Temporal Variability and Characterization of Aerosols across the Pakistan Region during the Winter Fog Periods

Khokhar, Muhammad Fahim; Yasmin, Naila; Chishtie, Farrukh; Shahid, Imran

doi:10.3390/atmos7050067

Cited by 43 publications

(26 citation statements)

References 76 publications

(69 reference statements)

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“…The duration of CAPs is found to increase with increasing valley width and depth until persistence occurs. This is consistent with phenomenological evidence which indicates that in small-scale terrain at mid and low latitudes, CAPs tend to be diurnal (Gustavsson et al, 1998;Clements et al, 2003 Sheridan et al, 2014), while persistent CAPs tend to be associated with a higher order of magnitude in terrain size, such as in the Salt Lake Valley area (Lareau et al 2013), the Alps (Zängl, 2003;2005b;Chemel et al, 2016;Largeron and Staquet, 2016), the Pyrenees (Pages et al 2017), and ranges of large mountains in the Middle East (Crosbie et al, 2014;Khokhar et al, 2016) and China (Bei et al, 2016;Song et al, 2016); a potential candidate also exists on Mars (Moehlmann et al 2009). The most likely physical causes of the behaviour with scale found here have already been broached, and are itemized below.…”

Section: Discussionsupporting

confidence: 86%

“…In addition, a high barrier surrounding a valley atmosphere may more readily encounter inversions formed away from the valley surface by subsidence or warm air advection aloft, which may impose a bouyancy "lid" confining air within the valley; Reeves and Stensrud (2009) show that increases in mid-level temperatures are associated with persistent CAP formation in a selection of deep basins in the Rocky Mountains. Numerous locations around the world which experience multi-day or "persistent" CAPs tend to lie in deeper valleys of calO(1,000 m) depth (Whiteman et al, 2001;Zängl, 2003;2005b;Billings et al, 2006;Lareau et al, 2013;Crosbie et al, 2014;Bei et al, 2016;Chemel et al, 2016;Khokhar et al, 2016;Largeron and Staquet, 2016;Song et al, 2016;Pages et al, 2017). Persistent CAPs in urbanized or industrialized areas have serious implications for accumulation of pollution and it is important to understand how and why this transition from small, diurnal CAPs to large, persistent CAPs occurs as a function of increasing scale and atmospheric conditions.…”

Section: Introductionmentioning

confidence: 99%

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Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

Sheridan

2019

Quart J Royal Meteoro Soc

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Idealized two‐dimensional simulations have been used to examine the dependence of valley cold‐air pool (CAP) behaviour, morphology and duration (persistence) on terrain scale. The simulations use terrain of constant aspect ratio but ranging from modest to Alpine scale, under midlatitude winter conditions with a finite background flow and incorporating a diurnal cycle. CAPs in smaller valleys are found to be more morphologically constrained, whereas larger valleys provide scope for more complex flow dynamics and more dramatic interplay of the external flow with the valley, including gravity wave breaking. Simulations of a valley within a plateau show that, as valley depth exceeds the depth‐scale of the nocturnal stable boundary layer, the CAP becomes increasingly confined and sheltered from the background flow (reduced mixing), becoming harder to remove by processes related to daytime insolation, and it can eventually become persistent (never removed). Both the surrounding terrain and gravity wave dynamics are found to play a part in the enhancement of sheltering. Therefore background atmospheric stability and wind are also crucial, summarized in a non‐dimensional valley depth parameter which largely determines CAP duration. More stable conditions, representative of (for instance) large‐scale subsidence or warm advection aloft, result in longer durations and a greater likelihood of persistence. Stronger nocturnal radiative cooling results in stronger CAPs, which are harder again to remove. Several mechanisms appear to act in concert to give rise to these behaviours and the dependence on non‐dimensional valley depth. Replacing the valley‐in‐plateau with a double hill enhances wave breaking, which complicates the relationship to non‐dimensional valley depth, reducing CAP duration significantly when subsidence or warm advection‐like effects are absent.

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Section: Discussionsupporting

confidence: 86%

Section: Introductionmentioning

confidence: 99%

Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

Sheridan

2019

Quart J Royal Meteoro Soc

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“…Using these retrievals, the mean AOD at various temporal resolutions for each region was calculated. Based on the local climatology the summer season comprises of the months June to September (JJAS), whereas December to February (DJF) corresponds to the winter season [74][75][76][77]. In addition, a number of statistical tests, including the modified Mann-Kendall (m-MK), Sen's slope (SS) estimator, and sequential Mann-Kendall (SqMK) tests were used to quantify the trends in annual and seasonal AOD time series [78][79][80][81][82].…”

Section: Methodsmentioning

confidence: 99%

Spatiotemporal Trends of Aerosols over Urban Regions in Pakistan and Their Possible Links to Meteorological Parameters

et al. 2020

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Aerosol optical depth (AOD) has become one of the most crucial parameters for climate change assessment on regional and global scales. The present study investigates trends in AOD using long-term data derived from moderate resolution imaging spectro-radiometer (MODIS) over twelve regions in Pakistan. Different statistical tests are used to assess the annual and seasonal trends in AOD. Results reveal increasing AOD trends over most of the selected regions with an obvious increase over the north and northeastern parts of the study area. Annually, increasing trends (0.0002–0.0047 year−1) were observed over seven regions, with three being statistically significant. All the selected regions experience increasing AOD trends during the winter season with six being statistically significant while during the summer season seven regions experience increasing AOD trends and the remaining five exhibit the converse with two being statistically significant. The changes in the sign and magnitude of AOD trends have been attributed to prevailing meteorological conditions. The decreasing rainfall and increasing temperature trends mostly support the increasing AOD trend over the selected regions. The high/low AOD phases during the study period may be ascribed to the anomalies in mid-tropospheric relative humidity and wind fields. The summer season is generally characterized by high AOD with peak values observed over the regions located in central plains, which can be attributed to the dense population and enhanced concentration of industrial and vehicular emissions over this part of the study area. The results derived from the present study give an insight into aerosol trends and could form the basis for aerosol-induced climate change assessment over the study area.

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“…The increase in the concentration of hygroscopic atmospheric particles can promote the formation of fog droplets even in unsaturated conditions, and the thickness of the fog layer will also increase with the increase of the concentration of atmospheric particles [14]. The intensity of fog will decrease with the decrease of aerosol concentration [15]. The hygroscopicity of atmospheric particles has a high dependence on particle size distribution and chemical composition, which changes with different conditions of relative humidity.…”

Section: Introductionmentioning

confidence: 99%

Fog Droplet Size Distribution and the Interaction between Fog Droplets and Fine Particles during Dense Fog in Tianjin, China

Liu

Wang

et al. 2020

Atmosphere

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From November 2016 to January 2017, there were large-scale dense fog processes in Tianjin area on the west coast of Bohai Bay, China, even strong dense fog with visibility less than 50 m occurred. Based on the observation data of fog droplet spectrum monitor, visibility sensor, environmental particle monitoring equipment and meteorological automatic station, the characteristics of fog droplet size distribution and the interaction between the fog droplets and fine particles during dense fog events were analyzed. The results show following characteristics: (1) The average concentration of fog droplets (Na), the average liquid water content (La) and the maximum liquid water content (Lmax) in the strong dense fog process are larger than those in the dense fog. The average spectrum of fog droplet size distribution conforms to Junge distribution, and they are all broad-spectrum fog with a spectrum width of about 45 μm. The average spectrum is similar to the dense fog of heavily industrialized inland in the world. (2) The maximum of fog droplet diameter during the formation stage have a good indication for the outbreak of strong dense fog. (3) The mass concentration of PM2.5 (CPM2.5) is ranged from 121–375 μg/m3, and the interaction between fog droplets and fine particles is analyzed. During the formation, development and maturity stages, fog process can scavenge atmospheric fine particles, and the scavenging efficiency of PM2.5 is more remarkable than PM10. When CPM2.5 does not exceed 350 μg/m3, the increase in the concentration of fine particles is conducive to the rapid growth of fog droplets and the sharp drop of visibility. However, when CPM2.5 exceeds the critical value, the increase has a negative feedback effect on the development of the fog process. More investigations and cases are necessary to fully assess the mechanisms related to the dense fog events in Tianjin area and further analysis will be done.

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Temporal Variability and Characterization of Aerosols across the Pakistan Region during the Winter Fog Periods

Abstract: Fog is a meteorological

Cited by 43 publications

References 76 publications

Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

Synoptic‐flow interaction with valley cold‐air pools and effects on cold‐air pool persistence: Influence of valley size and atmospheric stability

Spatiotemporal Trends of Aerosols over Urban Regions in Pakistan and Their Possible Links to Meteorological Parameters

Fog Droplet Size Distribution and the Interaction between Fog Droplets and Fine Particles during Dense Fog in Tianjin, China

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