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Impact of the March 2009 dust event in Saudi Arabia on aerosol optical properties, meteorological parameters, sky temperature and emissivity
Atmospheric Environment 45 (2011) 2164e2173
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Impact of the March 2009 dust event in Saudi Arabia on aerosol optical properties, meteorological parameters, sky temperature and emissivity A. Maghrabi a, *, B. Alharbi b, N. Tapper c a
National Centre for Mathematics and Physics, King Abdulaziz City For Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia National Environmental Technology Centre, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia c School of Geography and Environmental Science and Monash Weather and Climate Program, Monash University, Clayton 3800, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2010 Received in revised form 27 January 2011 Accepted 28 January 2011
On 10th March 2009 a widespread and severe dust storm event that lasted several hours struck Riyadh, and represented one of the most intense dust storms experienced in Saudi Arabia in the last two decades. This short-lived storm caused widespread and heavy dust deposition, zero visibility and total airport shutdown, as well as extensive damage to buildings, vehicles, power poles and trees across the city of Riyadh. Changes in Meteorological parameters, aerosol optical depth (AOD), Angstrom exponent a, infrared (IR) sky temperature and atmospheric emissivity were investigated before, during, and after the storm. The analysis showed significant changes in all of the above parameters due to this event. Shortly after the storm arrived, air pressure rapidly increased by 4 hPa, temperature decreased by 6 C, relative humidly increased from 10% to 30%, the wind direction became northerly and the wind speed increased to a maximum of 30 m s1. AOD at 550 nm increased from 0.396 to 1.71. The Angstrom exponent a rapidly decreased from 0.192 to 0.078. The mean AOD at 550 nm on the day of the storm was 0.953 higher than during the previous clear day, while a was 0.049 in comparison with 0.323 during the previous day. Theoretical simulations using SMART software showed remarkable changes in both spectral and broadband solar radiation components. The global and direct radiation components decreased by 42% and 68%, respectively, and the diffuse components increased by 44% in comparison with the previous clear day. IR sky temperatures and sky emissivity increased by 24 C and 0.3, respectively, 2 h after the arrival of the storm. The effect of aerosol loading by the storm on IR atmospheric emission was investigated using MODTRAN software. It was found that the effect of aerosols caused an increase of the atmospheric emission in the atmospheric window (8e14 mm) such that the window emissions resembled those of a blackbody and the atmospheric window was almost closed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Sky temperature Dust storm AOD MODTRAN Synoptic Meteorology
1. Introduction Dust is one of the major types of tropospheric aerosol. Dust particles affect both solar and terrestrial radiation, and are thus considered to be a significant climate-forcing factor and an important parameter in radiation budget studies (Satheesh and Ramanathan, 2000; Haywood and Boucher, 2000; Kaskaoutis et al., 2008; Jayaraman et al., 1998; Harrison et al., 2001). Dust storms are also considered to be a natural hazard that can affect daily life for short time intervals ranging from a few hours to a few days. Dust storms also cause a variety of other problems.
* Corresponding author. Tel.: þ966 1 481 4302; fax: þ966 1 481 3521. E-mail address: [email protected] (A. Maghrabi). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.01.071
One of the major problems associated with dust storms is the considerable reduction of visibility that limits various activities, increases traffic accidents, and may increase the occurrence of vertigo in aircraft pilots (Dayan et al., 1991; Kutiel and Furman, 2003). Other environmental impacts include reduced soil fertility in dust storm source areas and damage to crops (Fryrear, 1981), a reduction of solar radiation and resulting impairment of the efficiency of solar devices, damage to telecommunications and mechanical systems, dirt pollution, and air pollution (Kutiel and Furman, 2003; Mitchell, 1971). In addition, aerosols have significant impacts on human health (Longstreth et al., 1995; Bennett et al., 2006; Bennion et al., 2007), chemical/elemental fluxes between continents and oceanic biogeochemical cycles (Ozer et al., 2006). Goudie (2009) recently provided an up-to-date and comprehensive review on dust storms and their significance for many fields.
A. Maghrabi et al. / Atmospheric Environment 45 (2011) 2164e2173
In some seasons and for about 30% of the time, on average, parts of the Middle East, especially Saudi Arabia, are affected by dust storms. The frequency of dust storm occurrence peaks during the pre-monsoon season (MarcheMay), when dust aerosols are transported by southwesterly winds from the arid and semi-arid regions around the Arabian Sea (Ackerman and Cox, 1989; Middleton, 1986). Some of these regions include the alluvial plains of Iraq, the plateau of Eastern Jordan, the Jazirah of Eastern Syria, the plains of Dhofar, the adjacent interior eastern Yemen, and the Empty-Quarter desert centred in Saudi Arabia. In these regions, dust storms are a very frequent phenomenon and a better knowledge of their spatial and temporal distribution is of prime importance. In Saudi Arabia, dust storms are considered to be one of the most severe environmental problems. Several investigators have studied desert dust in the Middle East and Arabian Peninsula, including Saudi Arabia (e.g. Pease et al., 1998; Smirnov et al., 2002; Badarinath et al., 2010; Alharbi and Moied, 2005). All the previously mentioned studies have used either surface or satellite observations to characterize the large-scale dust loading of the atmosphere over the Arabian Peninsula. However, little work has been done to study the effect of these dust storms on meteorological parameters, solar and particularly infrared radiation in this region. On the 10th of March 2009 a dramatic windstorm moved over Riyadh accompanied by a strong dust storm. This short-lived, but intense, dust storm caused widespread and heavy dust deposition, badly affected visibility and air quality, caused a total airport shutdown, and caused damage to buildings, vehicles, power poles and trees over the city of Riyadh1. This large storm was clearly visible from space and is considered to be one of the heaviest recorded dust storms in the last two decades (Alharbi, 2009). The outbreak of the dust storm was associated with a cold frontal passage that coincided with the propagation of a pre-existing synoptic-scale upper tropospheric jet streak over the northern and central parts of Saudi Arabia (Fig. 1). The detailed description of the synoptic development and evolution of the disturbance will be the subject of another paper. This paper presents the impact of this severe storm on meteorological parameters, aerosol properties, and infrared atmospheric radiation. 2. Experimental site The study area of Riyadh lies in the central region of the Arabian Peninsula at (24 430 N; 46 400 E, 764 m a.s.l.). The central region is considered to be a vast eroded plateau consisting of areas of uplands, broad valleys and dry rivers. The area also contains a number of marshes, thought to be the remnants of inland seas that existed in ancient geological times. Riyadh is the capital and the largest city in Saudi Arabia; its population was 45,00,000 inhabitants according to the census of 2005. This purely urban area is one of the most polluted regions of the Kingdom, as it is surrounded by industrial areas and major traffic routes. The natural environment of the Empty-Quarter Desert lies beyond the urban area. The arid conditions and continentality prevailing at this site are responsible for large seasonal temperature differences, with cool winters and very hot summers. The area experiences extremely low humidity, particularly in the summer. The local aerosol sources are mainly the heavy traffic on the major roads of the city, together with the resuspension of material available on the ground, especially during the warm season when the reduced rainfall and dry terrain can increase the contribution of local mineral dust.
1 http://www.youtube.com/watch?v¼E5KHa5hxzQM, http://www.eosnap.com/ public/media/2009/03/duststorms/foto-full.jpg.
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The climate of the region exhibits four dominant seasons each year: winter (DecembereFebruary), pre-monsoon (MarcheMay), monsoon (JuneeAugust), and post-monsoon (SeptembereNovember). The pre-monsoon season, in which the present case study occurred, is characterized by frequent dust storms and long dry spells. 3. Observational data The observational dataset analysed here consists of IR long wave atmospheric radiation (presented in this study by sky temperature Tsky), surface and upper air meteorological data, and the aerosol optical depth (AOD) measurements. A brief description of how these data were collected and analysed is given below. 3.1. IR sky temperature measurements (Tsky) These temperatures were obtained using a single-pixel, broadband IR detector designed during collaborative work between King Abdulaziz City for Science and Technology (KACST) and the University of Adelaide, Australia. It was developed as an inexpensive tool for cloud monitoring and atmospheric radiation studies. The construction of this instrument, its calibration, and all related technical issues have been described in detail in a number of previous articles (Clay et al., 1998; Maghrabi, 2007; Maghrabi et al., 2009). The basic detector is a Heimann TPS 534 thermopile. In this study, its angular field of view was set to 3 (the full width at half maximum) using a Fresnel lens. The spectral range extended from 5.5 mm to over 50 mm if only the transmission of the lens was taken into account. The detector’s output was calibrated on a blackbody of known temperature that completely filled the field of view. The detector performance was tested over a long period of time and under various extreme conditions and showed a reliable and stable performance. Theoretical simulations using MODTRAN software (Berk et al., 1989) yielded good agreement with measured sky temperatures, with a root mean square error (RMSE) of approximately 1.5 C. The experimental error for Tsky measurements is approximately 2 C (Maghrabi et al., 2009). The detector was placed on the roof of the KACST main research institute building, 20 m above the ground, at a distance of 10 km from Riyadh Airport. Here, the IR monitor and the associated data logging system were installed in a purpose-built stand. The monitor was installed vertically at the top of the stand. Underneath, the logger sat inside a ventilated box. The data acquisition system was an XR5-8-A-SE data logger manufactured by Pace Scientific.2 The loggers have internal sensors to measure the relative humidity (RH) and the air temperature (T). The accuracies of the logger’s sensors are 2% humidity and 0.15 C at a temperature of 25 C. These recorded variables serve as a convenient local measure of the air temperature and RH at the site. The data from Riyadh airport were provided by the Presidency of Meteorological Environment (PME) and were used for comparison and for checking the accuracy of the air temperature and relative humidly measured by the loggers. No major differences were found between logger records and those obtained from the PME. For example, the logger data agree with PME screen temperatures to within 1 C. The data from the detector and the logger system were recorded at 10-min intervals. 3.2. Aerosol data The AOD measurements reported in this work were made with a CIMEL sun photometer (CE-318). This instrument was installed
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http://www.pace-sci.com/.
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Fig. 1. a: The surface synoptic pressures, 850 and 700 hPa pressure-heights and wind speeds in ms1 at 200 hPa on 10 and 11 March 2009. The surface synoptic pressures show the Mediterranean low-pressure area moving across the southern part of Iraq and northern part of Saudi Arabia. The wind speeds at 200 hPa show the upper jet streak over northern and central parts of Saudi Arabia. This figure was created using the NOAA/ESRL Physical Sciences Division website (http://www.cdc.noaa.gov/Composites/Day/). Accessed October 2009. b: METEOSAT infrared images showing the position and progression of the front on March 10, 2009 from 0000 UTC (top) to 1200 UTC (bottom) as a band of clouds on a northeastesouthwest axis. The position of the upper level jet stream can be seen as a band of clouds on a westeeast axis over the northern part of the Arabian Peninsula (From: NERC Satellite Receiving Station, Dundee University, Scotland: http://www.sat.dundee.ac.uk/, courtesy of EUMETSAT: http://www.eumetsat.de/) Accessed October 2009.
on the rooftop of Solar Village (24.91 N, 46.41 E, 764 m), approximately 15 km northwest of KACST. The instrument is an automatic tracking sun and sky scanning radiometer that makes direct sun measurements with a 1.2 full field of view every 15 min at 340, 380, 440, 500, 675, 870, 940 and 1020 nm (nominal) wavelengths. Seven of the eight bands are used to acquire AOD,
while that at 940 nm is used to retrieve total column water vapour in cm. The instrumentation, data acquisition, retrieval algorithms and calibration procedure conform to AERONET standards and are described in detail in numerous studies (e.g. Holben et al., 2001; Dubovik et al., 2000). Typically, the total uncertainty in AOD for
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Fig. 1. (continued).
a field instrument under cloud-free conditions, is w0.01 for l > 440 nm, and w0.02 for shorter wavelengths (Dubovik et al., 2002). In this study, the hourly-averaged Level 1.5 products were used, which are cloud-screened and quality assured (Smirnov et al., 2000). The Angstrom parameter was also computed from AOD measurements using the retrieval algorithms detailed in previous studies (e.g. Holben et al., 2001), at five wavelength ranges. 3.3. Meteorological and radiosonde data Standard meteorological observations such as air temperature, relative humidity, radiosonde data, and cloud information were obtained from Riyadh Airport records provided by PME. Air temperatures and relative humidity measured by the logger system at the IR site were used in this study. 4. Results and discussion 4.1. Meteorological parameters The hourly values of six key meteorological variables (relative humidity, temperature, visibility, wind speed and direction, and atmospheric pressure) for 8e12 March 2009 are plotted in Fig. 2.
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Referring to Fig. 2a and b, both T and RH reveal a diurnal cycle throughout the study period, showing opposite trends to one another as would normally be expected. Atmospheric pressure (Fig. 2c) shows a less clear diurnal cycle with notable variability from 1 h to the next during each day. Visibility (Fig. 2d) was in the 8e10 km range on the 8th and 9th of March and on the morning of the 10th, which is the normal maximum visibility in Riyadh at this time of the year (Alharbi, 2009). Fig. 2e and f shows that through most of the period from the morning of the 8th March until the morning on the 10th, wind directions were consistently southerly. During this period wind speed increased from w10 m s1 on the 8th to w20 m s1 on the 9th. On the day of the dust storm, and before the arrival of the storm, the weather was relatively benign; the temperature was w28 C, the air pressure had dropped to its minimum, the RH was 10%, and the local wind was relatively light from 160 to 180 (southerly). Around noon local time, with the arrival of the dust plume, there were dramatic changes in weather conditions. The wind swung to a northerly direction and wind speed rapidly increased to a maximum of 30 m s1. Following this, winds then began to drop once again. Air temperature dropped by about 6 C within an hour to reach 22 C at 13:00 on the 10th. The temperature continued to decrease, until it reached its daily minimum of about 14 C at 07:00 on the 11th. The air temperature then resumed its normal diurnal cycle (although temperatures remained cool on the 11th with a maximum of only 23 C). It is likely that this reduction in daytime temperature was caused by the cold air mass which hit the region and/or the reduced heating near the surface resulting from shortwave energy extinction by the additional aerosol loads arriving with the storm of the 10th. Relative humidity, on the other hand, increased dramatically with the arrival of the dust storm, reaching a maximum of 44% at 01:00 the 11th. The change in relative humidity is partly due to the cooler air temperatures, but is also likely due to the moisture brought to the region by the storm. The visibility deteriorated dramatically with the arrival of the dust storm and then remained at around 1 m for the 3 h following the event. It then increased to 6 km by 03:00 on the 11th. After another decrease to w2 km through the early hours of the morning it rose to w5e6 km and stayed at around this level for the rest of the study period. Several investigations of dust storms in the Arabian Gulf and adjacent Gulf countries (McNaughton, 1987) and other places around the world (Joseph et al., 1980; Pauley et al., 1996) have reported similar variations and characteristic changes in meteorology. 4.2. Spectral aerosol optical depth (AOD) Fig. 3 shows the average daily variation of the AOD at the seven considered wavelengths. With the exception of the day of the event, the general pattern shows a decrease in AOD values as wavelength increases. On the day of the event, the AOD values were 2e3-fold larger than on non-dusty days. Also, at this time the AOD showed a trend reversal in which AOD increased with wavelength. Fig. 4 shows the hourly variations of the AOD at 500 nm for the five days of the study period. On the morning of the 10th March, the AOD was around 0.396, until around midday when it jumped to 1.713, indicating the increased aerosol loading associated with the arrival of the dust storm. AODs remained at these high values for the next 3 h, then decreased gradually to reach a value of 0.919 by the end of the day. The mean AOD value for the day of the dust storm was 1.306 0.436. This value is about 270% higher than the March mean AOD values reported by Kambezidis and Kaskaoutis (2008) and 190% higher than the spring mean AOD values reported by Sabbah and Hasan (2008). The increase in average AOD was
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accompanied by a similar increase in the standard deviation, indicating greater variability in the atmospheric aerosol loading during that day. For example, the standard deviation during the day of the event was 0.436 at 500 nm compared with 0.047 on the preceding day. This large variability can be related to the properties of the air mass transporting the aerosols into the study area. The AOD values during this dust storm event were higher compared with intense dust episodes in other locations around the world, for example in Gwangju, Korea (Kim et al., 2006); Kwangju, South Korea (Ogunjobi et al., 2004); Arabian Sea (Badarinath et al., 2010) and Osaka, Japan (Yu et al., 2006). Values of AOD were similar to the intense dust outbreaks in Beijing and Inner Mongolia, China (Yu et al., 2006). However, other locations have shown higher AOD values during
dust outbreaks, for example in Hyderabad, India (Badarinath et al., 2007); West African Dakar (Ogunjobi et al., 2008); the Eastern Mediterranean (Kaskaoutis et al., 2008) and the Tengger Desert, China (Xina et al., 2005). The mean AOD On March 11th was 0.604 with a maximum of 0.711 at the end of the day. On March 12th, AOD was lower in comparison with the previous days, reaching a low value of 0.262 close to the minimum AOD levels observed before the storm. Similar patterns were observed in the other wavelength bands. The AOD values at all wavelengths for the days prior to the dust event (8e9th March) were relatively low compared with those after the dust event. For example, the average AOD at 870 nm during the 8th and 9th was 0.265 whereas during the 11th and 12th it was 0.466.
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Wavelength (µm) Fig. 3. Daily average spectral aerosol optical depths for the period from March 8th to March 12th 2009.
4.3. Angstrom exponents (a) The hourly variations of a values for the wavelength range 440e870 nm over the five days are shown in Fig. 5. Apart from on March 9th, there was a distinct diurnal variation in a: it decreased through the morning hours to reach a minimum in the early afternoon and then increases for the rest of the day. At around 10:00 on the morning of March 10th, a was 0.196, but then dropped dramatically to 0.078 at the time when the dust cloud arrived. a stayed near this value for the next 3e4 h and then began to increase to a value of 0.036 by the end of the day. The mean a value on March 10th was 0.043, which is about 139% lower than the March mean value obtained by Kambezidis and Kaskaoutis (2008) for the same wavelength range. On March 11th the variations of a were relatively small, while on the 12th a resumed its diurnal pattern and continued to increase. The mean a values for the two days before the dust event (8e9th March) were higher compared with those obtained after the dust event.
4.4. Attenuation of solar radiation by aerosols e SMARTS modelling As solar radiation passes through the atmosphere it is attenuated by the physical processes of scattering and absorption. Absorption takes place in different spectral ranges for each atmospheric gas, whilst scattering exists across the whole spectrum with wavelength dependence due to the specific characteristics of the scattering particles. Atmospheric aerosols modify the solar radiation spectrum by attenuating the solar energy reaching the ground, and by changing the direct and diffuse radiation levels. The effect of different aerosol loads on both the spectral and broadband solar
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radiation components have been investigated extensively both experimentally and theoretically by several researchers (Latha and Badarinath, 2005; Kaskaoutis et al., 2006; Leckner, 1978; Kambezidis and Kaskaoutis, 2008). The aim of this section is to investigate how the turbidity conditions produced by the March 10th dust storm affected the solar irradiance components. For this purpose the radiative transfer model SMARTS (version 2.9.2) (Gueymard, 2003) was used. In the present study, March 9th had the lowest AOD values among the five days and we assume this to be a low turbidity day. Therefore, modification of the global irradiance components caused by the dust storm at Riyadh on March 10th and 11th were compared relative to conditions on the 9th. The global, diffuse and directbeam irradiances were obtained from the SMARTS model in the spectral band 280 nme1100 nm (panels a, b, c, respectively in Fig. 6). These irradiances were obtained at the same zenith angle (air mass 1.5) and for standard ozone and water vapour concentrations, whereas an urban aerosol model was chosen as being most representative of the atmospheric conditions at Riyadh. The analysis includes three cases: (i) typical background conditions with low aerosols (AOD 500 ¼ 0.247), (ii) with AOD 500 ¼ 1.306 representing the daily mean on March 10th, and (iii) with AOD 500 ¼ 0.604 representing the daily mean on March 11th. For the low dust day, the spectral distribution of the three components is similar to that of clear skies (Iqbal, 1983). However the effect of high aerosol loadings on the spectral distribution of the solar irradiances differs significantly from one wavelength to another for the 10th and the 11th. For instance, the reduction of the global solar irradiance component due to the aerosols on March 10th was about 50% and 26% for wavelengths of 500 nm and 1000 nm, respectively. These reductions are relative to March 9th. Meanwhile, the corresponding reduction for direct irradiance was 80% and 51% for wavelengths of 500 nm and 1000 nm, respectively. On the other hand, the diffuse component at wavelengths below 450 nm was not much affected by the dust storm on the 10th. However, for wavelengths greater than 450 nm, the diffuse irradiance showed large increases, with a red shift of the spectral diffuse irradiance due to the storm on the 10th, relative to values observed on the clear day (the 9th). For example, the diffuse irradiance increased by 40% at 550 nm, and by about 62% and 140% for the wavelengths 600 nm and 832 nm, respectively. Similar findings have been reported in similar studies (Badarinath et al., 2007; Latha and Badarinath, 2005; Kambezidis and Kaskaoutis, 2008). Examination of the integrated solar radiation components across the whole spectrum (280e1100 nm broadband) suggested that the turbid conditions significantly reduced the global irradiance, by 42% and 15%, on the 10th and 11th March, respectively. The direct irradiance component was strongly reduced, by 68% and 29%, on March 10th and 11th, respectively, compared with the much clearer conditions on March 9th, whereas the turbid atmosphere
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strongly affected by both atmospheric water content and screen temperatures (e.g. Prata, 1996; Ruckstuhl et al., 2007). However, other parameters are also expected to have some effect on the atmospheric emission. Their effect varies in importance according to the wavelength, atmospheric conditions and other factors such as the geography of the site. For example, under cloudless conditions the effect of suspended dust in the atmosphere is suspected to influence the transmission of IR sky radiation within the atmospheric window, AW (8 mme14 mm), consequently increasing the total atmospheric radiation. However, this effect has not received much consideration in the literature. In the following two subsections, the effect of the aerosol particles brought to Riyadh by the dust storm on the IR atmospheric radiation will be investigated.
Fig. 6. Estimated spectral irradiances from the SMARTS model: (a) global; (b) diffuse and (c) direct-beam, for three turbidity conditions: i) non turbid clear conditions with AOD 500 ¼ 0.24 (9th March), ii) for AOD 500 ¼ 1.3 (10th March), and iii) for AOD 500 ¼ 0.604 (11th March.).
4.5.2. Sky temperature (Tsky) and atmospheric emissivity (3) Figs. 7 and 8 show the hourly variations of the IR sky temperatures and emissivity during the five-day study period. The atmospheric emissivity was calculated following the procedures described in Prata (1996). Apart from on the day of the event, both Tsky and 3 follow a diurnal cycle that is much clearer for the latter than for the former. They both decrease from early morning to the afternoon and then increase again during the rest of the day. Both show little variation during the night in comparison with the day. Some of this variability in the daytime measurements may be due to heating and subsequent re-radiation from the IR transmitting optics of the detector, leading to some bias in the measurements. This bias is estimated to be around 3 C (Maghrabi et al., 2009). On the day of the dust event and before the arrival of the dust cloud, both parameters followed their normal diurnal pattern. However, after the passage of the dust cloud, Tsky jumped from 6.9 C to a maximum of 17 C and 3 increased from 0.661 to reach a maximum of 0.969 by about 13:00e14:00 h. The atmosphere during this time was almost completely laden with dust and its emission resembled that of a blackbody at screen level temperature. Both Tsky and 3 remained at around their maximum for almost 2e3 h and then continuously declined, until reaching minimum values of 15.69 C and 0.69, respectively, by late afternoon on March 11th. For the rest of the period both parameters again followed their usual diurnal pattern. The mean diurnal increase of about 9 C and 0.13 in Tsky and 3, respectively, on March 10th compared with March 9th was a consequence of the higher turbidity on March 10th. The standard deviations of both Tsky and 3 were also higher, confirming that the atmosphere experienced more variability in its atmospheric aerosol content during the time of the dust storm event (for example
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had an opposite impact on the diffuse component and increased it by 54% and 43% on March 10th and 11th, respectively.
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4.5.1. Background Infrared (IR) atmospheric radiation (wavelength 4.0e100 mm) is a key term in the surface energy budget and is vitally important for climatological and meteorological studies, as well as in applications such as solar energy and agricultural meteorology. Previous studies have concentrated on the measurement and evaluation of the total and spectral IR radiative flux (see Kjaersgaard et al., 2007 and references therein). Most of these studies were limited to clear-sky conditions and only a few have attempted to include the effects of cloud cover (e.g. Iziomon et al., 2003). It has been well established previously that atmospheric radiation under normal conditions is
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4.5.3. MODTRAN simulations To obtain a better understanding of how the dusty conditions affected atmospheric radiation, the radiative transfer model MODTRAN (Berk et al., 1989) was used. Upper air data from radiosondes launched from Riyadh Airport at 03:00 and 15:00 LST on each of the five days were used to define the pressure, temperature, and relative humidity profiles over Riyadh during the period. These profiles were used as input into MODTRAN, which is a user-defined atmospheric model. The profiles of other atmospheric constituents (e.g. CH4, CO, O2, NO, etc) were set to those of the MODTRAN standard MLS model. The effect of aerosol on atmospheric emissions was examined by assigning different values of visibility, ranging from clear to totally turbid conditions. Visibility is related to the presence of aerosols in the atmosphere and has an effect on the visible range of wavelengths. The influence of aerosols may be extended to the IR part of the spectrum if the size of the aerosol particles becomes comparable with the IR wavelengths. We ran MODTRAN with several different visibilities, starting with a high visibility, and then subsequently increased the amount of aerosol (decreasing the visibility values). In each run the spectral distribution of the atmospheric radiation, AR, was obtained. The visibility values tested were 25 km, 10 km, 8 km, 5 km, 2.5 km, 1 km, 500 m, 100 m, 50 m, 10 m and 1 m. Figs. 9 and 10 show the spectral distribution of atmospheric radiation for the selected visibilities. Fig. 9 shows the spectral distribution of atmospheric radiation for visibilities between 25 km and 2.5 km. While the variability within the AW increased slightly to a maximum of 8% between visibilities of 25 km and 2.5 km, there was little variability outside of the AW. Generally speaking, no major changes were found in the spectral distribution of the AR for most wavelengths and the spectral distribution resembles that of the clear sky, with visibility ¼ 25 km in this case (Goody and Young, 1989). Fig. 10 shows the spectral distribution of the AR for the selected visibilities below 1 km. The spectral distribution curve for 25 km visibility was added for comparison purposes. Two interesting features can be noted from this figure. Firstly, the atmospheric emission outside the AW does not change greatly and remains steady. In this case the effect of the dust storm on the IR spectral distribution can be considered to be negligible. For instance, at a wavelength of 6 mm, the increase in atmospheric radiation as visibility reduces from 25 km to 1 m does not exceed
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the standard deviation of 3 was 0.11 on the 10th compared with 0.05 on the 9th).
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Fig. 9. MODTRAN-calculated clear-sky atmospheric spectra for different visibilities: (a) Visibility ¼ 25 km; (b) Visibility ¼ 8 km; (c) Visibility ¼ 5 km and (d) Visibility ¼ 2.5 km; Data are from the Riyadh dust storm of March 10th, 2009.
3%. Additionally, the increases in the atmospheric radiation at 25 mm between visibility ¼ 25 km and visibility ¼ 1 m was less than 1%. Secondly, the change of the spectral distribution inside the AW region is evident as we move towards lower visibilities (increasing the amount of aerosol). The increase in AR continues up to visibility ¼ 500 m. A dramatic increase in the atmospheric radiation in the AW region occurs for visibilities of 50 m and 10 m. For instance, the increases in AR in the AW region as visibility is reduced from 25 km to 50 m and from 25 km to 10 m are about 365% and 400%, respectively. With 1 m visibility (Fig. 10d) the atmospheric window is totally closed and the atmospheric emission resembles that of a blackbody. In this case the presence of larger sizes of aerosols effectively works as a lid that emits radiation in this part of the spectrum. Therefore, the main effect of the dust storm on the IR atmospheric radiation was to increase the atmospheric emission greatly inside the AW, causing the atmosphere to emit as a full blackbody.
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Fig. 11. MODTRAN-calculated integrated sky temperatures (5.5e50 mm) plotted against visibility. Data are from the Riyadh dust storm of March 10th, 2009.
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which is identical to the observed temperature of 16 C at that time (see Fig. 7).
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Fig. 10. MODTRAN-calculated clear-sky atmospheric spectra for visibility of 25 km compared with (a) Visibility ¼ 1 km; (b) Visibility ¼ 50 m; (c) Visibility 10 m and (d) Visibility ¼ 1 m. Data are from the Riyadh dust storm of March 10th, 2009.
Fig. 11 shows the integrated sky temperatures for the different values of visibility in the wavelength range 5.5 mme50 mm. This represents the wavelength response of our detection system. The sky temperature increases by 3.5 C as visibility reduces from 25 to 1.5 km. At these visibilities the effect of other meteorological parameters such as water vapour contents and air temperature are dominant. However, for visibilities below 1 km the sky temperature increases exponentially with decreasing visibility. For example, at a visibility of 500 m the sky temperature is 4.2 C, while at a visibility of 300 m the sky temperature is 0.8 C, equivalent to an increase of about 3.5 C. For a visibility of 1 m, close to the minimum observed value during the dust storm, the integrated sky temperature for the wavelength range 5.5 mme50 mm was 16.5 C,
On 10th March 2009 a severe and extensive dust storm event struck Riyadh and lasted for several hours. This event was caused by a cold front passage coinciding with the propagation of a preexisting synoptic-scale upper tropospheric jet streak over the northern and central parts of Saudi Arabia. The impact of this event on groundebased measurements of meteorological parameters, aerosol optical depth (AOD), Angstrom exponent a, infrared sky temperature (Tsky) and atmospheric emissivity (3) were analyzed. The results showed that the effects of this storm were associated with an increase in both atmospheric pressure and relative humidity, and a reduction in temperature and visibility, for the two days following the storm in comparison with conditions before the storm. The analysis of the AOD data showed that the AOD at l ¼ 500 nm increased by 330% immediately after the storm. The daily mean values of the AOD were higher by 429%, 144% and 52% for the 10th, 11th and 12th, respectively, in comparison with the day preceding the storm, as a result of the high aerosol loadings. At the time of the arrival of the dust cloud the Angstrom exponent plunged to negative values. a values on the event day and on the following days were lower than those before the dust storm. In terms of solar radiation, theoretical simulations using SMART software showed that there was a remarkable decrease in the broadband global and direct irradiances reaching the ground, amounting to 42% and 68%, respectively, in comparison with the previous clear day. The diffuse solar irradiance components increased by 44%. On studying the effect of this severe storm on the IR sky temperatures and sky emissivity, we found increases of 24 C and 0.3, respectively, immediately after the arrival of the storm. Both parameters remained higher after the event in comparison with their pre-event values. Theoretical investigations of the effect of this storm on the spectral and broadband IR atmospheric radiation were conducted using MODTRAN software. It was found that effects of the substantial aerosol loadings caused an increase in atmospheric emissions in the atmospheric window (between 8 and 14 mm), so that the emissions in this window resembled those of a blackbody and the atmospheric window was almost closed. There was good agreement between the integrated MODTRAN and measured sky temperatures during the peak of the event. Acknowledgments The authors would like to thank King Abdulaziz City for Science and Technology (KACST) for supporting this work. The Presidency
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of Meteorology and Environment (PME) is also acknowledged for providing the meteorological data. We also thank Dr. Nai Alabadi, the principle investigator for the solar village site, for his efforts maintaining the station. We also thank the two reviewers for their valuable comments and suggestions.
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Impact of the March 2009 dust event in Saudi Arabia on aerosol optical properties, meteorological parameters, sky temperature and emissivity A. Maghrabi a, *, B. Alharbi b, N. Tapper c a
National Centre for Mathematics and Physics, King Abdulaziz City For Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia National Environmental Technology Centre, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia c School of Geography and Environmental Science and Monash Weather and Climate Program, Monash University, Clayton 3800, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2010 Received in revised form 27 January 2011 Accepted 28 January 2011
On 10th March 2009 a widespread and severe dust storm event that lasted several hours struck Riyadh, and represented one of the most intense dust storms experienced in Saudi Arabia in the last two decades. This short-lived storm caused widespread and heavy dust deposition, zero visibility and total airport shutdown, as well as extensive damage to buildings, vehicles, power poles and trees across the city of Riyadh. Changes in Meteorological parameters, aerosol optical depth (AOD), Angstrom exponent a, infrared (IR) sky temperature and atmospheric emissivity were investigated before, during, and after the storm. The analysis showed significant changes in all of the above parameters due to this event. Shortly after the storm arrived, air pressure rapidly increased by 4 hPa, temperature decreased by 6 C, relative humidly increased from 10% to 30%, the wind direction became northerly and the wind speed increased to a maximum of 30 m s1. AOD at 550 nm increased from 0.396 to 1.71. The Angstrom exponent a rapidly decreased from 0.192 to 0.078. The mean AOD at 550 nm on the day of the storm was 0.953 higher than during the previous clear day, while a was 0.049 in comparison with 0.323 during the previous day. Theoretical simulations using SMART software showed remarkable changes in both spectral and broadband solar radiation components. The global and direct radiation components decreased by 42% and 68%, respectively, and the diffuse components increased by 44% in comparison with the previous clear day. IR sky temperatures and sky emissivity increased by 24 C and 0.3, respectively, 2 h after the arrival of the storm. The effect of aerosol loading by the storm on IR atmospheric emission was investigated using MODTRAN software. It was found that the effect of aerosols caused an increase of the atmospheric emission in the atmospheric window (8e14 mm) such that the window emissions resembled those of a blackbody and the atmospheric window was almost closed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Sky temperature Dust storm AOD MODTRAN Synoptic Meteorology
1. Introduction Dust is one of the major types of tropospheric aerosol. Dust particles affect both solar and terrestrial radiation, and are thus considered to be a significant climate-forcing factor and an important parameter in radiation budget studies (Satheesh and Ramanathan, 2000; Haywood and Boucher, 2000; Kaskaoutis et al., 2008; Jayaraman et al., 1998; Harrison et al., 2001). Dust storms are also considered to be a natural hazard that can affect daily life for short time intervals ranging from a few hours to a few days. Dust storms also cause a variety of other problems.
* Corresponding author. Tel.: þ966 1 481 4302; fax: þ966 1 481 3521. E-mail address: [email protected] (A. Maghrabi). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.01.071
One of the major problems associated with dust storms is the considerable reduction of visibility that limits various activities, increases traffic accidents, and may increase the occurrence of vertigo in aircraft pilots (Dayan et al., 1991; Kutiel and Furman, 2003). Other environmental impacts include reduced soil fertility in dust storm source areas and damage to crops (Fryrear, 1981), a reduction of solar radiation and resulting impairment of the efficiency of solar devices, damage to telecommunications and mechanical systems, dirt pollution, and air pollution (Kutiel and Furman, 2003; Mitchell, 1971). In addition, aerosols have significant impacts on human health (Longstreth et al., 1995; Bennett et al., 2006; Bennion et al., 2007), chemical/elemental fluxes between continents and oceanic biogeochemical cycles (Ozer et al., 2006). Goudie (2009) recently provided an up-to-date and comprehensive review on dust storms and their significance for many fields.
A. Maghrabi et al. / Atmospheric Environment 45 (2011) 2164e2173
In some seasons and for about 30% of the time, on average, parts of the Middle East, especially Saudi Arabia, are affected by dust storms. The frequency of dust storm occurrence peaks during the pre-monsoon season (MarcheMay), when dust aerosols are transported by southwesterly winds from the arid and semi-arid regions around the Arabian Sea (Ackerman and Cox, 1989; Middleton, 1986). Some of these regions include the alluvial plains of Iraq, the plateau of Eastern Jordan, the Jazirah of Eastern Syria, the plains of Dhofar, the adjacent interior eastern Yemen, and the Empty-Quarter desert centred in Saudi Arabia. In these regions, dust storms are a very frequent phenomenon and a better knowledge of their spatial and temporal distribution is of prime importance. In Saudi Arabia, dust storms are considered to be one of the most severe environmental problems. Several investigators have studied desert dust in the Middle East and Arabian Peninsula, including Saudi Arabia (e.g. Pease et al., 1998; Smirnov et al., 2002; Badarinath et al., 2010; Alharbi and Moied, 2005). All the previously mentioned studies have used either surface or satellite observations to characterize the large-scale dust loading of the atmosphere over the Arabian Peninsula. However, little work has been done to study the effect of these dust storms on meteorological parameters, solar and particularly infrared radiation in this region. On the 10th of March 2009 a dramatic windstorm moved over Riyadh accompanied by a strong dust storm. This short-lived, but intense, dust storm caused widespread and heavy dust deposition, badly affected visibility and air quality, caused a total airport shutdown, and caused damage to buildings, vehicles, power poles and trees over the city of Riyadh1. This large storm was clearly visible from space and is considered to be one of the heaviest recorded dust storms in the last two decades (Alharbi, 2009). The outbreak of the dust storm was associated with a cold frontal passage that coincided with the propagation of a pre-existing synoptic-scale upper tropospheric jet streak over the northern and central parts of Saudi Arabia (Fig. 1). The detailed description of the synoptic development and evolution of the disturbance will be the subject of another paper. This paper presents the impact of this severe storm on meteorological parameters, aerosol properties, and infrared atmospheric radiation. 2. Experimental site The study area of Riyadh lies in the central region of the Arabian Peninsula at (24 430 N; 46 400 E, 764 m a.s.l.). The central region is considered to be a vast eroded plateau consisting of areas of uplands, broad valleys and dry rivers. The area also contains a number of marshes, thought to be the remnants of inland seas that existed in ancient geological times. Riyadh is the capital and the largest city in Saudi Arabia; its population was 45,00,000 inhabitants according to the census of 2005. This purely urban area is one of the most polluted regions of the Kingdom, as it is surrounded by industrial areas and major traffic routes. The natural environment of the Empty-Quarter Desert lies beyond the urban area. The arid conditions and continentality prevailing at this site are responsible for large seasonal temperature differences, with cool winters and very hot summers. The area experiences extremely low humidity, particularly in the summer. The local aerosol sources are mainly the heavy traffic on the major roads of the city, together with the resuspension of material available on the ground, especially during the warm season when the reduced rainfall and dry terrain can increase the contribution of local mineral dust.
1 http://www.youtube.com/watch?v¼E5KHa5hxzQM, http://www.eosnap.com/ public/media/2009/03/duststorms/foto-full.jpg.
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The climate of the region exhibits four dominant seasons each year: winter (DecembereFebruary), pre-monsoon (MarcheMay), monsoon (JuneeAugust), and post-monsoon (SeptembereNovember). The pre-monsoon season, in which the present case study occurred, is characterized by frequent dust storms and long dry spells. 3. Observational data The observational dataset analysed here consists of IR long wave atmospheric radiation (presented in this study by sky temperature Tsky), surface and upper air meteorological data, and the aerosol optical depth (AOD) measurements. A brief description of how these data were collected and analysed is given below. 3.1. IR sky temperature measurements (Tsky) These temperatures were obtained using a single-pixel, broadband IR detector designed during collaborative work between King Abdulaziz City for Science and Technology (KACST) and the University of Adelaide, Australia. It was developed as an inexpensive tool for cloud monitoring and atmospheric radiation studies. The construction of this instrument, its calibration, and all related technical issues have been described in detail in a number of previous articles (Clay et al., 1998; Maghrabi, 2007; Maghrabi et al., 2009). The basic detector is a Heimann TPS 534 thermopile. In this study, its angular field of view was set to 3 (the full width at half maximum) using a Fresnel lens. The spectral range extended from 5.5 mm to over 50 mm if only the transmission of the lens was taken into account. The detector’s output was calibrated on a blackbody of known temperature that completely filled the field of view. The detector performance was tested over a long period of time and under various extreme conditions and showed a reliable and stable performance. Theoretical simulations using MODTRAN software (Berk et al., 1989) yielded good agreement with measured sky temperatures, with a root mean square error (RMSE) of approximately 1.5 C. The experimental error for Tsky measurements is approximately 2 C (Maghrabi et al., 2009). The detector was placed on the roof of the KACST main research institute building, 20 m above the ground, at a distance of 10 km from Riyadh Airport. Here, the IR monitor and the associated data logging system were installed in a purpose-built stand. The monitor was installed vertically at the top of the stand. Underneath, the logger sat inside a ventilated box. The data acquisition system was an XR5-8-A-SE data logger manufactured by Pace Scientific.2 The loggers have internal sensors to measure the relative humidity (RH) and the air temperature (T). The accuracies of the logger’s sensors are 2% humidity and 0.15 C at a temperature of 25 C. These recorded variables serve as a convenient local measure of the air temperature and RH at the site. The data from Riyadh airport were provided by the Presidency of Meteorological Environment (PME) and were used for comparison and for checking the accuracy of the air temperature and relative humidly measured by the loggers. No major differences were found between logger records and those obtained from the PME. For example, the logger data agree with PME screen temperatures to within 1 C. The data from the detector and the logger system were recorded at 10-min intervals. 3.2. Aerosol data The AOD measurements reported in this work were made with a CIMEL sun photometer (CE-318). This instrument was installed
2
http://www.pace-sci.com/.
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Fig. 1. a: The surface synoptic pressures, 850 and 700 hPa pressure-heights and wind speeds in ms1 at 200 hPa on 10 and 11 March 2009. The surface synoptic pressures show the Mediterranean low-pressure area moving across the southern part of Iraq and northern part of Saudi Arabia. The wind speeds at 200 hPa show the upper jet streak over northern and central parts of Saudi Arabia. This figure was created using the NOAA/ESRL Physical Sciences Division website (http://www.cdc.noaa.gov/Composites/Day/). Accessed October 2009. b: METEOSAT infrared images showing the position and progression of the front on March 10, 2009 from 0000 UTC (top) to 1200 UTC (bottom) as a band of clouds on a northeastesouthwest axis. The position of the upper level jet stream can be seen as a band of clouds on a westeeast axis over the northern part of the Arabian Peninsula (From: NERC Satellite Receiving Station, Dundee University, Scotland: http://www.sat.dundee.ac.uk/, courtesy of EUMETSAT: http://www.eumetsat.de/) Accessed October 2009.
on the rooftop of Solar Village (24.91 N, 46.41 E, 764 m), approximately 15 km northwest of KACST. The instrument is an automatic tracking sun and sky scanning radiometer that makes direct sun measurements with a 1.2 full field of view every 15 min at 340, 380, 440, 500, 675, 870, 940 and 1020 nm (nominal) wavelengths. Seven of the eight bands are used to acquire AOD,
while that at 940 nm is used to retrieve total column water vapour in cm. The instrumentation, data acquisition, retrieval algorithms and calibration procedure conform to AERONET standards and are described in detail in numerous studies (e.g. Holben et al., 2001; Dubovik et al., 2000). Typically, the total uncertainty in AOD for
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Fig. 1. (continued).
a field instrument under cloud-free conditions, is w0.01 for l > 440 nm, and w0.02 for shorter wavelengths (Dubovik et al., 2002). In this study, the hourly-averaged Level 1.5 products were used, which are cloud-screened and quality assured (Smirnov et al., 2000). The Angstrom parameter was also computed from AOD measurements using the retrieval algorithms detailed in previous studies (e.g. Holben et al., 2001), at five wavelength ranges. 3.3. Meteorological and radiosonde data Standard meteorological observations such as air temperature, relative humidity, radiosonde data, and cloud information were obtained from Riyadh Airport records provided by PME. Air temperatures and relative humidity measured by the logger system at the IR site were used in this study. 4. Results and discussion 4.1. Meteorological parameters The hourly values of six key meteorological variables (relative humidity, temperature, visibility, wind speed and direction, and atmospheric pressure) for 8e12 March 2009 are plotted in Fig. 2.
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Referring to Fig. 2a and b, both T and RH reveal a diurnal cycle throughout the study period, showing opposite trends to one another as would normally be expected. Atmospheric pressure (Fig. 2c) shows a less clear diurnal cycle with notable variability from 1 h to the next during each day. Visibility (Fig. 2d) was in the 8e10 km range on the 8th and 9th of March and on the morning of the 10th, which is the normal maximum visibility in Riyadh at this time of the year (Alharbi, 2009). Fig. 2e and f shows that through most of the period from the morning of the 8th March until the morning on the 10th, wind directions were consistently southerly. During this period wind speed increased from w10 m s1 on the 8th to w20 m s1 on the 9th. On the day of the dust storm, and before the arrival of the storm, the weather was relatively benign; the temperature was w28 C, the air pressure had dropped to its minimum, the RH was 10%, and the local wind was relatively light from 160 to 180 (southerly). Around noon local time, with the arrival of the dust plume, there were dramatic changes in weather conditions. The wind swung to a northerly direction and wind speed rapidly increased to a maximum of 30 m s1. Following this, winds then began to drop once again. Air temperature dropped by about 6 C within an hour to reach 22 C at 13:00 on the 10th. The temperature continued to decrease, until it reached its daily minimum of about 14 C at 07:00 on the 11th. The air temperature then resumed its normal diurnal cycle (although temperatures remained cool on the 11th with a maximum of only 23 C). It is likely that this reduction in daytime temperature was caused by the cold air mass which hit the region and/or the reduced heating near the surface resulting from shortwave energy extinction by the additional aerosol loads arriving with the storm of the 10th. Relative humidity, on the other hand, increased dramatically with the arrival of the dust storm, reaching a maximum of 44% at 01:00 the 11th. The change in relative humidity is partly due to the cooler air temperatures, but is also likely due to the moisture brought to the region by the storm. The visibility deteriorated dramatically with the arrival of the dust storm and then remained at around 1 m for the 3 h following the event. It then increased to 6 km by 03:00 on the 11th. After another decrease to w2 km through the early hours of the morning it rose to w5e6 km and stayed at around this level for the rest of the study period. Several investigations of dust storms in the Arabian Gulf and adjacent Gulf countries (McNaughton, 1987) and other places around the world (Joseph et al., 1980; Pauley et al., 1996) have reported similar variations and characteristic changes in meteorology. 4.2. Spectral aerosol optical depth (AOD) Fig. 3 shows the average daily variation of the AOD at the seven considered wavelengths. With the exception of the day of the event, the general pattern shows a decrease in AOD values as wavelength increases. On the day of the event, the AOD values were 2e3-fold larger than on non-dusty days. Also, at this time the AOD showed a trend reversal in which AOD increased with wavelength. Fig. 4 shows the hourly variations of the AOD at 500 nm for the five days of the study period. On the morning of the 10th March, the AOD was around 0.396, until around midday when it jumped to 1.713, indicating the increased aerosol loading associated with the arrival of the dust storm. AODs remained at these high values for the next 3 h, then decreased gradually to reach a value of 0.919 by the end of the day. The mean AOD value for the day of the dust storm was 1.306 0.436. This value is about 270% higher than the March mean AOD values reported by Kambezidis and Kaskaoutis (2008) and 190% higher than the spring mean AOD values reported by Sabbah and Hasan (2008). The increase in average AOD was
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accompanied by a similar increase in the standard deviation, indicating greater variability in the atmospheric aerosol loading during that day. For example, the standard deviation during the day of the event was 0.436 at 500 nm compared with 0.047 on the preceding day. This large variability can be related to the properties of the air mass transporting the aerosols into the study area. The AOD values during this dust storm event were higher compared with intense dust episodes in other locations around the world, for example in Gwangju, Korea (Kim et al., 2006); Kwangju, South Korea (Ogunjobi et al., 2004); Arabian Sea (Badarinath et al., 2010) and Osaka, Japan (Yu et al., 2006). Values of AOD were similar to the intense dust outbreaks in Beijing and Inner Mongolia, China (Yu et al., 2006). However, other locations have shown higher AOD values during
dust outbreaks, for example in Hyderabad, India (Badarinath et al., 2007); West African Dakar (Ogunjobi et al., 2008); the Eastern Mediterranean (Kaskaoutis et al., 2008) and the Tengger Desert, China (Xina et al., 2005). The mean AOD On March 11th was 0.604 with a maximum of 0.711 at the end of the day. On March 12th, AOD was lower in comparison with the previous days, reaching a low value of 0.262 close to the minimum AOD levels observed before the storm. Similar patterns were observed in the other wavelength bands. The AOD values at all wavelengths for the days prior to the dust event (8e9th March) were relatively low compared with those after the dust event. For example, the average AOD at 870 nm during the 8th and 9th was 0.265 whereas during the 11th and 12th it was 0.466.
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Wavelength (µm) Fig. 3. Daily average spectral aerosol optical depths for the period from March 8th to March 12th 2009.
4.3. Angstrom exponents (a) The hourly variations of a values for the wavelength range 440e870 nm over the five days are shown in Fig. 5. Apart from on March 9th, there was a distinct diurnal variation in a: it decreased through the morning hours to reach a minimum in the early afternoon and then increases for the rest of the day. At around 10:00 on the morning of March 10th, a was 0.196, but then dropped dramatically to 0.078 at the time when the dust cloud arrived. a stayed near this value for the next 3e4 h and then began to increase to a value of 0.036 by the end of the day. The mean a value on March 10th was 0.043, which is about 139% lower than the March mean value obtained by Kambezidis and Kaskaoutis (2008) for the same wavelength range. On March 11th the variations of a were relatively small, while on the 12th a resumed its diurnal pattern and continued to increase. The mean a values for the two days before the dust event (8e9th March) were higher compared with those obtained after the dust event.
4.4. Attenuation of solar radiation by aerosols e SMARTS modelling As solar radiation passes through the atmosphere it is attenuated by the physical processes of scattering and absorption. Absorption takes place in different spectral ranges for each atmospheric gas, whilst scattering exists across the whole spectrum with wavelength dependence due to the specific characteristics of the scattering particles. Atmospheric aerosols modify the solar radiation spectrum by attenuating the solar energy reaching the ground, and by changing the direct and diffuse radiation levels. The effect of different aerosol loads on both the spectral and broadband solar
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radiation components have been investigated extensively both experimentally and theoretically by several researchers (Latha and Badarinath, 2005; Kaskaoutis et al., 2006; Leckner, 1978; Kambezidis and Kaskaoutis, 2008). The aim of this section is to investigate how the turbidity conditions produced by the March 10th dust storm affected the solar irradiance components. For this purpose the radiative transfer model SMARTS (version 2.9.2) (Gueymard, 2003) was used. In the present study, March 9th had the lowest AOD values among the five days and we assume this to be a low turbidity day. Therefore, modification of the global irradiance components caused by the dust storm at Riyadh on March 10th and 11th were compared relative to conditions on the 9th. The global, diffuse and directbeam irradiances were obtained from the SMARTS model in the spectral band 280 nme1100 nm (panels a, b, c, respectively in Fig. 6). These irradiances were obtained at the same zenith angle (air mass 1.5) and for standard ozone and water vapour concentrations, whereas an urban aerosol model was chosen as being most representative of the atmospheric conditions at Riyadh. The analysis includes three cases: (i) typical background conditions with low aerosols (AOD 500 ¼ 0.247), (ii) with AOD 500 ¼ 1.306 representing the daily mean on March 10th, and (iii) with AOD 500 ¼ 0.604 representing the daily mean on March 11th. For the low dust day, the spectral distribution of the three components is similar to that of clear skies (Iqbal, 1983). However the effect of high aerosol loadings on the spectral distribution of the solar irradiances differs significantly from one wavelength to another for the 10th and the 11th. For instance, the reduction of the global solar irradiance component due to the aerosols on March 10th was about 50% and 26% for wavelengths of 500 nm and 1000 nm, respectively. These reductions are relative to March 9th. Meanwhile, the corresponding reduction for direct irradiance was 80% and 51% for wavelengths of 500 nm and 1000 nm, respectively. On the other hand, the diffuse component at wavelengths below 450 nm was not much affected by the dust storm on the 10th. However, for wavelengths greater than 450 nm, the diffuse irradiance showed large increases, with a red shift of the spectral diffuse irradiance due to the storm on the 10th, relative to values observed on the clear day (the 9th). For example, the diffuse irradiance increased by 40% at 550 nm, and by about 62% and 140% for the wavelengths 600 nm and 832 nm, respectively. Similar findings have been reported in similar studies (Badarinath et al., 2007; Latha and Badarinath, 2005; Kambezidis and Kaskaoutis, 2008). Examination of the integrated solar radiation components across the whole spectrum (280e1100 nm broadband) suggested that the turbid conditions significantly reduced the global irradiance, by 42% and 15%, on the 10th and 11th March, respectively. The direct irradiance component was strongly reduced, by 68% and 29%, on March 10th and 11th, respectively, compared with the much clearer conditions on March 9th, whereas the turbid atmosphere
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strongly affected by both atmospheric water content and screen temperatures (e.g. Prata, 1996; Ruckstuhl et al., 2007). However, other parameters are also expected to have some effect on the atmospheric emission. Their effect varies in importance according to the wavelength, atmospheric conditions and other factors such as the geography of the site. For example, under cloudless conditions the effect of suspended dust in the atmosphere is suspected to influence the transmission of IR sky radiation within the atmospheric window, AW (8 mme14 mm), consequently increasing the total atmospheric radiation. However, this effect has not received much consideration in the literature. In the following two subsections, the effect of the aerosol particles brought to Riyadh by the dust storm on the IR atmospheric radiation will be investigated.
Fig. 6. Estimated spectral irradiances from the SMARTS model: (a) global; (b) diffuse and (c) direct-beam, for three turbidity conditions: i) non turbid clear conditions with AOD 500 ¼ 0.24 (9th March), ii) for AOD 500 ¼ 1.3 (10th March), and iii) for AOD 500 ¼ 0.604 (11th March.).
4.5.2. Sky temperature (Tsky) and atmospheric emissivity (3) Figs. 7 and 8 show the hourly variations of the IR sky temperatures and emissivity during the five-day study period. The atmospheric emissivity was calculated following the procedures described in Prata (1996). Apart from on the day of the event, both Tsky and 3 follow a diurnal cycle that is much clearer for the latter than for the former. They both decrease from early morning to the afternoon and then increase again during the rest of the day. Both show little variation during the night in comparison with the day. Some of this variability in the daytime measurements may be due to heating and subsequent re-radiation from the IR transmitting optics of the detector, leading to some bias in the measurements. This bias is estimated to be around 3 C (Maghrabi et al., 2009). On the day of the dust event and before the arrival of the dust cloud, both parameters followed their normal diurnal pattern. However, after the passage of the dust cloud, Tsky jumped from 6.9 C to a maximum of 17 C and 3 increased from 0.661 to reach a maximum of 0.969 by about 13:00e14:00 h. The atmosphere during this time was almost completely laden with dust and its emission resembled that of a blackbody at screen level temperature. Both Tsky and 3 remained at around their maximum for almost 2e3 h and then continuously declined, until reaching minimum values of 15.69 C and 0.69, respectively, by late afternoon on March 11th. For the rest of the period both parameters again followed their usual diurnal pattern. The mean diurnal increase of about 9 C and 0.13 in Tsky and 3, respectively, on March 10th compared with March 9th was a consequence of the higher turbidity on March 10th. The standard deviations of both Tsky and 3 were also higher, confirming that the atmosphere experienced more variability in its atmospheric aerosol content during the time of the dust storm event (for example
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had an opposite impact on the diffuse component and increased it by 54% and 43% on March 10th and 11th, respectively.
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4.5.1. Background Infrared (IR) atmospheric radiation (wavelength 4.0e100 mm) is a key term in the surface energy budget and is vitally important for climatological and meteorological studies, as well as in applications such as solar energy and agricultural meteorology. Previous studies have concentrated on the measurement and evaluation of the total and spectral IR radiative flux (see Kjaersgaard et al., 2007 and references therein). Most of these studies were limited to clear-sky conditions and only a few have attempted to include the effects of cloud cover (e.g. Iziomon et al., 2003). It has been well established previously that atmospheric radiation under normal conditions is
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4.5.3. MODTRAN simulations To obtain a better understanding of how the dusty conditions affected atmospheric radiation, the radiative transfer model MODTRAN (Berk et al., 1989) was used. Upper air data from radiosondes launched from Riyadh Airport at 03:00 and 15:00 LST on each of the five days were used to define the pressure, temperature, and relative humidity profiles over Riyadh during the period. These profiles were used as input into MODTRAN, which is a user-defined atmospheric model. The profiles of other atmospheric constituents (e.g. CH4, CO, O2, NO, etc) were set to those of the MODTRAN standard MLS model. The effect of aerosol on atmospheric emissions was examined by assigning different values of visibility, ranging from clear to totally turbid conditions. Visibility is related to the presence of aerosols in the atmosphere and has an effect on the visible range of wavelengths. The influence of aerosols may be extended to the IR part of the spectrum if the size of the aerosol particles becomes comparable with the IR wavelengths. We ran MODTRAN with several different visibilities, starting with a high visibility, and then subsequently increased the amount of aerosol (decreasing the visibility values). In each run the spectral distribution of the atmospheric radiation, AR, was obtained. The visibility values tested were 25 km, 10 km, 8 km, 5 km, 2.5 km, 1 km, 500 m, 100 m, 50 m, 10 m and 1 m. Figs. 9 and 10 show the spectral distribution of atmospheric radiation for the selected visibilities. Fig. 9 shows the spectral distribution of atmospheric radiation for visibilities between 25 km and 2.5 km. While the variability within the AW increased slightly to a maximum of 8% between visibilities of 25 km and 2.5 km, there was little variability outside of the AW. Generally speaking, no major changes were found in the spectral distribution of the AR for most wavelengths and the spectral distribution resembles that of the clear sky, with visibility ¼ 25 km in this case (Goody and Young, 1989). Fig. 10 shows the spectral distribution of the AR for the selected visibilities below 1 km. The spectral distribution curve for 25 km visibility was added for comparison purposes. Two interesting features can be noted from this figure. Firstly, the atmospheric emission outside the AW does not change greatly and remains steady. In this case the effect of the dust storm on the IR spectral distribution can be considered to be negligible. For instance, at a wavelength of 6 mm, the increase in atmospheric radiation as visibility reduces from 25 km to 1 m does not exceed
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the standard deviation of 3 was 0.11 on the 10th compared with 0.05 on the 9th).
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Fig. 9. MODTRAN-calculated clear-sky atmospheric spectra for different visibilities: (a) Visibility ¼ 25 km; (b) Visibility ¼ 8 km; (c) Visibility ¼ 5 km and (d) Visibility ¼ 2.5 km; Data are from the Riyadh dust storm of March 10th, 2009.
3%. Additionally, the increases in the atmospheric radiation at 25 mm between visibility ¼ 25 km and visibility ¼ 1 m was less than 1%. Secondly, the change of the spectral distribution inside the AW region is evident as we move towards lower visibilities (increasing the amount of aerosol). The increase in AR continues up to visibility ¼ 500 m. A dramatic increase in the atmospheric radiation in the AW region occurs for visibilities of 50 m and 10 m. For instance, the increases in AR in the AW region as visibility is reduced from 25 km to 50 m and from 25 km to 10 m are about 365% and 400%, respectively. With 1 m visibility (Fig. 10d) the atmospheric window is totally closed and the atmospheric emission resembles that of a blackbody. In this case the presence of larger sizes of aerosols effectively works as a lid that emits radiation in this part of the spectrum. Therefore, the main effect of the dust storm on the IR atmospheric radiation was to increase the atmospheric emission greatly inside the AW, causing the atmosphere to emit as a full blackbody.
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Fig. 10. MODTRAN-calculated clear-sky atmospheric spectra for visibility of 25 km compared with (a) Visibility ¼ 1 km; (b) Visibility ¼ 50 m; (c) Visibility 10 m and (d) Visibility ¼ 1 m. Data are from the Riyadh dust storm of March 10th, 2009.
Fig. 11 shows the integrated sky temperatures for the different values of visibility in the wavelength range 5.5 mme50 mm. This represents the wavelength response of our detection system. The sky temperature increases by 3.5 C as visibility reduces from 25 to 1.5 km. At these visibilities the effect of other meteorological parameters such as water vapour contents and air temperature are dominant. However, for visibilities below 1 km the sky temperature increases exponentially with decreasing visibility. For example, at a visibility of 500 m the sky temperature is 4.2 C, while at a visibility of 300 m the sky temperature is 0.8 C, equivalent to an increase of about 3.5 C. For a visibility of 1 m, close to the minimum observed value during the dust storm, the integrated sky temperature for the wavelength range 5.5 mme50 mm was 16.5 C,
On 10th March 2009 a severe and extensive dust storm event struck Riyadh and lasted for several hours. This event was caused by a cold front passage coinciding with the propagation of a preexisting synoptic-scale upper tropospheric jet streak over the northern and central parts of Saudi Arabia. The impact of this event on groundebased measurements of meteorological parameters, aerosol optical depth (AOD), Angstrom exponent a, infrared sky temperature (Tsky) and atmospheric emissivity (3) were analyzed. The results showed that the effects of this storm were associated with an increase in both atmospheric pressure and relative humidity, and a reduction in temperature and visibility, for the two days following the storm in comparison with conditions before the storm. The analysis of the AOD data showed that the AOD at l ¼ 500 nm increased by 330% immediately after the storm. The daily mean values of the AOD were higher by 429%, 144% and 52% for the 10th, 11th and 12th, respectively, in comparison with the day preceding the storm, as a result of the high aerosol loadings. At the time of the arrival of the dust cloud the Angstrom exponent plunged to negative values. a values on the event day and on the following days were lower than those before the dust storm. In terms of solar radiation, theoretical simulations using SMART software showed that there was a remarkable decrease in the broadband global and direct irradiances reaching the ground, amounting to 42% and 68%, respectively, in comparison with the previous clear day. The diffuse solar irradiance components increased by 44%. On studying the effect of this severe storm on the IR sky temperatures and sky emissivity, we found increases of 24 C and 0.3, respectively, immediately after the arrival of the storm. Both parameters remained higher after the event in comparison with their pre-event values. Theoretical investigations of the effect of this storm on the spectral and broadband IR atmospheric radiation were conducted using MODTRAN software. It was found that effects of the substantial aerosol loadings caused an increase in atmospheric emissions in the atmospheric window (between 8 and 14 mm), so that the emissions in this window resembled those of a blackbody and the atmospheric window was almost closed. There was good agreement between the integrated MODTRAN and measured sky temperatures during the peak of the event. Acknowledgments The authors would like to thank King Abdulaziz City for Science and Technology (KACST) for supporting this work. The Presidency
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of Meteorology and Environment (PME) is also acknowledged for providing the meteorological data. We also thank Dr. Nai Alabadi, the principle investigator for the solar village site, for his efforts maintaining the station. We also thank the two reviewers for their valuable comments and suggestions.
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