Assessment of dust activity and dust-plume pathways over Jazmurian Basin, southeast Iran

Aeolian Research 24 (2017) 145–160

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Assessment of dust activity and dust-plume pathways over Jazmurian Basin, southeast Iran A. Rashki a,⇑, M. Arjmand a, D.G. Kaskaoutis b a b

Department of Desert and Arid Zones Managements, Faculty of Natural Resources and Environment, Ferdowsi University of Mashhad, Mashhad, Iran Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, GR-11810 Athens, Greece

a r t i c l e

i n f o

Article history: Received 13 November 2016 Revised 14 January 2017 Accepted 16 January 2017

Keywords: Jazmurian Basin Dust storms Meteorology Transport pathways Southeast Iran

a b s t r a c t Jazmurian (or hamun-e Jaz Murian) is a dried lake located in a topographic-low basin in southeast Iran and a major source for high dust emissions under favorable weather conditions. This work examines for the first time the dust activity over the basin by classifying the dust events (DEs, visibility <10 km) and dust-storm events (DSEs, visibility <1 km) based on observations at five local meteorological stations during the period 1990–2013. Analysis of the temporal evolution, seasonality, frequency and persistence (duration) of the DEs and DSEs, along with examination of the backward and forward air-mass trajectories in the Jazmurian Basin are the main objectives of the present study. The DEs exhibit maximum frequency in June–July and lowest in autumn and winter, while the DSEs peak mostly during March–May also presenting large variability between the stations. The frequency of both the DEs and DSEs increases during 2001–2004 due to a prolonged drought over southeastern Iran, while no significant tendency is found during the period 1990–2013. Further, the DEs and DSEs exhibit a clear diurnal pattern with highest frequency between 15:30 and 18:30 LST due to thermal convection and transported dust plumes. The analysis reveals an average frequency of 12.7 dust-storm days per year, while the DSEs last for 5.1 h, on average, during the dust-storm days. The dust storms originating from Jazmurian affect mostly the northern coast of the Arabian Sea (Makran mountains), the Oman Sea, the southeastern Arabian Peninsula and the western Pakistan, while air masses from the arid/desert areas of central-eastern Iran and Arabia seem to further aggravate the dust-aerosol loading over Jazmurian. Ó 2017 Published by Elsevier B.V.

1. Introduction Dust storms, as recurring phenomena, are considered as natural hazards over the arid and semi-arid areas of the Earth (e.g., Goudie and Middleton, 2001; Yang et al., 2008; Rezazadeh et al., 2013). Dust storms may have both natural and human-induced causes and affect meteorological and climatic conditions from local to global scales (Prospero and Lamb, 2003; Rodriguez et al., 2015; Solmon et al., 2015), radiation and energy budget (Antón et al., 2011; Jish Prakash et al., 2014; Kumar et al., 2015; Valenzuela et al., 2015), ocean bio-geochemistry (Jickells et al., 2005; Jickells and Moore, 2015), ecosystems (Lin, 2002), human health (Pope, 2003; Martiny and Chiapello, 2013; García-Pando et al., 2014) and socio-economic issues (Kurosaki and Mikami, 2003; Sharifikia, 2013). Impaired visibility and high particulate matter (PM) concentrations, as consequences of the dust storms over urban environments, have been linked to traffic accidents and ⇑ Corresponding author. E-mail address: [email protected] (A. Rashki). http://dx.doi.org/10.1016/j.aeolia.2017.01.002 1875-9637/Ó 2017 Published by Elsevier B.V.

health hazards, i.e., cardiovascular and pulmonary diseases, chronic asthma, eye and nose irritation and breathing impairment (Chang et al., 2006; Holyoak et al., 2011; Sajani et al., 2011; Watanabe et al., 2011; Tam et al., 2012). Analyses based on satellite remote sensing over the globe (Prospero et al., 2002; Washington et al., 2003; Engelstaedter et al., 2006; Baddock et al., 2009; Christopher et al., 2011; Ginoux et al., 2012) highlights the importance of topographic-low basins of internal drainage (e.g. Bodélé depression in Chad, Taoudenni in Mali, Etosha Pan in Namimbia, Great Salt Lake in USA, Eyre in Australia, Tarim in west China, Aral and Balkhash in Kazakhstan, Sistan and Qom in Iran) for large dust emissions with serious impacts in the regional atmospheric environment. Furthermore, these drainage basins are very prone to saline dust storms and entrainment of large concentrations of fine-grain saline and alkaline material into the atmosphere, such as sodium chloride (NaCl), sodium sulfate (NaSO4) and other potentially toxic components (Small et al., 2001; Micklin, 2007; Abuduwaili et al., 2010; Liu et al., 2011; Gholampour et al., 2015). Numerous studies have examined the dust activity from local to global scales as a function of meteorological conditions, land

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use – land cover changes and topographic characteristics (e.g. Schepanski et al., 2009; Prezerakos et al., 2010; Gkikas et al., 2012; Pey et al., 2013; Bryant, 2013; Guan et al., 2015). In this respect, Wang et al. (2011) examined sand and dust storms over the globe in 2008 and identified four regions i.e., north Africa, Middle East, Mongolia and northwest China with highest frequency. McGowan and Clark (2008) analyzed the pathways of the dust plumes originated from the Australia’s arid lake Eyre by means of Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, finding that the dust plumes may impact the atmospheric composition and air quality thousands of kilometers downwind by transferring significant amounts of dust. Tan et al. (2012) analyzed daily observations at 43 meteorological stations in Inner Mongolia, China from 2000 to 2007 finding that annual and seasonal variations of dust storms are related to local and regional weather conditions, especially wind speed. They also examined the transport routes and altitude layers of the dust storms at two stations with the highest dust-storm frequency and simulated the dust impact on the Yellow river and East China Sea. Ge et al. (2016) assessed the potential dust trajectories in Ebinur lake, a dry region in northwest China, using the HYSPLIT model during the period 1978–2013. The results showed a distinct seasonality in dust activity maximizing in spring and summer. Alam et al. (2011) examined the spatial-temporal variability and the transport routes of dust over selected cities in Pakistan finding that the dusty air masses in winter had a longer pathway than summer; the highest dust concentrations in summer was a result of more frequent and intense dust storms coming mostly from Iranian deserts, southern Afghanistan and eastern Arabian Peninsula. Focusing on Iran, Cao et al. (2015) identified the main dust sources and areas prone to desertification based on satellite remote sensing, while Rezazadeh et al. (2013) examined the most dustaffected areas over the Middle East and Iran based on visibility data from meteorological stations. Both studies revealed that the most dust-affected areas are the Khuzestan Province in southwest Iran and the Sistan Basin in southeast Iran that borders Afghanistan. These regions have also attracted the highest scientific interest in terms of meteorological regimes controlling dust storms, Total Suspended Particulate (TSP) and PM concentrations, dust mineralogy, health impacts and socio-economic effects (e.g. Miri et al., 2007; Zarasvandi, 2009; Najafi et al., 2014; Rashki et al., 2012, 2013a,b; Sharifikia, 2013; Kaskaoutis et al., 2015a, 2016). Some studies have been also performed over Iran examining the transport routes of the dust storms occurring across the country. In this respect, Vishkaee et al. (2011, 2012) showed that the dust storms affecting the northwest part of Iran were mostly originated from western directions i.e., the Iraqi plains and Syrian desert, with lower contributions from central Asia (Aral Basin and Karakum desert) and internal sources in Iran, i.e., dried salt lakes around Qom and Dasht-e-Kavir. These findings have been verified by Masoumi et al. (2013), who quantified the contribution of each sector on dust frequency and optical properties. Similarly, the severe dusty conditions over Khuzestan are attributed to dust transported from the Iraqi plains and Fertile Crescent (Najafi et al., 2014; Notaro et al., 2015), while the Sistan Basin is considered as the most active and intense dust-source region in southwest Asia (Middleton, 1986; Kaskaoutis et al., 2015a). Furthermore, Baaqhideh and Ahmadi (2013) studied the dust evolution over west and southwest Iran via statistical analysis in 11 synoptic stations finding that the highest frequency of dust storms is in July and August. Another interesting area for examining dust emissions, concentrations, mineralogy and chemical composition is the Urmia lake in northwest Iran due to significant land use – land cover changes as a result of lake desiccation and desertification (Gholampour et al., 2015, 2016). Except for the Sistan Basin, the eastern part of Iran has remained nearly unexplored with respect to dust-storm evolution,

despite the existence of an hyper-arid topographic-low basin the Dasht-e-Lut, which is one of the major desert areas in Iran and southwest Asia (Goudie and Middleton, 2006; Ginoux et al., 2012). The southern edge of the Dasht-e-Lut forms a closed drainage basin consisting of the Jazmurian ephemeral lake, which has been totally dry during the last 15 years. The alluvial silt and saline material that has been left in the dried lake beds after desiccation is the source area for frequent and intense dust storms that strongly impact the neighboring towns and villages. The present study is the first that identifies the dust events (DEs; visibility <10 km), dust-storm events (DSEs; visibility <1 km) and examines their frequency, seasonality, duration and temporal evolution during the period 1990–2013 based on meteorological observations and visibility records at five stations located around the Jazmurian Basin. Furthermore, the areas that are mostly impacted by the dust storms originating from the Jazmurian Basin are identified via trajectory analysis. The results have significant importance for the atmospheric dust science over the southwest Asia and for aerosol studies over the northern Arabian Sea, Oman and United Arab Emirates that are mostly affected by the Jazmurian dust storms. 2. Study area Jazmurian is a low depression (topographic-low basin) in southeast Iran straddling the provinces of Kerman and Sistan – Baluchistan (see Fig. 1). It covers an area of 69,600 km2, with 300 km length and maximum width of 140 km lying at a mean elevation of 360 m. The Jazmurian Basin is surrounded by high rocky and arid mountainous ranges in excess of 2000–2500 m and is covered by a seasonal lake (hamun-e Jaz Murian), which may remain almost totally dry in years with very low precipitation. In contrast, during wetter years it can hold water during winter and spring, but shrinks in summer. Two rivers flow into the Jazmurian Basin i.e., the Bampur River from the east and the Halil Rud River from the west. However, these small rivers are not capable to bring much water to the central parts of the basin in order to feed the lake since their waters are largely or even totally removed for agricultural irrigation. In some years, during the spring season when the rivers overflow, the central basin is flooded and a very shallow (20–30 cm) slightly saline lake forms; this lake has changeable contours and dimensions with reed growth along the shore. Jazmurian lake has been covered mostly by silty-clay sediments and due to the lack of infiltration some parts of the marshy land become a seasonal lake in certain periods. The annual precipitation in the western part of the basin is about 200 mm, while other parts are very dry with average annual precipitation of less than 100 mm. In contrast, evaporation is very high and reaches to 2500 mm per year due to very hot climate (Gandomkar, 2009). An extensive drought in the beginning of 2000s has resulted in the complete drying of the lake, which has been transformed to a desert area. West and northwest winds seem to have a great impact in creating sand dunes that are mostly barchans, in the south and southeast parts of the basin. West, east and south margins of Jazmurian are considered as the most prone dust-emission regions, while the northwest-to-southeast orientation of the basin (see Fig. 1) plays an important role for the channelization of the wind, dust emissions and transport. 3. Dataset and methodology 3.1. Meteorological parameters and dust classification In order to assess the long-term dust characteristics, meteorological data from five stations located in the Jazmurian Basin (i.e., Bam, Jiroft, Kahnuj, Khash and Iranshahr) and belonging to the Iran

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Fig. 1. Geographic map of the Jazmurian drainage basin which is defined by red. The wind roses at the five meteorological stations during the period 1990–2013 are also plotted.

the DEs according to their intensity following Rezazadeh et al. (2013) as: (i) dust-in-suspension, which refers to widespread dust that is not raised near the station with visibility above 10 km, (ii) blowing dust raised at the station during the time of observation corresponding to visibility of 1–10 km, (iii) dust storm with limiting visibility between 200 and 1000 m and (iv), severe dust storm caused by violent winds and huge quantities of airborne dust/sand with visibility from a few meters to 200 m. In this study, cases with visibility below 10 km are defined as DEs, while those with visibility below 1 km as DSEs at each meteorological station. A dusty day (DD) or dust-storm day (DSD) is defined as a day with at least one DE or DSE, respectively. Since the DEs and DSEs correspond to 3-h time intervals, a DD (or DSD) may have a maximum of 8 DEs (or DSEs) if it is a persistent dusty day. The duration of the DEs and DSEs during the dusty days is defined as DE/DD  3 (or DSE/DSD  3), assuming that each event lasts 3 h (time interval between the meteorological observations). Therefore, the duration of the DEs or DSEs shows how persistent they are during the dusty days. Since each station cannot separately represent the dust characteristics over the whole Jazmurian Basin, the DEs and DSEs at each meteorological station were merged to form the dataset over the whole Jazmurian Basin. In this respect, a DE (or DSE) in the Jazmurian Basin is considered if at least one DE (or DSE) is reported at any one of the stations on a certain time interval. Similarly, the DDs (or DSDs) over the basin were defined if at least one DD or DSD was observed at any station. The duration of the DEs (or DSEs) in the Jazmurian Basin was calculated with the same procedure used

Meteorological organization (IRIMO) were used. The meteorological data consist of three-hours recordings of air temperature, relative humidity (RH), wind speed and direction, atmospheric pressure, horizontal visibility and precipitation during the period 1990–2013. Table 1 summarizes the main topographic characteristics and the meteorological parameters at each station during 1990–2013, while detailed analysis of the meteorological conditions over the sites is beyond the scope of the present study. The dataset includes a total of 8766 days with 3-h meteorological observations from which the DEs and DSEs were identified at each station. There are no missing data in the 3-h meteorological observations that are able to harm the data series, climatology and statistics at the five stations. Except of the automatic meteorological recordings, the station observers systematically monitor the prevailing atmospheric conditions and keep statistics (eight times per day at 3-h intervals) of several codes established by the World Meteorological Organization (WMO, 2005) for the monitoring of dust conditions around a station. For the nighttime visibility recordings there are light indicators at different distances that help observers to determine the visibility. The codes that are related to dust events, i.e., 6–9, 30–35 and 98 are summarized and fully explained in Table 2, and have been also marked down in each station’s log during the examined period 1990–2013. For the identification of the DEs at each station, the codes were isolated and the DEs have been considered regardless of their severity and code numbers. The measured horizontal visibility was considered as an index to group

Table 1 Topographic characteristics and meteorological parameters at the five stations in the Jazmurian Basin during the period 1990–2013. Stations

Latitude (degree)

Longitude (degree)

Elevation (m)

Wind speed (ms 1)

RH (%)

Temperature (°C)

Pressure (hpa)

Precipitation (mm)

Distance to Jazmurian lake (km)

Bam Iranshahr Jiroft Kahnuj Khash

29.06 N 27.12 N 28.35 N 27.58 N 28.13 N

58.21 60.42 57.48 57.42 61.12

1066.9 591.1 601.0 469.7 1394

3.09 2.56 1.16 3.61 2.64

25.46 27.92 43.11 39.09 28.59

23.68 27.24 25.04 27.30 21.33

893.75 942.58 938.81 952.61 856.88

55.2 107.4 17.9 190.0 159.4

208.84 165.17 185.68 150.01 235.67

E E E E E

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Table 2 WMO-SYNOP weather codes for identification of dust events. Code

Description

06

Widespread dust in suspension in the air, raised by wind at or near the station at time of observation Dust or sand raised by wind at or near the station at the time of observation, but no well-developed dust whirl(s) and no dust storm or sandstorm seen Well-developed dust whirl(s) or sand whirl(s) seen at or near the station during the preceding hour or at the time of observation, but no dust storm or sandstorm Dust storm or sandstorm within sight at the time of observation or at the station during the proceeding hour Slight or moderate dust storm or sandstorm has decreased during the preceeding hour Slight or moderate dust storm or sandstorm no appreciable change during the preceding hour Slight or moderate dust storm or sandstorm has begun or has increased during the preceding hour Severe dust storm or sandstorm has decreased during the preceeding hour Severe dust storm or sandstorm no appreciable change during the preceding hour Severe dust storm or sandstorm has begun or has increased during the preceding hour Thunderstorm combined with dust storm or sandstorm at time of observation

07

08

09 30 31 32 33 34 35 98

at each of the stations and constitutes an average situation over the basin. 3.2. Backward and forward air-mass trajectories – HYSPLIT model The HYSPLIT model (Draxler and Hessm, 1998; Draxler and Rolph, 2003) was used to examine the backward and forward air masses that are associated with the Jazmurian dust storms. Meteorology data from NCEP/NCAR reanalysis (available 1948 – present at http://ready.arl.noaa.gov/HYSPLIT_traj.php) were used for the trajectory simulations and for the vertical velocity field, which is very important for studying the altitudinal variation of the air masses along their transport pathways (Rashki et al., 2015). Therefore, the altitudinal variation along the air-mass transport is examined in order to give special information if the air-mass interacts or not with the boundary layer, thus being able to uplift dust from the ground and/or to impact air quality via dust dry deposition. More specifically, HYSPLIT was run for 72-h forward trajectories with a time interval of 6 h, starting at the elevation of 500 m above ground level at the central pixel (27.15° N, 60.59° E) of the Jazmurian Basin. These forward trajectories were computed on the dust-storm days aiming at identifying the impacted areas by the dust storms originating from Jazmurian. The starting altitude of 500 m was selected in order the dusty air masses to be able to overcome the coastal Makran mountains (1500 m) following Rashki et al. (2015), and it is assumed representative (well within the boundary layer) for air-masses originating from Jazmurian. The backward air-mass trajectories were also computed at 500 m over each meteorological station on the DSDs aiming to identify the regions and transport routes that may contribute to the deterioration of air quality (so, the ending altitude is within the boundary layer) over the Jazmurian Basin. 4. Results 4.1. Annual variability of DEs and DSEs over Jazmurian Basin Using the methodology described in Section 3.1, the DEs and DSEs were identified for 3-h observations at each meteorological station, and then, were merged at the five stations and for various time intervals computing diurnal, daily, monthly, seasonal and

annual data series. The temporal variation of the frequency of the DEs (Fig. 2a) reveals a wide annual range with maximum in spring and summer months and minimum in autumn and winter also associated with a significant inter-annual variability during 1990–2013. More specifically, for certain years like 2001, 2003, 2006 and 2008 the frequency of the DEs maximizes, with an annual maximum of 714 DEs in 2003. The period with the maximum frequency of DEs (2001–2003) is consisted with the extreme drought that hit southeastern Iran during 2000–2004 and resulted in desiccation of the Jazmurian lake and Hamoun lakes in Sistan (Rashki et al., 2013a) transforming both basins into saline dried playas with considerable increase in dust emissions (Rashki et al., 2012). Moreover, the large number of 119 DEs during June 2008 was attributed to favorable meteorological conditions (persistent intense winds and desiccation of the Hamouns) for dust emissions from the arid areas of Sistan, Jazmurian and Makran mountains that resulted in abnormal aerosol loading over the north and central parts of the Arabian Sea (Prijith et al., 2013; Kaskaoutis et al., 2014). In contrast, in some years like 1996 (231 DEs) and 2013 (193 DEs) the frequency of the DEs was much lower due to suppression dust emissions during spring and summer as a result of increased precipitation. It is observed that in 1996 the precipitation is higher (196 mm in the first half of the year), while in 2013 there is rainfall during the summer season (20 mm in June– September) (see Fig. 2c). As a result, the maximum number of 130 DEs during the summer months of 2001 and 2003 dropped to 40 DEs during the summer months of the years with the lowest dust activity. Based on the observations at the five meteorological stations, 9919 DEs were identified during 1990–2013, indicating a mean of 413 DEs with a total duration of 1239 h per year. These DEs correspond to 14.1%, on average, of the total 3-h meteorological observations per annum. On the other hand, the temporal evolution of the monthly DSEs (Fig. 2b) during the period 1990–2013 reveals significant intraseasonal and very large inter-annual fluctuation and, in general, a different pattern to that found for the DEs. The DSEs maximize (15–18 events) in different months during the years 1995, 2003, 2007, 2008, 2012, while there is a long period (1996–2000) with very low occurrence of DSEs (below 17 DSEs per year) over the Jazmurian Basin, which coincides well with the period of lowest dust activity in the Sistan Basin (Rashki et al., 2013a). The total number of the DSEs over the Jazmurian Basin during 1990–2013 is 521. Both DEs and DSEs do not exhibit any tendency during 1990– 2013, with some suggestion of correlation with monthly precipitation (r = 0.22 for DEs and r = 0.03 for DSEs). However, detailed examination of the role of precipitation (annual or seasonal) on the dust activity at each station and in the whole Jazmurian Basin is beyond the scope of the present work. Tables 3 and 4 summarize the slope values computed from the linear regressions of the long-term trends of DEs and DSEs, respectively on monthly, seasonal and annual basis at each station separately and in the Jazmurian Basin. In the vast majority of the cases, the slope values are very close to zero indicating negligible trends in the frequency of the DEs and DSEs from 1990 to 2013. On an annual basis, the highest increasing trend for DSEs is detected at Bam station (0.4 per year), while Iranshahr and Khash exhibit negative trends ( 0.16 per year) (Table 4). The annual trend in the Jamurian Basin is found to be negative for the DEs ( 0.75 per year) and positive (0.10 per year) for the DSEs. On the other hand, increasing trends in DEs ranging from 2.1 to 3.2 are found at Iranshahr and Jiroft, respectively, while Kahnouj exhibits a negative trend ( 4.0 per year). It is worth noting that the Jazmurian Basin exhibits mostly declining trends in the frequency of DEs, but without any statistically significance, except for autumn (Table 3). The annual fluctuation of the DEs in the Jazmurian Basin during 1990–2013 reveals the presence of 413.3 ± 123.7 DEs per year, on

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DE

135 90 45 0

20

DSE

15 10 5

Precipitation (mm)

0

120 100 80 60 40 20 0

1990

1991

1992

1993

1994

1995 1996

1997

1998

1999

2000

2001 2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

Fig. 2. Multi-year variability of the frequency of monthly DEs, DSEs and monthly accumulated precipitation in the Jazmurian Basin during 1990–2013. The monthly accumulated precipitation corresponds to the mean value at the five stations.

Table 3 Slope values of the linear regression trends for the DEs at the five meteorological stations and in the Jazmurian Basin on monthly, seasonal and annual basis during the period 1990–2013. The statistically significant trends at 95% confidence level are highlighted in bold. Bam

Iranshahr

Jiroft

Kahnouj

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

0.18 0.30 0.22 0.02 0.39 0.21 0.03 0.01 0.26 0.21 0.22 0.09

0.03 0.06 0.02 0.12 0.17 0.58 0.71 0.40 0.38 0.12 0.02 0.00

0.12 0.46 0.73 0.28 0.26 0.64 0.35 0.29 0.08 0.08 0.13 0.06

0.06 0.16 0.15 0.88 0.39 0.18 0.69 0.68 0.70 0.04 0.09 0.12

0.04 0.02 0.18 0.20 0.63 0.06 0.18 0.14 0.06 0.02 0.03 0.30

0.23 0.32 0.30 0.45 0.46 0.77 0.14 0.34 0.56 0.20 0.28 0.23

Spring Summer Autumn Winter

0.16 0.28 0.34 0.70

0.63 1.48 0.10 0.12

1.19 0.72 0.01 1.32

1.46 2.07 0.25 0.25

0.89 0.38 0.34 0.24

0.60 0.58 1.05 0.32

2.12

3.24

4.03

1.86

0.75

Annual

0.07

Khash

Jazmurian Basin

Table 4 Same as in Table 3, but for the DSEs. Bam

Iranshahr

Jiroft

Kahnuj

Khash

Jazmurian Basin

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

0.08 0.03 0.22 0.02 0.04 0.05 0.00 0.02 0.00 0.00 0.01 0.01

0.05 0.02 0.02 0.06 0.00 0.00 0.07 0.05 0.02 0.00 0.00 0.02

0.06 0.07 0.02 0.01 0.03 0.03 0.01 0.00 0.05 0.00 0.03 0.01

0.10 0.02 0.04 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00

0.00 0.00 0.01 0.01 0.09 0.01 0.04 0.01 0.02 0.00 0.00 0.00

0.23 0.06 0.16 0.04 0.15 0.09 0.10 0.05 0.06 0.00 0.02 0.02

Spring Summer Autumn Winter

0.03 0.03 0.02 0.33

0.06 0.14 0.02 0.05

0.01 0.04 0.03 0.11

0.00 0.02 0.00 0.17

0.09 0.06 0.00 0.01

0.03 0.06 0.08 0.28

Annual

0.40

0.16

0.05

0.15

0.16

0.10

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average, exhibiting a remarkable year-to-year fluctuation from 193 DEs (in 2013) to 714 DEs (in 2003) (Fig. 3a). On the other hand, the annual-mean frequency of the DSEs is found to be 21.7 ± 10.3, ranging from 4 in 1998 and 2011 to 40 in 2003 and 2012 (Fig. 3b). The DSEs correspond to the 5.2% of the DEs in the Jazmurian Basin, exhibiting a low frequency compared to the Sistan Basin (Rashki et al., 2012; Kaskaoutis et al., 2015a). An important feature of the annual fluctuation patterns is that the dust activity exhibits different seasonality and temporal variation depending on its intensity. Therefore, the dustiest years concerning the DEs (i.e., frequency of DEs above the mean + 1 stdev) are the 2001, 2003, 2004 and 2008, while the highest frequencies of the DSEs are noted in 2003, 2004, 2007 and 2012. Similar inconsistency is also found for the less dusty years. This indicates that the dust storms over the Jazmurian Basin are mostly a result of local meteorological and topographic factors and/or short-lasting violent phenomena. This will be also shown by examining the seasonal and monthly variations of the DEs and DSEs (Section 4.2). 4.2. Seasonal variability of DEs and DSEs over Jazmurian Basin The seasonal trends in DEs over the Jazmurian Basin reveal negative values in spring and autumn and positive in winter and summer (Table 3). Kahnouj and Khash stations exhibit negative tendencies in all seasons, as well as Bam (except of winter), while

700 (a) 600

Mean = 413.3 SD = 123.7 Min = 193 Max = 714

+1SD=537.0

Frequency of DEs

500 400 -1SD=289.5

300 200 100 0

8 6 0 0 4 2 8 2 6 0 2 4 199 199 199 199 199 200 200 200 200 200 201 201

Year

45 40

Frequency of DSEs

35

(b)

M ean = 21.7 SD = 10.3 M in = 4 M ax = 40

+1SD=32.0

30 25 20 15

-1SD =11.4

10 5 0 19

90 992 994 996 998 000 002 004 006 008 010 012 2 2 2 2 2 2 2 1 1 1 1

Y ear

Fig. 3. Annual frequency of the DEs (a) and DSEs (b) in the Jazmurian Basin during 1990–2013. The mean and one-standard deviation values are also given and plotted in the graphs.

Iranshahr and Jiroft exhibit mostly positive trends. On the other hand, the seasonal evolution of the DSEs shows that in all seasons except winter the trends are negative but not statistically significant (Table 4). In synopsis, the regression analysis reveals that the frequencies of occurrence for both DEs and DSEs rarely exhibit statistically significant trends, indicating low tendency in dust activity over Jazmurian during 1990–2013, but remarkable variability between the years and stations. The variability of the seasonal DEs in the Jazmurian Basin during 1990–2013 is shown in Fig. 4a. The frequency of the DEs in spring (3131) and summer (4470) is more than double compared to the other seasons (1124 in autumn and 1194 in winter). The drought period in the beginning of the 2000s in southeast Iran seems to impact the DEs only during spring and summer by increasing their frequency and leaves nearly unaffected the other seasons. In contrast, the temporal variation of the seasonal DSEs in the Jazmurian Basin during 1990–2013 (Fig. 4b) reveals large variability from year to year as well as between the seasons and a more complicated pattern compared to the DEs (Fig. 4a). In general, spring and summer show a tendency for more DSEs during 2001–2004 due to drought conditions, but the maximum frequencies of the DSEs occur in other years highlighting localized effects on these episodes. In this respect, the multi-year variability of the DSEs in autumn is very low, with frequencies usually below 5 DSEs per year except of 1995, which shows 14 DSEs. It is noticed that from 2005 to 2013 (except of 2007) there was no DSE in autumn over Jazmurian. Further, winter exhibits a totally different variability and trend in DSEs with a prolonged non-active duststorm period from mid-1990s to 2006 (except of 2003). Afterwards, the winter DSEs present high frequency in 2008 and 2012 with 19 and 17 DSEs, respectively, resulting in the increasing trend (Table 4), which would vanish if these years were excluded. On seasonal-mean basis, the DEs over the Jazmurian Basin during 1990–2013 exhibit the highest frequency in summer (45.1%) and spring (31.6%) and they are rare in winter (12%) and autumn (11.3%) (Fig. 5a). In contrast, the frequency of the DSEs (Fig. 5b) maximizes in spring (43.2%) against summer (28.6%), while an important increase is shown in winter (19.6%) compared to the DEs. The DSEs in autumn present even lower frequency (8.6%) compared to the DEs. Therefore, there is a shift in the frequency of the DSEs towards late winter and early spring compared to the summer maximum for the DEs. This is a very important finding, since several studies over the Middle East and southwest Asia showed that the dust emissions present their highest frequency in late winter and spring over the western parts of the region (Prospero et al., 2002; Kim et al., 2011; Awad and Mashat, 2013; Najafi et al., 2014; Rezazadeh et al., 2013), while over eastern Iran the highest frequency of dust events occurs in summer favored by the presence of the strong Levar wind (Alizadeh Choobari et al., 2013, 2014; Kaskaoutis et al., 2015a). Therefore, although the seasonality of the DEs (Fig. 5a) in the Jazmurian Basin is in general agreement with previous studies that show a maximum frequency during summer, the high frequency of the DSEs during spring and winter (Fig. 5b) shows great similarity with the western parts of the Middle East and Arabia, where the influence of the EastMediterranean troughs and frontal systems may cause intense dust storms during winter (Najafi et al., 2014; Mashat et al., 2016). Table 5 summarizes the seasonal-mean frequencies for the DEs and DSEs at the five meteorological stations during the period 1990–2013. On annual-mean basis, the largest number of DEs (159.5 ± 64.4) is shown at Iranshahr, located in the southeastern edge of the Jazmurian Basin, followed by Bam located in the northwest part (149.2 ± 94.4), while the DEs in Jiroft and Kahnouj are about 50–55 per year. The DSEs constitute the 5.2% of the DEs in the Jazmurian Basin; this fraction ranges from 3.2% in Kahnouj to 6.9% in Jiroft on annual-mean basis. The large differences in

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350

Frequency of DEs

300

Winter Spring Summer Autumn

(a)

250 200 150 100 50 0 1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

2012

Year 35

(b)

Winter Spring Summer Autumn

Frequency of DSEs

30 25 20 15 10 5 0 1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

2012

Year Fig. 4. Annual variability of the seasonal DEs (a) and DSEs (b) in the Jazmurian Basin during the period 1990–2013.

Fig. 5. Percentage (%) frequency distribution of the seasonal DEs (a) and DSEs (b) in the Jazmurian Basin during the period 1990–2013.

seasonality and number of DEs, and especially DSEs, between the stations is attributed to local factors and specific topographic, surface and soil-moisture conditions along with the human disturbances that dominate at each station. Therefore, locality seems to play an important role in dust emissions, seasonality of dust activity, visibility impairment and air quality assessment around the Jazmurian Basin. 4.3. Monthly variability of DEs and DSEs over Jazmurian Basin It has been seen that the DEs and DSEs exhibit a distinct seasonality that differs depending on the intensity of the dusty conditions in the Jazmurian Basin. Fig. 6a, b shows the monthly-mean vari-

ability of the frequency of the DEs and DSEs, respectively at each of the five stations. The DEs exhibit a well-defined annual variation with autumn/winter minima and spring/summer maxima for all the stations, but with large differences between them. The stations Jiroft and Kahnouj exhibit a relative small monthly variability with less than 10 DEs in all months (Fig. 6a). In contrast, Bam and Iranshahr although exhibit low numbers of DEs (below 10) during September–February, the maximum frequency in June–August months may reach up to 25–30 events. The respective monthly-mean variability of the DSEs (Fig. 6b) is much more complicated without exhibiting a clear annual pattern and with large differences between the stations, highlighting the major role of locality in the intense dust emissions. However, in most stations

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Table 5 Seasonal and annual mean frequency of the DEs and DSEs at the five meteorological stations in the Jazmurian Basin during 1990–2013. Station

Winter

Spring

Summer

Autumn

Annual

Bam

DSE DE

1.6 ± 0.8 16.6 ± 17.7

3.8 ± 3.3 51.1 ± 43.9

1.2 ± 0.7 66.4 ± 43.7

0.3 ± 1.4 15.1 ± 11.9

6.9 ± 5.4 149.2 ± 94.4

Iranshahr

DSE DE

1.4 ± 1.7 16.7 ± 10.7

2.6 ± 2.8 48.6 ± 26

2.7 ± 2.1 78.5 ± 36.2

0.4 ± 1.3 15.7 ± 5.4

7.1 ± 4.5 159.5 ± 64.4

Jiroft

DSE DE

1.0 ± 3.1 6.7 ± 21.3

1.1 ± 1.6 20.4 ± 19.5

0.6 ± 2.1 18.7 ± 14.8

0.8 ± 1.3 6.6 ± 5.3

3.6 ± 4.3 52.3 ± 50.9

Kahnouj

DSE DE

0.6 ± 3.4 6.1 ± 12.5

0.8 ± 0.5 18.7 ± 20.3

0.4 ± 0.4 25.9 ± 31.6

0.1 ± 0.4 5.6 ± 5.3

1.9 ± 3.3 56.2 ± 60.6

Khash

DSE DE

0.8 ± 1.8 18.7 ± 12

2.8 ± 1.7 37.8 ± 25.7

1.5 ± 2.1 46.7 ± 24.2

0.4 ± 1.1 11.9 ± 5.2

5.6 ± 3.8 115.2 ± 55

35 30

Bam Iranshahr Jiroft Kahnouj Khash

(a)

20 15 10 5 0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month 2,0 Bam Iranshahr Jiroft Kahnouj Khash

(b) Frequency of DSEs

1,6

4.4. Diurnal variability of DEs and DSEs over Jazmurian Basin In addition to the seasonal and monthly variability, both DEs and DSEs exhibit a well-defined diurnal pattern over the Jazmurian Basin as shown in Fig. 8. Therefore, on annual mean basis, about 60% of the DEs and DSEs occur during the three 3-h time intervals from morning (9:00 UTC) to late afternoon (18:00 UTC;+3:30 LST), while the maximum frequencies for both DEs and DSEs (22–24%) occur during the time interval 12:00–15:00 UTC (15:30–18:30

1,2

18 0,8

DEs DSEs

16 14

0,4

0,0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Fig. 6. Monthly-averaged frequency of DEs (a) and DSEs (b) at each station around the Jazmurian Basin during 1990–2013.

Frequency (%)

Frequency of DEs

25

On monthly-mean basis, July contributes more (17.1%) to the annual DEs, followed by June (14.1%) and August (13.9%), while the lowest occurrence of the DEs occurs in November and December (2.2%) (Fig. 7). On the other hand, March contributes 17.3%, on average, to the annual DSEs followed by May (15.0%) and August (10.9%), while an important fraction (8–9%) of the annual DSEs occur in January and February (Fig. 7). The autumn months exhibit the lowest dust activity over the Jazmurian Basin regardless of their intensity (see also Fig. 5a, b). The (%) frequency of the DSEs is larger compared to the DEs from November to May (Fig. 7), indicating a shift in the frequency of the DSEs towards winter and spring months. As stated above, this may be attributed to the influence of the East-Mediterranean troughs and frontal systems that favor dust storms over the eastern Mediterranean, Middle East and Arabia during the February–April period (Gkikas et al., 2012; Kaskaoutis et al., 2012; Najafi et al., 2014; Jish Prakash et al., 2014), which is objective of future research.

12 10 8 6 4 2

the largest frequency of the DSEs is shown between March and May with a significant decrease afterwards, except of the eastern stations (Iranshahr and Khash), which exhibit peaks in August implying more influence by the northern Levar wind during summer.

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Fig. 7. Monthly-mean frequency distribution (%) of the DEs and DSEs in the Jazmurian Basin during 1990–2013.

A. Rashki et al. / Aeolian Research 24 (2017) 145–160

LST). More or less similar diurnal patterns exist in all stations, since the majority of the DEs and DSEs occur during 12:00–15:00 UTC (Table 6). In contrast, the dust activity lowers significantly during nighttime with only 5–10% of the DEs and/or DSEs occurring in these time intervals (Fig. 8). The diurnal variation of the dust activity over the Jazmurian Basin is mostly driven by the local meteorological conditions, i.e., the increase in the surface wind during noon-to-early afternoon hours (not shown) as a result of the radiative heating of the ground and the increase in convective activity and buoyancy. Therefore, the thermal heating of the ground and the upward movement of the air favors the blowing of stronger surface winds, thus favoring dust uplift and transport (Bou Karam et al., 2008). Higher wind intensities during noon-to-early afternoon hours associated with increase in dust emissions and PM10 concentrations were also observed over Zahedan (Rashki et al., 2013b), located approximately 400 km northeast from Jazmurian. On the other hand, the wind speed maximizes during the morning hours over the Sistan Basin resulting in uplift and southward transport of large quantities of dust from the dried Hamoun lake beds (Alizadeh Choobari et al., 2013; Kaskaoutis et al., 2015a). The Jazmurian Basin is in the downwind direction of the Sistan dust storms, especially during spring and summer as will be seen in Section 4.6. Taking into account that the dust storms over Sistan are associated with wind speeds of 15–20 m s 1 (Kaskaoutis et al., 2015a), and Sistan is located 600 km north from Jazmurian, it is estimated that the Sistan dusty air masses need about 8–10 h to reach Jazmurian and, therefore, influence the basin during afternoon hours. It should be noted that more or less similar diurnal patterns for the DEs and DSEs were found in all seasons, while detailed investigation of the role of wind on dust activity over the basin is subject of future work. 4.5. Duration of the DEs and DSEs over Jazmurian Basin An important characteristic of the dust events apart from their intensity is their duration, which strongly affects the dry deposition, the persistence of the visibility degradation and PM concentrations near the ground (Rashki et al., 2012). Therefore, the duration of the DEs and DSEs over the Jazmurian Basin is an important parameter for the deterioration of air quality and impacts on human health, and very crucial to be accurately forecasted in order to take appropriate measures for mitigating the effects. The temporal evolution of the DEs, dusty days (DDs) and duration of the DEs are shown in Fig. 9a, b on annual and monthly basis, respectively.

24

DEs DSEs

Frequency (%)

20 16 12 8 4 0

:30 0/3

:30 3/6

:30 6/9

0 30 30 30 30 2:3 00: 21: 18: 15: 9/1 21/ 18/ 15/ 12/

UTC/LST Fig. 8. Mean diurnal frequency (%) distribution of the DEs and DSEs in the Jazmurian Basin during 1990–2013.

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The same analysis for the DSEs, dust-storm days (DSDs) and duration of the DSEs is shown in Fig. 10a, b. Since the temporal variability of the DEs and DSEs was well documented in the previous sections, here we focus on the DDs, DSDs and duration of the events. The DDs in the Jazmurian Basin range from 91 in 1996 to 203 in 2003, exhibiting a mean of 145.5 ± 26.7 per year (Fig. 9a). In general consistency with the annual evolution of the DEs, the frequency of the DDs increases after 2000 due to drought conditions over eastern Iran and the complete dryness of the Jamurian lake. During the last decade a decreasing tendency of the DDs is observed similar to that found in Sistan (Rashki et al., 2012). On annual basis, the duration of the DEs is found to be 8.4 ± 1.3 h, corresponding to 2.8 DEs on each dusty day. The duration of the DEs does not exhibit a significant trend during 1990– 2013, except of continuous peaks and gaps (7 to 11 h) from the end-1990s to mid-2000s and a decreasing tendency afterwards (Fig. 9a). On monthly basis, the duration of the DEs is higher in spring and summer (9 h) and drops to 6.5 h in autumn. Further, the DDs exhibit an increasing tendency from January to July and a decrease afterwards, following the annual pattern of the DEs (Fig. 9b). On the other hand, the analysis reveals 12.7 ± 5.4 DSDs per year (305 during the period 1990–2013) in the Jazmurian Basin, while the DSEs last for 5.1 ± 1.2 h, on average, which corresponds to a mean of 1.7 DSEs per DSD. The DSDs are more frequent during the beginning of the 2000s (highest frequency of 21–22 DSDs in 2003–2004) due to prolonged dry conditions, while the mid1990s exhibit the lowest number (<10 DSDs per year) of DSDs (Fig. 10a) due to wetter weather conditions and higher precipitation (see Fig. 1). Similarly to the present findings, Rashki et al. (2013a) found much lower frequency of dusty days in the Sistan Basin during mid-1990s due to increased precipitation, water coverage in the lakes, soil moisture and living (green) vegetation of the area. Furthermore, a general decreasing tendency in DSDs, as was also seen for the DEs and DDs (Fig. 9a), is observed during the last decade in agreement with the declining trend in Sistan (Rashki et al., 2012, 2013a). The duration of the DSEs does not exhibit a significant tendency during 1990–2013, ranging from 3 h to 8.5 h, on average. The most persistent dust storms over the Jazmurian Basin were observed in 1995, 1996 and 2012 with a mean duration of 7.1, 7.8 and 8.6 h, respectively. On monthly-mean basis, the DSDs over the Jazmurian Basin are more frequent from March to August, ranging from 1.3 and 2.0 days per month, compared to less than 0.8 days per month during autumn and winter (Fig. 10b). October exhibits the lowest frequency of DSDs (0.2) and the lowest duration of 3 h, since dust storms were absent in some of the stations i.e., Bam, Khash and Kahnooj. The duration of the dust storms shows a continuous decreasing tendency from January (6.7 h) to August (3.7 h). The highest duration of the DSEs during winter is an important finding for the dust activity over the Jazmurian Basin. Therefore, despite the lower frequency of the DSDs, the large duration of the dust storms during winter contributes to accumulation of dust aerosols over the region and to deterioration of air quality that is a threat to the local population, indicating that the Jazmurian Basin remains an active dust source throughout the year. Analysis of the visibility records at Zabol, in the Sistan Basin, revealed the presence of 356 dust-storm days (vis <1 km) in the summer months (June–September) during the period 2001–2012, corresponding to approximately 30 DSDs per season or 25% of the summer days were classified as DSDs (Kaskaoutis et al., 2015a). Furthermore, Alizadeh Choobari et al. (2013) reported an average of 167 dusty days per year over Sistan, but without classifying them according to their severity, while Middleton (1986) reported more than 30 dust storms per year originating from Sis-

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Table 6 Mean diurnal frequency distribution (%) of the DEs and DSEs at the 3-h time intervals in the five meteorological stations during 1990–2013. 00:00 UTC corresponds to the time interval 00:00–03:00 UTC and same for the rest of the columns. Station

UTC LST

00:00 03:30

03:00 06:30

06:00 09:30

09:00 12:30

12:00 15:30

15:00 18:30

18:00 21:30

21:00 00:30

Bam

DSE (%) DE (%)

9.6 7.1

10.2 6.8

12.0 11.1

12.7 18.4

20.5 21.1

14.5 16.3

10.2 10.2

10.2 9.2

Iranshahr

DSE (%) DE (%)

3.5 1.8

9.4 6.7

8.2 7.9

12.9 16.1

29.8 28.6

24.6 25.1

7.0 9.4

4.7 4.4

Jiroft

DSE (%) DE (%)

4.4 0.9

10.3 8.4

10.3 13.5

7.4 19.7

32.4 27.0

25.0 26.1

7.4 3.1

2.9 1.2

Kahnouj

DSE (%) DE (%)

2.3 2.6

9.3 4.8

20.9 10.7

20.9 17.9

23.3 22.5

20.9 26.5

2.3 9.9

0.0 5.1

Khash

DSE (%) DE (%)

3.0 3.5

14.2 5.8

17.9 11.2

21.6 23.8

19.4 26.0

21.6 20.0

2.2 5.3

0.0 4.3

35

18

30

18

25

15

20

12

15

9

10

6

5

3

9

200

6

100

3

0

0

Year

80

60

(b)

DE DD Duration

12

30

9

20

6

10

3

0

0

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

3,5

18

40

21

0

24

4,0

21

15

DSE DSD Duration

Year

24

50

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

(a)

Duration (hrs)

12

300

70

Duration (hrs)

15 400

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Frequency DE, DD

21

24

Month Fig. 9. Annual (a) and monthly (b) variability of the DEs, DDs and duration of the DEs in the Jazmurian Basin during 1990–2013.

tan. The present results reveal that the frequency of the DSDs (12.7 ± 5.4 per year) or even the DDs (145.5 ± 26.7 per year) in the Jazmurian Basin is much lower compared to Sistan. 4.6. Backward air-mass pathways on DSDs over Jazmourian Basin This section examines the transport pathways and the source areas that influence the aerosol loading and visibility at the meteorological stations in the Jazmurian Basin focusing on the DSDs during the period 1990–2013. The back-trajectory pathways ending at the northwestern station of Bam and the southeastern

DSE DSD Duration

(b)

21

3,0

18

2,5

15

2,0

12

1,5

9

1,0

6

0,5

3

0,0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Duration (hrs)

500

40

Frequency DSE, DSD

(a)

24

Duration (hrs)

Frequency DE, DD

600

DE DD Duration

Frequency DSE, DSD

700

0

Month Fig. 10. Same as in Fig. 9, but for the DSEs, DSDs and duration of the DSEs.

station of Iranshahr at 500 m agl are shown in Figs. 11 and 12, respectively. It should be noted that the back-trajectory pathways for the rest of the stations are more or less similar to Figs. 11 and 12. To the best of our knowledge, the current work is the first that examines the source regions that affect the atmospheric conditions, as well as the dust-transport pathways and the areas affected by the dust storms originating from the Jazmurian Basin. The majority of air masses reaching Bam from western directions in winter and autumn traverse the Middle East and centralsouth Iran (Fig. 11). Although these air masses are travelling at high altitudes (>3–4 km) above the arid areas of the Middle East,

A. Rashki et al. / Aeolian Research 24 (2017) 145–160

in certain cases they may carry significant amounts of dust from these regions, since several frontal dust storms occur during winter (e.g., Awad and Mashat, 2013; Najafi et al., 2014; Hamidi et al., 2013; Awad et al., 2016). In contrast, the air masses originating from central Iran and the southeastern Arabian Peninsula travel at low altitudes (below 1500 m) and are capable of transferring significant amounts of dust, especially from the Dasht-i-Kavir and Dasht-i-Lut deserts in Iran and the Rub-Al-Khali desert in Saudi Arabia. In spring, there is an increase in trajectories coming from the north, mostly originating from the Karakum desert in Turkmenistan and traversing along the eastern side of Iran. These air masses are able to transfer significant amounts of dust from the Karakum desert and Aral Basin (Masoumi et al., 2013; Kaskaoutis et al., 2015b), as well as from Sistan, and affect the temporal evolution of the dust storms in Bam. The fraction of air masses from the north increases during summer due to intensification of the Levar wind along eastern Iran, while air masses from northern directions during the rare DSDs in autumn are nearly absent. It is characteristic that the air masses coming from northern directions either are travelling at lower altitudes, as shown in spring, or present a decreasing altitude as they approach Bam (summer). At Iranshahr station (Fig. 12), south and southwest air masses dominate during the winter DSDs, which originate mostly from the Oman and Rub-Al-Khali deserts. These air masses are enriched with dust aerosols since they are travelling within the lower boundary layer and are able to entrain dust (Meloni et al., 2007; Kaskaoutis et al., 2012). Furthermore, before reaching Iranshahr they are crossing the arid Jazmurian Basin, which may also contribute to the DSDs over Iranshahr in winter. In spring, the air masses originate from three well-defined directions, i.e., north, west and south, while in summer there is a shift to dusty air masses coming from the north. The north air masses are travelling at higher altitudes for distances far away from Iranshahr and within the boundary layer (<1.5–2 km) as they approach the mea-

155

suring site, being able to carry large amounts of dust aerosol from the Karakum desert and Sistan. Finally, the few dust storms in autumn are mostly associated with air masses coming from southern Iran and Oman. 4.7. Transport pathways and affected areas from the Jazmurian dust storms Fig. 13 shows the three-days forward air-mass trajectories on the DSDs in each season. The air masses originate at 500 m agl from the central Jazmurian Basin (27.15° N, 60.59° E) where the dry lake exists and are mostly travelling towards the east and south during winter and affecting southeast Iran, west Pakistan, the north Arabian Sea and the Omani coast. The dusty air masses preferably travel at lower altitudes (below 2 km) in winter, while in spring and summer they rise to higher altitudes and transport longer distances away from the source. In spring, the dust storms travel nearly all directions around the Jazmurian Basin due to changing atmospheric circulation and mostly affect the nearby areas, as well as the Persian Gulf countries, i.e., the United Arab Emirates, Oman and Saudi Arabia and south Afghanistan at altitudes above 2.5 km. In certain circumstances, they may also reach central-north Pakistan and west India at higher altitudes without exhibiting significant impact on air quality and PM concentrations. In summer, the north Levar wind pushes the dust storms originating from the Jazmurian Basin in southward directions affecting the southern Iran and the Makran mountains, the eastern Arabian Peninsula, Oman Sea and the north and central Arabian Sea. In certain circumstances, the Persian Gulf and coastal Pakistan are also affected since the dust storms are travelling within the boundary layer (<2 km), thus favoring dust deposition over these areas. It is characteristic that the summer air masses rise in altitude over the northern coast of the Arabian Sea (24°–25° N) where the Intertropical Convergence Zone exists (Kaskaoutis et al., 2015b),

Fig. 11. 72-h backward air-mass trajectories ending at 500 m above Bam on the dust-storm days of each season during 1990–2013. The trajectory pathways are color-scaled according to their altitude.

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Fig. 12. Same as in Fig. 11, but for the Iranshahr station.

Fig. 13. Three-days forward air-mass trajectories starting from the Jazmurian Basin at 500 m above ground level on the dust-storm days during 1990–2013. The trajectory pathways are color-scaled according to their altitude.

A. Rashki et al. / Aeolian Research 24 (2017) 145–160

and this zone of strong convergence moves the dusty air masses to upper altitudes (>2.5 km). This prevents the central Arabian Sea from being largely impacted by the Iranian dust storms as was documented in previous works (Kaskaoutis et al., 2015b; Rashki et al., 2015). In spite of the great similarities, the Jazmurian dust-storm pathways in summer do not exhibit a shift to northeastern directions after passing over the north coast of the Arabian Sea, as shown for the dusty air masses originating from Sistan (Rashki et al., 2015). This suggests that different atmospheric systems control the Sistan and Jazmurian air-mass pathways, since the thermal low over western Pakistan (Alizadeh Choobari et al., 2014; Kaskaoutis et al., 2015a) does not seem to significantly affect the dusty air masses originating from Jazmurian. Finally, the dust storms during autumn, generally follow the winter pattern mostly affecting southeast Iran, the Makran mountains and Oman Sea, travelling at altitudes <1.5 km. Fig. 14 presents the density plot of the 3-days forward air-mass trajectories at 1°  1° spatial resolution, i.e., the total number of the dusty air-masses crossing each 1°  1° pixel; the arrows show the preferable directions of the dust storms originated from the Jazmurian Basin in each season. This provides useful information about the preferred pathways that the dust storms follow and indicates how often a downwind region is affected by Jazmurian dust storms. In general, the higher density, associated with larger impact, is shown over areas neighboring the Jazmurian Basin, where the largest dust particles are deposited. Further, the Oman Sea, northern part of the Arabian Sea and western Pakistan are strongly impacted, as observed during the severe dust storm on 10 October 2001 (Walker et al., 2009). In addition to the multiple impacts on the downwind affected areas, on desertification and damage of ecosystems due to dust deposition, on air quality and human health, the Jazmurian dust storms cause severe soil loss from the basin and removal of organic matter and soil nutrients, thereby reducing agricultural productivity (Gerivani et al., 2011).

157

In synopsis, the analysis reveals that except for the Arabian Peninsula and the Sistan Basin, which are the main dust sources that affect the north Arabian Sea (Tindal and Pease, 1999; Kaskaoutis et al., 2014), the Jazmurian dust storms may also have a significant impact.

5. Discussions and further challenges The monthly, annual and multiyear variations in the frequency of the DEs and DSEs are driven by changes in wind erosion, uplift and transport of dust as well as by long-term trends in dust activity due to possible changes in regional climate (e.g., drought, desertification). The variation in dust activity is also a function of local and regional weather conditions, soil moisture, living vegetation cover of the area, texture, roughness and human-induced land disturbances (e.g., Mahowald et al., 2005; Israelevich et al., 2012; Ridley et al., 2014; Salvador et al., 2014; Guan et al., 2015). The wind erosion over Jazmurian is controlled by the intensity and duration of the local winds, the percentage of water coverage in the lake, local and regional precipitation and the river’s discharge, in general similarity with the Sistan Basin (Rashki et al., 2013a) and other drainage basins over the globe (Washington et al., 2006; Bryant et al., 2007; Ge et al., 2016). Besides the influence of the local/regional meteorology on dust activity over the Jazmurian Basin, changes in synoptic weather systems and, more specifically, in the pressure gradient between the Caspian Sea and Hindu Kush, result in remarkable changes in the wind intensity over eastern Iran and, in turn, in the frequency and severity of the DEs (Kaskaoutis et al., 2015a, 2016). Moreover, the wet and dry spells over Iran may also coincide with cold and warm phases of the El-Nino Southern Oscillation (ENSO) as reported by previous studies (Soltani and Gholipoor, 2006; Abolhasan and Maryam, 2013; Zoljoodi and Didevarasl, 2013). In this respect, the extreme

Fig. 14. Density plot of the 3-day forward air-mass trajectories shown in Fig. 13. The arrows show the main dust-plume pathways in each season. The legend refers to the number of air masses at each 1°  1° grid. The trajectory densities were drawn by using the ArcMap GIS9.3@ software.

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drought during 1999–2001 was found to be related to prolonged duration of La Nina phase (Mathew et al., 2002). The complicated topography of the Jazmurian Basin, with several ranges of rocky-arid mountains, results in significant differences in meteorological parameters and frequency of DEs between the stations. The basin has a northwest-southeast direction (see Fig. 1), while the winds at each station show a rather west-northwest dominance but with considerable differences between them. In this respect, Iranshahr is in the downwind direction of the arid Jazmurian Basin and, therefore, exhibits larger number of DEs and DSEs (Table 5, Fig. 6a, b). The prevailing winds in Bam are from northern directions (Fig. 1) and, therefore, this station is downwind of the Dasht-e-Lut desert lying just north of Jazmurian, which can explain the frequent DEs and DSEs (Table 5, Fig. 6a, b). In contrast, Kahnuj is located in upwind direction of the Jazmurian Basin and is mostly influenced by south-westerlies from the Persian Gulf, thus exhibiting less DEs (Table 5, Fig. 6a, b). An important finding that was revealed from the current study is the influence of the Sistan dust storms on the frequency and temporal variability of the Jazmurian DEs. This issue was very briefly examined in the present work and constitutes a real challenge for further research in order to evaluate the impact of dust plumes transported from the Sistan Basin and to determine which stations in the Jazmurian Basin are mostly affected. Moreover, the relative contribution of the local processes and the long-range transported plumes to the afternoon peaks in dust activity over the basin, which contrasts the morning peak in dust emissions in the western Sahara (Schepanski et al., 2009; Allen and Washington, 2014), should be examined in more detail. In addition, the challenges of evaluating the role of rainfall in the water coverage in the Jazmurian lake and the effects of precipitation and wind on the frequency and long-term trends in dust emissions over the basin are under examination along with the atmospheric circulation systems that are associated with dust emissions from the Jazmurian Basin.

6. Conclusions This work identified and analyzed for the first time the dust events (DEs; vis <10 km) and dust-storm events (DSEs; vis <1 km) at five meteorological stations around the Jazmurian Basin in southeastern Iran. The analysis was based on 3-h meteorological observations and visibility records and provided an assessment of the temporal evolution of the DEs and DSEs at the stations, and in the Jazmurian Basin as a whole by merging the data from the five stations during the period 1990–2013. In total, 9919 DEs were identified in the Jazmurian Basin during 1990–2013, corresponding to 413 DEs, on average, per year i.e., 14.1% of the total 3-h visibility recordings. The annual DEs may reach 700 in certain years (e.g., 2001, 2003) of extreme drought. On the other hand, the DSEs were found to be 521 (21.7 ± 10.3 per year), corresponding to 5.2% of the DEs. Both DEs and DSEs increased during 2001– 2004 following an extreme and prolonged drought over southeastern Iran that resulted in the complete drying of the Jazmurian lake. However, no significant trend was found for both DEs and DSEs during the period 1990–2013. The DEs were more frequent in summer (45%) and spring (31%) and rather rare in winter (12%) and autumn (11%), while the DSEs exhibited higher frequency in spring (41%), followed by summer (28%) and winter (20%). The seasonal and monthly variation of the DSEs was much more complicated than the DEs, with larger variability between the stations, indicating significant impact of local factors on dust storm emissions. The period June to August exhibited the highest frequency of DEs, while March and May showed the largest number of DSEs in the Jazmurian Basin. On a diurnal basis, both DEs and DSEs exhibited

their highest frequency in the afternoon (after 15:00 LST) due to the development of thermal advections and impact of transported dust plumes from Sistan, especially in late spring and summer seasons. The dusty days in the Jazmurian Basin, i.e., days with at least one DE at any station, were found to be 145.5 ± 26.7 per year, on average, while the annual-mean dust-storm days were 12.7 ± 5.4. The DEs lasted for 8.4 h compared to 5.1 h for the DSEs per dusty day, without exhibiting a significant annual or monthly variability, except for the larger duration of the DSEs in winter. The forward air-mass trajectories originating from Jazmurian on the dust-storm days showed preferable directions towards the east and south, impacting western Pakistan, the Oman Sea, the north Arabian Sea, the southeast Arabian Peninsula and the neighboring regions of southeast Iran. The dusty air masses usually travel within the lower boundary layer (<1.5 km) thus deteriorating the air quality in the impacted areas by deposition of dust particles. The central part of the Arabian Sea is rather rarely affected, since the dust storms are blocked by the Inter-Tropical Convergence Zone and are forced to higher altitudes over the marine environment. Acknowledgements Authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.ready.noaa.gov) used in this publication. Our special thanks go to the personnel of the five meteorological stations (Bam, Iranshahr, Jiroft, Khash and Kahnuj) for their efforts in maintaining the meteorological instruments and providing 3-h continuous observations. Valuable comments by editor and reviewers are highly appreciated. References Abolhasan, G., Maryam, N., 2013. Case Study: ENSO Events, rainfall variability and the potential of SOI for the seasonal precipitation predictions in Iran. Am. J. Clim. Change 2, 34–45. Abuduwaili, J., Liu, D., Wu, G., 2010. Saline dust storms and their ecological impacts in arid regions. J. Arid Land 2, 144–150. Alam, K., Trautmann, T., Blaschke, T., 2011. Aerosol optical properties and radiative forcing over mega-city Karachi. Atmos. Res. 101, 773–782. Alizadeh Choobari, O., Zawar-Reza, P., Sturman, A., 2013. Low level jet intensification by mineral dust aerosols. Ann. Geophys. 31, 625–632. Alizadeh Choobari, O., Peyman, Z.-R., Sturman, A., 2014. The global distribution of mineral dust and its impacts on the climate system: a review. Atmos. Res. 138, 152–165. Allen, C.J.T., Washington, R., 2014. The low-level jet dust emission mechanism in the central Sahara: observations from Bordj-Badji Mokhtar during the June 2011 Fennec Intensive Observation Period. J. Geophys. Res. 119, 2990–3015. http:// dx.doi.org/10.1002/2013JD020594. Antón, M., Gil, J.E., Fernández-Gálvez, J., Lyamani, H., Valenzuela, A., Foyo-Moreno, I., Olmo, F.J., Alados-Arboledas, L., 2011. Evaluation of the aerosol forcing efficiency in the UV erythemal range at Granada, Spain. J. Geophys. Res. 116, D20214. http://dx.doi.org/10.1029/2011JD016112. Awad, A.M., Mashat, A.-W.S., 2013. Synoptic features associated with dust transition processes from North Africa to Asia. Arab. J. Geosc. 7, 2451–2467. Awad, A.M., Mashat, A.S., Alamoudi, A.O., Assiri, M.E., 2016. Synoptic study of the seasonal variability of dust cases observed by the TOMS satellite over northern Saudi Arabia. Theor. Appl. Climatol. 124, 1099–1117. Baaqhideh, D., Ahmadi, H., 2013. Dust hazard analysis and its trends in the West and South West of Iran. J. Rescue 2, 43–57 (in Persian). Baddock, M.C., Bullard, J.E., Bryant, R.G., 2009. Dust source identification using MODIS: a comparison of techniques applied to the Lake Eyre Basin, Australia. Rem. Sens. Environ. 113, 1511–1528. Bou Karam, D., Flamant, C., Knippertz, P., Reitebuch, O., Pelon, J., Chong, M., Dabas, A., 2008. Dust emissions over the Sahel associated with the West African monsoon intertropical discontinuity region: a representative case-study. Q. J. R. Meteorol. Soc. 134, 621–634. Bryant, R.G., 2013. Recent advances in our understanding of dust source emission processes. Prog. Phys. Geogr. 37, 397–421. Bryant, R.G., Bigg, G.R., Mahowald, N.M., Eckardt, F.D., Ross, S.G., 2007. Dust emission response to climate in southern Africa. J. Geophys. Res. 112, D09207. http://dx.doi.org/10.1029/2005JD007025. Cao, H., Liu, J., Wang, G., Yang, G., Luo, L., 2015. Identification of sand and dust storm source areas in Iran. J. Arid Land 7, 567–578.

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