revisiting period of MODIS is one or two days. It

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comprises of 36 spectral bands with a spatial resolution of 250, 500 and 1000 m (Draxler and

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Rolph, 2011). MODIS collects lots of information about the earth’s surface, ocean and 5

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atmosphere both on micro and macro scales. MODIS data are downloaded from different

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sources. Among these sources, website of Level-1 and Atmosphere Archive & Distribution

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System (LAADS) is used for worldwide atmospheric products. Land and surface products are

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procured from the United States Geological Survey (USGS) data center. Similarly, Sea surface

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and ocean products are available at Goddard Space Flight Center (GSFC). In this study MODIS

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8-day snow product (MOD10A2) is used to study the change in snow cover over the study area.

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The aerosol product MOD04 is used to study the variations in aerosol concentrations.

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2.2.2 CALIOP

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The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite

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launched in April 2006, provides vertical profile of aerosols and clouds (Kumar et al., 2012).

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CALIPSO come across the equator at 13:30 (Winker et al., 2010). This is the only satellite

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sensor that has LIDAR. CALIPSO carries cloud Aerosol-Lidar with Orthogonal Polarization

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(CALIOP) that operates at wavelengths of 532 nm and 1064 nm with attenuated backscattering

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to give detailed information of aerosols and clouds globally. It works with CloudSat satellite and

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provides 3-D perspectives of clouds and aerosol particles that is unique in the world (Winker et

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al., 2009). The data is downloaded from the website: www.calipso.larc.nasa.gov.

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2.3 Atmospheric and dust Models

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This study utilized various numerical models, namely, HYSPLIT, WRF-Chem and NAAPS

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model. The output results of WRF-Chem and NAAPS models in terms of dust concentrations are

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investigated that show the dust dispersion in the study area.

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The Global Forecast System (GFS) is a weather forecast dynamical model developed by

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National Centers for Environmental Prediction (NCEP) operational since May 2007 (Saha et al.,

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2006). Many variables such as atmospheric and land surface variables can be procured from this

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dataset. The GFS analysis data with 0.5 degree resolution is used for synoptic analysis. The GFS

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model is a global coupled model includes atmospheric, oceanic, land surface and sea-ice model.

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2.3.1 HYSPLIT Model

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The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and

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Rolph, 2003) is used to calculate the trajectory of the particles and pollutants existing in the

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atmosphere. It can trace and forecast the particles of radioactive substances, fire smoke, dust, and 6

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pollutants arisen from permanent and moveable sources, and volcanic ashes too. Furthermore,

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for the purpose of computation of advection, dispersion and particles transfer routes in the

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atmosphere, this model applies Lagrangian and Eulerian method to compute the density of

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pollutant particles in the air. The best advantage of this model is to determine the source of the

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aerosol particle by using the backward trajectory method.

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2.3.2 WRF-Chem Model

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The WRF-Chem model is the WRF numerical prediction model that is associated with the

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atmospheric chemistry. Additionally, this model includes quantities relevant to the atmospheric

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chemistry simultaneously with meteorological quantities such as wind, precipitation and

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temperature as an output, too. Moreover, it is also used for study of air quality, aerosol, and the

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relationship between clouds and atmospheric chemistry. The general structure of the WRF–

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Chem model is shown in Fig 2.

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WRF/Chem

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WPS

Physics

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Statistical data

geogrid

metgri d

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Gridedd data

Dynamics

ungrib Chemistry

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Figure 2. The general structure of WRF–Chem model.

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The Chemistry parts of WRF–Chem models have 5 different dust schemes: GOCART (Goddard

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Global Ozone Chemistry Aerosol Radiation and Transport) (Ginoux, 2001), AFWA (Air Force

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Weather Agency) (Jones, 2012, Shao, 2001, Liu & Shao, 2004, Shao, 2011). In this study, the

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AFWA dust scheme was used, because it has better performance in the proposed study area.

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In the AFWA scheme, the dust saltation flux
  of each particle size with diameter  is

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calculated as follows (Kawamura, 1951):

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  = 

∗ ∗   1 +   1 −   1

∗ ∗

 = 
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where in equation (1), C is dimensional tuning constant,  implies air density, ∗ is friction

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velocity and ∗ shows threshold friction velocity. AFWA dust emission scheme, in contrast to

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the GOCART scheme, which uses 10-meter wind speed, calculates threshold friction velocity. In

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this scheme, the vertical dust flux is calculated based on Marticorena and Bergametti (1995),

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which has a correction coefficient (Gillett and Morales, 1979): !"#$ = % × '() 3 % = 10 .- .%0 123 4

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where !"#$ is bulk vertical dust flux, G is a constant parameter and % is efficiency factor.

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Regarding the extensive study area, the WRF-Chem model uses a 21 km horizontal resolution

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respectively, with 120 and 101 points at x and y axes and 30 vertical levels (altitude). The GFS

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data with 0.5 degree resolution have been used as initial and boundary condition. Other schemes,

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used in the implementation of the model are displayed in Table 1.

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Table1. WRF-Chem model schemes used in this study.

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Microphysics Longwave radiation Shortwave radiation Surface physics Planetary boundary layer Cumulus

WRF Single-Moment 5-class scheme RRTM scheme (Mlawer et al., 1997) Goddard shortwave (Chou and Suarez, 1998) Noah Land Surface Model Yonsei University scheme (Noh et al., 2002) Grell 3D

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2.3.3 NAAPS Model

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NAAPS is a global transport model that predicts smoke, sea salt and dust aerosol for 25 vertical

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levels, 4 times a day for 144 hours. It presents an operational forecast of the aerosols across the 8

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world and uses NOGAPS atmospheric model data. The NAAPS model has 94 land use types, of

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which 8 types are related to the potential dust sources. Land use in the model has been

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determined by using the surface-covering characteristics data. In general, the intensity of erosion

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and dust particle emission depends on wind speed and thermal static stability in the atmospheric

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boundary layer. Dust emission is restricted to arid zones with soil moisture less than 0.3.

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Most of the time, when the friction velocity exceeds from a threshold value, then the snow depth

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is less than the threshold value, and the surface moisture will be less than a critical value, thus

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dust emissions will be occurred . The dust emission flux equation is:

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. !5#67 = 89: ∗ 5

where 9: is without unit and the erodible fraction, ∗ is the friction velocity with the threshold value of 0.6 m/s, and c is a constant and its value is 4.5 × 102< (Walker et al., 2009). 3. Results and Discussions

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3.1 The spatio-temporal variation in visibility and precipitation

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Airborne dust has significantly deteriorated air quality during the study period. Noticeable

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visibility reduction happened at synoptic stations located in an extensive part of the central and

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eastern regions of Iraq, southwest (SW) of Iran, and east and north of Saudi Arabia as a result of

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rising dust particles in these regions from 19 to 21 January 2018. The visibility at 12 UTC on the

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20th January is displayed in figure 3. The visibility values have been reduced to less than 1km in

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SW Iran, eastern Iraq and Saudi-Arabia. The visibility in the extensive regions of the northern

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and central Iraq, and in the west of Iran has been found to be lower than 5km. Mahowald et

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al.(2007) analyzed visibility in the arid regions to assess the anthropogenic impact on dust

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emission using meteorological station data. Baltaci (2017) analyzed spatial and temporal

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variation of the extreme Saharan dust event and found that visibility decreased to 3–4 km in

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various parts of Turkey, particularly in the city of Istanbul.

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Figure 3. The visibility (m) on 20th January at 12 UTC.

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Figure 4 shows, the total precipitation of each synoptic station in the provinces, including

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Kermanshah, Ilam, Khuzestan, and Busheher, located in the west and SW of Iran, occurred on 3

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days from 18 to 20 January 2018. The results show that the precipitation was occurred in almost

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all stations of this region on these days. However, most precipitation has been observed in

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eastern provinces of Ilam and Khuzestan.

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Figure 4. Total precipitation (mm) of each synoptic station in Kermanshah, Ilam, Khuzestan, and Busheher provinces from 18 to 20 January 2018.

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Figure 5 shows the temporal variation in visibility related to dust code in five cities including

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Kermanshah, Ilam, Ahwaz, Behbahan, and Omidiyeh. The results revealed that the visibility has

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been dropped below 500 m in all the cities except Kermanshah during the study period. A sever

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visibility drop has started in Ahwaz at 09 UTC on the19th, and visibility reduction has occurred

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in the two cities of Omidiyeh and Behbahan respectively at 06 and 15 UTC on the 20th January,

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2018.

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Figure 5. Visibility (m) in Kermanshah, Ilam, Ahwaz, Behbahan, and Omidiyeh cities from 18 to 22 January 2018.

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According to the daily precipitation (see Figure 6), the high precipitation is about 15mm, which

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is reported by Ilam synoptic station on the 19th January, while a considerable visibility drop due

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to dust storm was occurred in this city at the end of the same day. It is worth to mention that both

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visibility drop and precipitation occurred in some cities on the 20th January.

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Figure 6. Daily precipitation (mm) in Ilam and Ahwaz from 18 to 22 January 2018.

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3.2 Analysis of snow, rainfall and AOD

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The true color image of MODIS sensor on Terra Satellite, along with the snow and rain rate from

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IMERG (Integrated Multi-satellitE Retrievals for GPM) on 19th January 2018 were obtained

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from website: https://worldview.earthdata.nasa.gov. The average AOD (Aerosol Optical Depth)

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of Terra MODIS (https:// Giovanni.gsfc.nasa.Gov) from 18 to 21 January 2018 are displayed in

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Figure 7. A severe rainfall occurred in the region of Eastern Syria, Northern Iraq and the west

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and SW of Iran. The average AOD also shows higher values across Iraq, especially in its NE,

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central and SE areas, Kuwait, West of Persian Gulf, North and East of Saudi-Arabia, as well as,

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the west and SW of Iran, proving a higher density of aerosols in this region during 18 to 21

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January 2018. Alam et al. (2014) and Prasad & Singh (2007) observed that the aerosol optical

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parameters (AOD, Volume Size distribution, and Single Scattering Albedo) change significantly

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during dust events. Čanić et al. (2009) analyzed Saharan dust impact on precipitation in Croatia

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and found an increase of calcium concentration in precipitation due to more frequent mud rain.

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b)

Figure 7. (a) The true color image of MODIS on Terra Satellite with the snow and rain rate on 19th January 2018. (b) Average AOD of Terra MODIS with Dark target and Deep blue algorithms from 18 to 21 Jan 2018.

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Figure 8 shows the vertical feature mask of the CALIPSO satellite on 21th January 2018

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(https://www-calipso.larc.nasa.gov/products). The image shows a thick cloud mass at 34.55ºN

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and 44ºE with the bottom height of 7km and the dust particles in this region. The most dust

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concentration in the area is at altitudes lower than 1km, but a little amount of dust can be seen

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just below the clouds (7km altitude). b)

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Figure 8. Vertical feature mask of CALIPSO satellite on 21th January 2018.

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3.3 Analysis of sea level pressure, temperature and geopotential height

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Synoptic patterns have an important effect on dust formation and transport. Many studies

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investigated dust storm from the synoptic point of view. For example, Klose et al.(2010) 14

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examined the synoptic patterns, which caused dust emission from the Sahara and transport of

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dust particles to Sahel by Using the ERA-Interim Reanalysis data. The images of the satellite and

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the mean sea level pressure at 00 UTC on 19 and 20 January 2018 are shown in Figure 9. On

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19th January, a strong dynamic low pressure center over Syrian desert is conformed, of which its

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trough extended up to the south of Iraq and Saudi-Arabia. Due to cloud mass and the cycle of

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isobar contours, which can be a sign of the front in this region; warm, cold and occluded fronts

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related to the low pressure center are shown in Figure 9. On 19th most areas of Khuzestan

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Province in southwest of Iran are affected as a result of the severe wind blowing before the warm

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front, which led to a dust transport from south of Khuzestan regions. While, in many cases south

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easterly winds is occurred before reaching the cold front to the region, it leads to form the

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prefrontal dust storm in the region in this season of the year. On the 20th January, influencing the

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ridges of high pressure from the west, the center of low pressure is weakened and moved to the

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east, caused the contour at a pressure level 1014hPa near to Iraq. On this day a cold front covered

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SW Iran, and with more heavy dust arisen, from the central areas of Iraq, dust particles entered

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the west and SW of Iran. Meanwhile, due to a heavy mass of cloud relevant to the occluded front

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and the low pressure center, it affected the west and NW of Iran’s Provinces leading to

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precipitation in these regions. The clouds related to occluded front cause rainfall in the western

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part of Iran on the early 19th January. At the end of the same day, heavy clouds moved to the east

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and strong winds arisen more dust particles from dust sources in the southwest of Iran and

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central areas of Iraq. The visibility reduced drastically in western Iran on 20th January.

a)

b)

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Figure 9. The images of the satellite and the mean sea level pressure at 00 UTC on (a)19 and (b) 20 January 2018. 15

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The 850hPa geopotential height, and the temperature at 00UTC on 19th and 20th January are

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displayed in Figure 10. On 19th January, in the cold front area located in the Central regions of

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Iraq, the height contours crossed the isotherms, with a noticeable inclination, which caused a

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sever baroclinicity in this region. On this day, a lower pressure was observed at lower altitude,

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indicating the system is weakened. On 20th, the low altitude center is transferred at the higher

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speed towards the east relative to the surface low pressure and arrived towards eastern Iraq.

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Figure 10. The geopotential height (m), and temperature ℃ at 850hPa at 00UTC on the (a) 19th (b) 20th January.

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At the 500hPa level (see Figure 11), on 19th. January, a low altitude center set up over eastern

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Syria, which gradually weakens and moves towards the east. This system was shifted to west of

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Iran on 20th January.

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a)

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b)

Figure 11. The geopotential height (m) at 500hPa at 00UTC on the a) 19th b) 20th January.

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3.3 Model based analysis

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HYSPLIT and WRF-Chem models were used in order to understand the origin, transport of dust

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and dust concentration in the study region. Figure 12 shows the back trajectory of HYSPLIT

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Model, at 12UTC at a height of 500 m above the ground on 19th and 20th January, 2018. The

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model is run by GDAS data with 0.5 degree horizontal resolution for 18 hours. On 19th January,

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the model shows the change of wind direction and a front covering east of Iraq. Furthermore,

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relying on the model output it can be expressed that on this day the dust existing in the regions of

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the west and SW Iran mainly transported from inside Iran. However, the dust within the east and

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central Iraq transferred from the central and SW Iraq and from north of Saudi Arabia. On 20th

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January the model output revealed that the dust is transported from central Iraq and eastern Syria

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to eastern Iraq and western Iran.

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a)

b)

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Figure 12. The output of HYSPLIT Model in backward trajectory method, at 12UTC on the days of (a) 19th and (b) 20th January 2018.

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The WRF-Chem Model output of dust concentration, with the AFWA dust emission scheme, is

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shown in Figure 13. At 06 UTC on the 19th January, the model depicted the formation of the

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prefrontal dust, arisen from SW Iran, located in Province Khuzestan and eastern Iraq, which is

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due to severe southeasterly winds. Also, a little amount of dust behind a cold front is shown at

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eastern Syria and western Iraq. At 12 UTC on the same day, the dust concentration is drastically

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increased at the south of Syria and Iraq, central Iraq, and SW Iran. On the day of 20th at 00 UTC,

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with the transfer of the cold front towards the east, the dust mass arrived west of Iran regions,

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due to westerly winds impact, consequently an intense dust is observed in the region. At 12UTC,

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the concentration of dust at the west and SW Iran is decreased, but the dust mass is developed

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and covered areas of central Iran. The comparison of model results and satellite images revealed

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that model simulated the dust pattern and its transfer procedure very well. Eltahan et al. (2018

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used WRF-Chem model to simulate dust events over Egypt and observed that the model results

368

are closer to the satellite and Aerosol Robotic Network observations. 18

369 370 371 372

a)

b)

c)

d)

373 374 375

Figure 13. The surface dust concentration (µg/m3) of WRF-Chem model output, on 19th January at a) 06UTC, b) 12UTC and on the 20th January at c) 00UTC, d) 12UTC.

376 377

The vertical cross-section of relative humidity and the dust concentration, along with the wind

378

and the potential temperature are displayed in Figure 14. At 12 UTC, a center of maximum dust

379

concentration is observed near the surface around the region Hoor al-Azim, one of the major dust

380

sources in the boundary of Iran and Iraq (at 48ºE longitude). Above this dust mass, a relatively 19

381

high humidity (more than 70%) existing at the mid-levels of the atmosphere, expressing the

382

formation of a cloud mass in these regions, which can block the vertical dispersion of the dust

383

particles to the higher altitudes of the atmosphere. On the other hand, a dust mass with less

384

intensity at the eastern border of Iraq is formed. In the region between the two masses, where the

385

dust concentration is very low, the intensive gradient of the relative humidity, an inclination of

386

the isentropic contours and a sudden direction change of wind in the higher altitudes indicated

387

the existence of the front. At 12 UTC on the 20th January, the two dust masses integrated and

388

consequently the dust concentration increased in the whole region. Also, vertical dust dispersion

389

in the atmosphere, shows the dust particles affected the Zagros Heights up to about 4km

390

(between 51ºE to 63ºE longitude), due to the relative humidity drop in the mid-levels of the

391

atmosphere. The intensive gradient of the relative humidity at the geographical longitude, which

392

is observed about 54°E on this day, expressing the inclination of the isentropic contours resulted

393

in transferring the front towards the east in this region.

a)

b)

20

c)

394 395 396

d)

Figure 14. Vertical cross-section of relative humidity and the dust concentration along the latitude of 31ºN at 12UTC on a) 19th January b) 20th January, and vertical cross-section of wind and potential temperature along the same latitude at 12UTC on c) 19th January d) 20th January.

397 398

The PM10 concentration of WRF-Chem model output and the dust monitoring stations in the

399

two cities of Ahwaz and Khoramshahr in Province Khuzestan-Iran is shown in Figure 15.

400

However, the model output in Ahwaz city is more similar to the ground observation. The highest

401

concentration is 5538 > ⁄? at 9UTC on 19 Jan, based on the data observed in Ahwaz, while

402

the model estimated concentration is 4682 > ⁄? with 3 hour delay, also the model showed

403

reduction of dust concentration at the next coming hours with less inclination relative to the

404

observation. In Khoramshahr city the difference between PM10 measured and the model output

405

is little more than Ahvaz. The high PM10 concentration observed at 00 UTC on 20th January is

406

4380> ⁄? , but the model maximum observed concentration is 3188.69> ⁄? at 12UTC on

407

19th

408

concentration is acceptable, and indeed the model is correctly simulated the trend in

409

concentration changes. Kuo et al. (2013) found that particulate matter (PM2.5) was responsible

410

for reducing visibility in various locations in Taiwan. Similarly, during the dust events, PM10

411

was also responsible to reduce visibility up to 3–4 km in various parts of Turkey (Baltaci, 2017).

412

Jugder et al. (2011) reported that during the heavy dust storm at Zamyn-Uud (Mangolia) the dust

413

concentrations reached to 1228 µg/ m3, consequently reduced the visibility in the range from 300

414

to 700 m.

January. On the whole, it can be concluded that the model simulation of PM10

21

415 416 417

Figure 15. The PM10 concentration of WRF-Chem model output and the dust monitoring stations in the two cities of Ahwaz and Khoramshahr from 17th to 21th January.

418 419

Figure 16 shows that the WRF-Chem model has estimated noticeable values of precipitation on

420

northern Iraq and the west and SW Iran from 18 to 21 January 2018. Having compared the model

421

output with the precipitation data reported by synoptic stations in the west and SW Iran and the

422

satellite data, it can be concluded that the model precipitation simulation is in good agreement in

423

the region.

424 425

22

426 427 428

Figure 16. Accumulated precipitation of WRF-Chem model output from 18th to 21th January 2018.

429 430

The NAAPS Model output is presented for city Basra, located on SE Iraq is shown in Figure 17.

431

The result shows that on the day of 20th January the dust concentration close to the surface is

432

increased and the vertical dispersion of dust particles is reached to the mid-levels of the

433

atmosphere. The vertical profile of dust on 21 January showed good agreement with CALIPSO

434

image displayed in Figure 8.

435

The dust pattern from NAAPS model output for the 20th January showed plenty of dust across

436

Iraq, the west and SW Iran, and the north and east Saudi Arabia. Having compared NAAPS and

437

WRF-Chem model outputs, it can be concluded that the dust pattern of both models is identical,

438

however, the dust concentration values obtained from NAAPS model is much less than the

439

WRF-Chem model. The dust estimated by NAAPS in the region is from 640 to 1260 > ⁄? ,

440

while for WRF-Chem model, the dust concentration is more than 2000> ⁄? , which is closer to

441

the PM10 concentration observed at some monitoring stations in the region.

442 443 444

23

b)

a)

445 446 447

Figure 17. NAAPS Model output of a) vertical profile of dust concentration for Basra city from 18th to 21th January b) surface dust concentration on 20th January 2018 at 00UTC.

448 449

4.Conclusion

450

While the snow and rainfall occurred simultaneously with the dust storm in the eastern and

451

central Iraq, and the west and SW Iran on 19th and 20th January 2018, which were an

452

unprecedented event. Regarding the high concentration of the dust particles in the atmosphere,

453

effected the eastern and central Iraq and the west and SW Iran, together with the winter’s cold

454

weather, they also led to several problems in these regions. The visibility in the SW Iran, the east

455

Iraq, and Saudi Arabia reduce to less than 1km, also in the vast areas in northern and central Iraq,

456

as well as in western Iran, the visibility has been below 5km. Moreover, the satellite data have

457

displayed severe rainfall in the regions of eastern Syria, the northern Iraq and west and SW Iran,

458

also in this area snowfall is observed. The average AOD on the days between 18 and 21 January

459

2018 shows high values across Iraq, Kuwait, western Persian Gulf, and north and east Saudi

460

Arabia and also in the west and SW Iran, actually, indicating the high amount of the aerosol in

461

the region. The image of a vertical feature mask of the CALIPSO satellite on the 21th January

462

2018, shows the thick cloud mass with the bottom height of 7km and the dust particles in this

463

area. The most dust concentration is at an altitude lower than 1km, but the little amount of dust

464

can be seen just below the clouds. 24

465 466

On the basis of the synoptic analysis, a strong dynamic low pressure is conformed, which its

467

troughs, reached the countries comprising Iraq and Saudi Arabia, besides, the cold and warm

468

fronts which effected this region can be observed accordingly. On this day the dust particles from

469

the southern regions of Khuzestan, transported due to southeasterly sever winds, blown in front

470

of the cold front, consequently affected most parts of this province. Also, in many dust storms,

471

the south easterly winds cause formation of prefrontal dust before reaching the cold front in this

472

region during this season. On the 20th January the cold front is set up over SW Iran and the dust

473

with more intensity is arisen from central regions of Iraq arrives in SE Iran. Additionally, the

474

west and NW Iran are affected due to a massive cloud relevant to occluded front and low

475

pressure center and therefore, it leads to rainfall in these regions.

476

Concerning the simulation of the WRF-Chem model, the model is correctly simulated the pattern

477

of the dust and its formation procedure as from eastern Syria, central and eastern Iraq, as well as,

478

SW Iran; also it has correctly shown the location of the fronts and the wind direction change,

479

which is effective in the dust particle transport in the atmosphere. Furthermore, having compared

480

the PM10 concentration data in Khoramshahr and Ahwaz, the model has well estimated the

481

changes trend, particularly in Ahwaz, but it has determined the maximum values a bit less, of

482

course. However, on the whole it can be concluded that the WRF-Chem model output is

483

acceptable at simulation stages, based on the atmospheric quantities, including wind speed,

484

precipitation and the location of the cold and warm fronts, and also the dust concentration.

485

Consequently, on the basis of the outputs of the two models, i.e. WRF-Chem and NAAPS, it can

486

be concluded that the pattern of the dust and the vertical transfer procedure concerning each

487

model is very identical, however, the NAAPS Model underestimated the dust concentration

488

quantities.

489 490

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30

Highlights

1.

Occurrence of rain and snowfall during dust episode in the Middle East.

2.

The visibility has been reduced to less than 500 m in the study period.

3.

The model shows the formation of the prefrontal dust in the region.