Impact of Dust Storm on the Atmospheric Boundary Layer over Ahmedabad, a Western Indian Region .

The present study focuses on investigating the impacts of a sudden dust storm on the atmospheric boundary layer (ABL) over Ahmedabad (23.02°N, 72.57°E), an urban site located in the western region of India. The accumulation of dust in the atmosphere during the dust storm, originating from the Thar Desert in Rajasthan, led to the decrease in surface temperature as a consequence of dust-radiation interaction. Ambient particulate matter data obtained from Air Quality (AQ) station at Ahmedabad showed a spike of 118.5% and 44.5% in PM10 and PM2.5 concentrations, respectively during the event in comparison to the previous control day. Sudden exposure to an anomalous increase in particulate aerosols may cause severe impacts on human health. These surface forcing have been reected in the stable nocturnal ABL. Backscatter signals recorded by ground-based Ceilometer Lidar at Physical Research Laboratory (PRL), showed that ABL was shallow and collapsed during the dust storm episode. Turbulence was detected in the ABL during the event which further assisted in the vertical mixing of dust aerosols in the ABL. These aerosols got trapped within the residual layer, preventing further percolation in the free atmosphere. Such sub-grid scale changes in the ABL during the dust storm were not reected in the boundary layer height (BLH) obtained from the ERA-5 reanalysis dataset. A signicant association between the ABL and the local radiative budget has been found. It has been substantiated by Coupled Ocean-Atmosphere Radiative Transfer Model (COART) simulations that showed a cooling of the surface. Thus, this study is important as it can be taken as feedback to improve local climate models with respect to dust storm meteorology.


Introduction
Dust aerosols are complex tiny entities suspended in a small fraction of the atmosphere, however, contributing substantially to the earth's radiative budget as these particles are responsible for both re ecting and absorbing shortwave and longwave radiations (Houghton and T., 1986;Kok et al., 2017).
The presence of enormous amounts of dust aerosols in the atmosphere affects the optical properties of the atmosphere such as visibility (Auvermann et al., 2006, Baddock et al., 2014, biogeochemical process and severely impact human health (Zhang et al., 2014;Querol et al., 2019). A huge in ux of dust is usually witnessed during dust storm episodes over a region. Dust storms are sporadic meteorological events that transport dust and sand from the hotspots and deposit to areas far away from the source owing to high wind speeds. Dust storms modulate the near-surface atmospheric dynamics and have secondary impacts on human health (Han et al. 2013). In addition to mineral dust and sand, dust storms are also responsible for carrying microbes causing harm to humans (Hua et al. 2007;Arnold 2020).
Several studies have been dedicated to understanding the impact of dust storms on the atmosphere at varying scales (Iwasaka et al. 1983;Husar et al. 2001;Yang et al. 2008;Patel and Kumar 2015 and references therein). Despite the efforts, the robust understanding remains elusive. There are several techniques and methods to detect and monitor dust storms (Muhammad Akhlaq et al. 2012). Satellite observations are used to monitor the dust storms (Badarinath et al. 2007;Sharma et al. 2009;Mishra et al. 2015;Di et al. 2016;Sabbah et al. 2018; Abhiram and Satapathy 2020), however intricate local effects are usually obscured by such long distance remote sensing with modest vertical resolution, cloud presence, density and re ectivity of dust plume and so on. Wang et al. (2013) have used wind pro ling radar to study the dust storm over Taklimakan desert in detail. Ming et al. (2019) have used a groundbased multimeter wave radar to detect dust storm in the same desert region, where re ectivity factors of dust storms were better estimated in real time in comparison with satellite remote sensing. However, the spatial movement of the dust storm cannot be comprehended by ground based instruments. Thus combined study of satellite and ground observations can give a clearer picture of the dust storms. Chakravarty et al. (2021) have made synergetic use of satellite, radar, ground-based observations and models to study a severe dust storm in Northern India. Aher et al. (2014) have used multiple platforms to study the effect of dust storm on aerosols in western India. The western semi-arid Indian region is frequented by dust storm episodes during pre-monsoon season, originating from the desert in Rajasthan and middle-eastern countries (Dey et al. 2004;Santra et al. 2010;Yadav et al. 2017b).
Previous studies by Sanwlani et al. (2011), Sharma et al. (2012, Singh et al. (2016), Soni et al. (2018), Chhabra et al. (2021) and references therein, have reported the study of dust storms in the western India region using satellites, aerosol spectrometers and so on, with major focus on air quality and radiative forcing. However, little is known about the impact of dust storms on the atmospheric boundary layer over this region. Atmospheric boundary layer is the lowermost region of the Earth's atmosphere that plays a crucial role in exchanging heat, energy and momentum with the free troposphere. Being closest to the Earth's surface, the boundary layer interacts with the biosphere and plays a signi cant role in the dispersion of pollutants and impacts the radiative budget of the planet (Garratt 1994;Li et al. 2021). Heinold et al. (2008) have studied the dust radiative response on the Saharan boundary layer using a regional dust model LM-MUSCAT. Alizadeh Choobari et al. (2012) have studied feedback of windblown mineral dust and the planetary boundary layer using WRF-CHEM regional model, during a dust event in Australia. Rémy et al. (2015) have reported the modi cation of boundary layer meteorology during a dust storm event in the east Mediterranean using MACC-II, WGNE and ECMWF models. Chen et al. (2017) have used the WRF-CHEM model to understand the impacts of soil dust on the boundary layer meteorology during a severe dust storm in East Asia. However, with the inherent uncertainties of the models, groundbased remote sensing instruments have an upper hand to measure boundary layer dynamics accurately during dust storm events (Todd et al. 2008;Wang et al. 2008;Yuan et al. 2019;Wu et al. 2020). The usual in-situ methodologies for measuring boundary layer characteristics such as radiosondes cannot be deployed during a dust storm event as strong winds will sway the radiosonde from its desired location.
Ground-based Ceilometer lidars have proved to be signi cantly helpful in measuring boundary layer characteristics during dust storm events (Zhang et al. 2005;Luo et al. 2014;Uzan et al. 2020). Previous studies by Schafer et al. (2004), Emeis and Schäfer (2006), Herrera-Mejía and Hoyos (2019), and references therein have used this instrument to study the characteristics of the boundary layer.
This study is a rst-of-a-kind report of the impact of a dust storm on the atmospheric boundary layer in western Indian region. Ground-based observations of the boundary layer have higher precision than reanalysis datasets. Previous studies have reported the limitations of ERA-5 in simulating the atmospheric boundary layer (Chen et al. 2020a). The boundary layer is crucial in the atmosphere as it helps in dispersing the atmospheric pollutants (Kotthaus et al. 2018). In this study, additional information about the surface forcings and ambient particulate matter has been studied from SAFAR Air Quality Station. The dust storm impact on the local boundary layer further in uenced the local radiative budget that has been supported by the COART (Coupled Ocean Atmosphere Radiative Transfer) model. This study has been organized as follows: Sect. 2 contains a description of the data sources used in this study, followed by results and discussions in Sect. 3. Section 4 includes the conclusion.

Data Sources
Ceilometer Lidar An all-weather Ceilometer Lidar (CL31) is operational at Physical Research Laboratory (PRL) (23.02°N, 72.5°E). This lidar is mono-axial, wherein the transmitting of laser and receiving of backscatter signal is done using one lens only. The lidar transmits a laser at 910 nm wavelength that can probe the atmosphere up to a height of 7.5 km. It has a very high vertical resolution of ~10 meters and can record consecutive lidar signals at a temporal resolution of ~16 seconds. More information about this CL31 Ceilometer can be found at http://www.vaisala.com . For this study, the gradient method has been used to derive the boundary layer from the backscatter signals (Hennemuth and Lammert, 2006;Münkel et al., 2007;Lin et al., 2012).

Moderate Resolution Imaging Spectroradiometer (MODIS)
In this study, visible satellite images for 26-28 April 2021 have been obtained from MODIS, an important instrument on-board the Terra/Aqua satellites. MODIS acquires data in 36 spectral bands within the wavelength range of 0.4~14.4 µm covering visible, NIR, and TIR bands. More information about MODIS can be found at https://modis.gsfc.nasa.gov/about/. Aerosol Optical Depth across 470, 550, and 660 nm derived from the deep blue algorithm of MODIS retrievals has also been used in this study to estimate the impact of a dust storm on the aerosol loading over the observation site. AOD from MODIS has also been used as input to the COART model run to study the impact of a dust storm on the radiative budget.

SAFAR AQ Station
An Ambient Air Quality Monitoring Station has been installed at Space Applications Centre, Ahmedabad, under Ministry of Earth Sciences, System of Air Quality and Weather Forecasting And Research (SAFAR) programme of Govt. of India. This SAFAR station is located at an urban location in western Ahmedabad and is 4 km away from the measurement site (Chhabra et al. 2020). In this study, meteorological parameters such as temperature, relative humidity, wind speed have been taken from this station. In addition, values of PM10. PM2.5 and PM1 have also been obtained from the AQ station.
More information can be found at https://www.ecmwf.int/en/forecasts/datasets/reanalysisdatasets/era5. In this study, atmospheric boundary layer height over Ahmedabad for 26-28 April has been obtained from ERA-5 for comparison with Ceilometer observations. A one-hour moving average lter has been applied on Ceilometer observations in order to compare with hourly ERA-5 data.
Coupled Ocean-Atmosphere Radiative Transfer (COART) Model In this study, the Coupled Ocean and Atmosphere Radiative Transfer (COART) model has been used for simulating radiative forcing estimates for the dust storm event over Ahmedabad. It calculates radiance and irradiance ( uxes) at different levels in the atmosphere and ocean. Model simulations for radiative forcing uxes have been performed for pre to post dust storm event period using the online available  Figure 1 that the observational site was swept by a dust storm on April 27. The dust storm appears in a light brown shade, partially obscuring the dark land targets on 27 April. The storm has originated from the desert plains of Rajasthan, a North-western Indian region, reported by Indian Meteorological Department (IMD), Gujarat. Figure 2 shows the one-day back trajectory of the air mass, calculated using HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model, ending at 500 m from the surface over Ahmedabad, with a new trajectory starting every 3 hours. An in ux of air mass from the north of Gujarat and Rajasthan has been observed on 27 April at 1500 UTC, which coincides with the dust storm arrival time over Ahmedabad. Several places in Gujarat (adjacent to the place of origin) experienced rainfall according to IMD reports. However, due to lesser moisture content over Ahmedabad, this took the form of a dust storm on 27 April. This sudden dust storm had several implications on the local atmosphere, radiation budget as well as human health.
In this study, we have taken 26 and 28 April as the control days and 27 April as event day. The meteorological parameters obtained from SAFAR AQ station in Ahmedabad on all three days have been studied in order to understand the impact during a dust storm. Reduced solar radiation due to the dust layer in the atmosphere led to a decrease in the surface temperature. Figure 3a shows that temperature decreases to 30.7 °C during the dust storm episode associated with cold air advection, correspondingly the relative humidity shows a small peak during the same period. From Figure 3c, strong winds of 5m/s ( ) can be observed after 8 PM (local time, UTC + 5.5 hours ) on 27 April over the observational site as compared to control days (Romanic and Hangan 2020). Rémy et al. (2015), in their study of the boundary layer during a dust storm in the eastern Mediterranean, found similar meteorological conditions wherein local surface winds increased due to the thermal wind effect. Reduced maximum temperature of the surface during the night also forced stronger surface winds.
Next, in this study, we have studied the changes in aerosol loading over Ahmedabad due to the dust storm. We have used ambient Particulate Matter (PM) data from the AQ station located in Ahmedabad. Figures 3d and 3e show the concentration of PM10 and PM 2.5 over the observational site during all control and event days. PM10 and PM2.5 concentrations surge on the event day by 118.5% and 44.5%, respectively, as compared to the previous control day during night-time. It has been reported in previous studies that PM10, PM2.5 values in urban areas are usually low during the daytime due to reduced tra c and dispersion in the mixed layer and the concentration of particulate matter aerosols peaks in the night as the boundary layer gets shallow (Yadav et al. 2014;Liu et al. 2015). During this dust storm event, longrange transportation of coarse desert dust in addition to changes in boundary layer contributed to such high values of PM10 and PM2.5 (746.5 and 273.8 ,respectively) . PM1 values increased during the dust episode. However, no signi cant changes in PM1 have been observed as compared to control days. A similar increase in PM10/PM2.5 during pre-monsoon season in Udaipur has been reported by Yadav et al. (2017) due to dust storm events. Querol et al. (2019), Achilleos et al. (2014), Aryal et al. (2012), Farahani and Arhami (2020) and references therein have also reported elevated PM concentrations during dust storm events.
Studies in the past have reported adverse health issues such as premature mortality, exacerbate bronchitis, asthma attacks, other respiratory symptoms, cardiovascular attacks, etc., if humans are exposed to the sudden increase of particulate matters (Wilson and Suh, 1997;Bell et al., 2007;Kim et al., 2015;Kumar Rai, 2015;Abel et al. 2018;Chhabra et al., 2020) Another direct impact of the dust storm can be seen in the atmospheric boundary layer over this region. Backscatter from the aerosols has been continuously recorded by the Ceilometer stationed at Physical Research Laboratory, Ahmedabad. Figure 4 shows the range-time intensity plot of the lidar backscatter during 26-28 April 2021. A well-formed boundary layer can be observed on 26 April, with a stable, dense nocturnal boundary layer after the sunset. Similar signatures of the convective mixed layer can be observed on 27 April, except for the shallow and collapse of the boundary layer after 8 PM. The boundary layer, however, rebuilds on the next day. The time of boundary layer collapse is not a mere coincidence with the occurrence of the dust storm (Choi et al. 2008). This dust storm event had a direct impact on the local radiative budget. Desert mineral dust is cooling in nature as they re ect the incoming solar radiation (Mallet et al. 2009), thus reducing the surface temperature (Satheesh and Krishna Moorthy 2005). However, some studies by Kok et al., (2017), Klingmüller et al., (2019), and references therein have reported the uncertainty of dust aerosols in the radiative budget.
Surface temperature and humidity are driving factors of the boundary layer (Garratt 1994). Thus, the decrease in surface temperature, due to the cooling effect of the dust layer, may have led to the sudden collapse of the stable nocturnal boundary layer after 8 PM (local time). However, strong backscatter from the dust in the residual layer within 3.5-4 km can be observed on the event day. The inversion layer prevented dust from percolating in the upper troposphere. Similar observations of dust layer trapped within the inversion layer have been reported by (Kawai et al. 2019) over the Gobi Desert. Aerosols in the nocturnal residual layer on control days show different signatures than that observed on the event day. Strong attenuated backscatter signals can be observed around 1-1.5 km on 26 and 28 April during the night, indicating accumulation of aerosols within that height range. But almost uniform backscatter is recorded by the Ceilometer from the residual layer on 27 April, indicating proper vertical mixing of dust aerosols. Wavelet analysis of the backscatter received by the Ceilometer indicated the presence of signi cant turbulence around 8 PM on the event day, as shown in Figure 5. Only the night time spectrum has been shown in Figure 5 to get a clear picture of the effect of turbulence on the vertical mixing in the residual layer (Qiao et al. 2016;Chen et al. 2020;Polnikov 2020). High-speed winds observed during the episode aided dust particles from the surface to suspend in the air. The unstable boundary layer further encouraged dust to rise higher in the air. The oating dust layer cleared the next morning after the dust storm, as seen in Figure 4. This can be substantiated by lower PM values on 28 April (Figure 3 d-f) as compared to the dust storm episode.
Further, we have compared the boundary layer obtained from Ceilometer with ERA-5 reanalysis. Figure 6 shows the diurnal variation of the boundary layer from ERA5 during 26-28 April. The convective boundary layer height during daytime is comparable with the Ceilometer retrievals, however, modulations of the nocturnal boundary layer during the dust episode have not been represented in the ERA-5 reanalysis dataset. Moreover, the stable nocturnal boundary layer height has been highly overestimated in ERA5 in comparison to ground-based observations.
The association of the collapse of the boundary layer with the local radiative budget has been studied using the COART model. Figure 7 illustrates dynamics of the estimated radiative uxes of diffused down, direct down, total downward, total upward, and the upward/downward ratio during pre (25 th April) to dust storm event day (27 April) to post-event (29 April). The diffused downward ux increased with increasing aerosol loading in the atmosphere, with a peak on the dust event day. The direct downward ux decreased by 25.1% during 25-27 April and gradually increased post-event. The high AOT due to heavy dust events resulted in alterations in radiative forcing balance by intercepting the sunlight reaching the earth's surface. This is marked by a 94% increase in the up/down ratio during 25-27 April of the study period. Post-dust storm event, this up/down ratio gradually decreased with reduced AOT, thus indicating a 'local cooling effect' due to the scattering-type of aerosols in the atmosphere resulting from a thick dust storm. Chhabra et al. (2021) also reported a 'surface cooling effect' with radiative forcing effects of dust aerosols over Ahmedabad and their likely impacts on the radiative budget at a regional scale. Using WRF Chem model simulations, Kedia et al., (2018) estimated dust storm-induced cooling effect at the surface in shortwave resulted from severe dust storm over India and surrounding oceanic regions of the Arabian Sea and Bay of Bengal.

Conclusion
This is a case study on the implications of a sudden dust storm on the boundary layer over Ahmedabad. During this episode of a dust storm, the surface temperature dropped, accompanied by a rise in relative humidity and strong surface winds. These winds forced the surface dust-up in the air. The aerosol loading over this region has been studied using Air Quality data over Ahmedabad. PM10 and PM2.5 concentrations hiked by 118.5% and 44.5%, respectively as compared to the previous control day. Sudden exposure to high concentrations of particulate matter in the ambient air can cause serious health issues. In addition to changes in the meteorological parameters, the nocturnal boundary layer collapsed during the dust storm episode. The unstable boundary layer further encouraged dust higher in the air. Wind shear and differential heating in the boundary layer gave rise to turbulence during the dust storm episode, which aided in vertical mixing throughout the residual layer. Dust aerosols got trapped in the inversion layer, which prevented the percolation of dust in a free atmosphere. The sudden accumulation of dust particles in the atmosphere reduces the surface temperature over this region as dust particles are cooling in nature. The forcing in surface temperature has been re ected in the boundary layer dynamics. This episode had a signi cant impact on the local radiative budget, causing surface cooling.
Declarations Figure 1 Visual satellite images obtained from MODIS onboard TERRA on 26-28 April 2021. The image obtained for 27 April shows dust cover in and around the observation site.
Page 17/20 One-day back-trajectory of air mass ending at 500 m above the surface over Ahmedabad during 15UTC April 27, 2021.
Page 18/20 Range-Time intensity plot of the backscatter received by Ceilometer over Ahmedabad.

Figure 5
Wavelet power spectrum of the backscatter signals received by the Ceilometer on 26, 27 and 28 April,

Figure 7
Aerosol Radiative forcing Integrated uxes (Wm-2) estimated for dust storm event over Ahmedabad.