IASTA-2010

E–O–1

Aerosol Radiative Forcing over an Urban Location: Observations and Model Estimates

Rohit Srivastava* and S. Ramachandran

Physical Research Laboratory,Ahmedabad, India * rohits@prl.res.in

Introduction

Atmospheric aerosols play an important role in earth – atmosphere radiation balance. They scatter and absorb the incoming solar radiation, and outgoing terrestrial radiation.

The role of aerosols in the radiation budget is one of the largest source of uncertainty in validating the model prediction of climate change [IPCC, 2007]. Generally the aerosol radiative forcing estimated by the modeled fluxes with external mixing of aerosols provide the lower bound of the forcing i.e., more cooling than the homogeneous internal mixture, with core - shell result in between [Lesins et al., 2002]. According to the simulation for elemental carbon forcing, the core - shell mixing state shows 50% higher forcing than external and 40% lower forcing than well internally mixed aerosols [Jacobson, 2000]. The uncertainly in the observed flux is lesser than the fluxed estimated by models based on several assumptions. The down – welling fluxes and aerosol optical parameters are simultaneously measured and aerosol radiative forcing will be estimated using these aerosol parameters in radiative transfer model and differences between them will be studied.

Measurements and Methodology

Ground reaching broad-band global and diffuse fluxes in wavelength range 0.31 to 2.8 ìm are measured over an urban location Ahmedabad (23.03oN, 72.55oE) during 2008 using set of pyranometers (Kipp and Zonen Model CM21). Down – welling fluxes are measured at every five minute resolution during each clear sky day of year 2008. The absolute accuracy in pyranometer measured fluxes is 5%, however the uncertainty due to the directional response can be ±10 Wm-2. Simultaneous measurements of aerosol optical depth (AOD) at five wavelengths (0.38, 0.44, 0.5, 0.675, 0.88 ìm) using Microtops II sunphotometer are also conducted. Column ozone and water vapour are also observed using Microtops II ozonometer. The absolute uncertainty in the AOD values are less than 0.03 at all wavelengths.

Measurement Location and Meteorology

All the measurements are performed at the campus of the Physical Research Laboratory (23.03oN, 72.55oE, 50 m amsl), located in the west of the Ahmedabad an urban city with large and small scale industries and variety of vehicles. Thar desert and the Arabian Sea are present in the northwest and southwest of the Ahmedabad respectively, which are the major sources of the mineral dust and sea salt. The location is affected with both

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anthropologically produced sub-micron aerosols along with the naturally produced coarser mineral dust and sea salt aerosols.

During winter (December – February), winds over Ahmedabad are either north easterly or north westerly, which transport aerosols of continental origin and land derived dust particles, while during pre- monsoon (March – May) and monsoon season (June – September), the winds are north-westerly or south-westerly which transport aerosols of marine origin. During post-monsoon (October – November) season the winds are calm and from the random direction.

Results and Discussion

The diurnal variations of total and diffuse fluxes along with the aerosol optical depths on 06, 16 January and 14 May 2008 are shown in figure 1. Higher AOD and lower diffuse flux are observed on 16 Jan when compared to 06 Jan and 14 May. The enhancement in diffuse flux on 06 Jan is observed due to large scattering of the solar flux by the aerosols. On 06 Jan total flux is lower than that on 16 Jan, because the direct flux is reduced due the large aerosol extinction (absorption and scattering) of the direct flux and hence total flux is also reduced. The total flux is higher in May, because of the higher solar insolation in May. The diffuse flux on May 14 is high in comparison to that on 06 Jan while the AOD is low. The more scattering of solar flux occur due to the dominance of scattering aerosols (e.g. Sea salt) over the location during May which are transported by the south- westerly winds. Thus aerosols significantly modify the direct and diffuse fluxes.

Aerosol optical properties are also be used to simulate total, direct and diffuse fluxes. The measured AOD, SSA, and black carbon mass concentration are used to constrain the output of OPAC model [Hess et al., 1998] considering external mixture of the aerosols, and hence aerosol optical depth, single scattering albedo along with asymmetry parameter are estimated for the entire shortwave region. The aerosol optical parameters such as AOD, SSA and asymmetry parameter are then used as inputs in the radiative transfer model (SBDART) [Ricchiazzi et al., 1998] to estimate fluxes at earth’s surface, atmosphere and top of the atmosphere.

Figure 1. Diurnal variations of ground reaching broad-band total, diffuse fluxes and aerosol optical depths at 0.5 ìm over an urban location Ahmedabad (23.03oN, 72.55oE) on (a) January 06, (b) January 16, and (c) May 14, 2008

Fluxes will also be estimated by models using the measured optical parameters of aerosols and aerosol radiative forcing will be calculated. It is anticipated that the comparison between the observed and model estimated fluxes will provide the better understanding of

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the aerosol’s impact on the radiation balance regarding the states of compositional mixing of the aerosols.

Summary

The observed and model estimated fluxes and the radiative effects of aerosols during different seasons are studied. The comparison between them will lead to information of state of compositional mixing of the aerosols. Down - welling total and diffuse fluxes simultaneously with the AOD are measured over and urban location Ahmedabad. Higher AOD shows the enhancement in the diffuse fluxes and reduction in total fluxes. However, the dominance of particular type aerosols, e.g. scattering aerosols can also increase the diffuse flux. The fluxes, using measured aerosols optical parameters into the radiative transfer model, will be simulated and aerosol radiative forcing will be calculated and compared with observed one. Detailed results obtained on the observed and model estimated fluxes and aerosol radiative forcing and their seasonal variability will be presented and discussed.

References

Jacobson, M.,Z. (2001), Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols, Nature, 409, 695-697.

Hess, M., P., Koepke., and I., Schult (1998 ), Optical properties of aerosols and clouds: the software package OPAC, Bull. Amer. Meteor. Soc., 79, 831— 844.

Lesins, G., P. Chylek, U. Lohmann, (2002), A study of internal and external mixing scenarios and its effect on aerosol optical properties and direct radiative forcing, J. Geophys. Res., 107, doi:10.1029/201JD000973.

Ricchiazzi, P., S. Yang, C. Gautier, and D. Sowle (1998), SBDART: A research and teaching tool for plane- parellel radiative transfer in the Earth’s atmosphere, Bull. Am. Meteorol. Soc., 79, 2101 — 2114.

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E–O–2

Modulation of Spectral AOD by Stratospheric Quasi Biennial Oscillations

*S Naseema Beegum1, K Krishna Moorthy1, S Suresh Babu1,

R Ramakrishna Reddy2, K Rama Gopal2 and Y Nazeer Ahmed3

1Space Physics Laboratory,VSSC,Trivandrum – 695022, India

2Department of Physics, Sri Krishnadevaraya University,Anantpur – 515003, India

3National Physical Laboratory, Dr. K S Krishnan Marg, New Delhi – 110012,India

* Corresponding author email address: bsnaseema@yahoo.co.in

Introduction

Quasi Biennial Oscillations (QBO) are downward propagating easterlies or westerlies in the wind field over the equatorial stratosphere having a variable period in the range of ~ 21-32 months and have an approximate Gaussian distribution about the equator with a half-width of ~12° latitude. In the present study, we examine continuous, long- term time series data of aerosol optical depths (AOD) at 4 tropical stations, over Asia and Africa, to delineate signatures of long-period oscillations. Among the four stations, two of them are ISRO-GBP network observatories of Trivandrum (8.55° N, 76.9° E, TVM) and Anantapur (14.7° N, 77.6° E, ATP) in India (South Asia), with measurements of spectral AODs using Multi-Wavelength Radiometer, and the other two are AERONET stations of Ouagadougou (12.2 ° N, 1.4 °W, OGU, in Africa) and Solar Village (24.98 °N, 46.38 °E, SV, in Arabia) with CIMEL sun-photometer measurements.

Results and Discussions

The time series of monthly mean AOD at 500 nm showed the presence of strong oscillations with periods of 2-3 years (Quasi Biennial Oscillations, QBO) in addition to the annual oscillations. The corresponding monthly mean zonal wind speed U, at 50 hPa level (NCEP) also showed similar oscillations.

In order to resolve the dominant periodicities, wavelet analysis was performed on the time series data and is shown in the Figure 1 (left panel) and the corresponding zonal winds (at 50 hPa) on the right panel. All the four stations revealed strong annual component (AO, period ~12 months) in both AOD and U and significant signatures of QBO with periodicity in the range around 20 to 32 months. To quantify these, we estimated the amplitudes as well as phases of these periodicities and from the amplitudes, the percentage contribution of each periodicities to the climatological mean was calculated. It is observed that the QBO, which is basically a stratospheric phenomenon, contributed as much as 10 % to 19 % to the annual mean AOD at 500nm; with the highest contribution (19 %) at OGU and least (10 %) at SV. This could be due to the presence of the anomalous meridional circulation associated with the stratospheric QBOU. During the easterly (westerly) phase of the QBOU, the contrasting thermal regimes in the lower and upper stratosphere favor equatorial convection (subsidence) and an

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off equatorial subsidence (convection); hence a cooler and higher (warmer and lower) tropopause near the equator and a warmer and lower (cooler and higher) tropopause at the off equatorial regions. Hence, during the westerly phase of the QBOU, the divergence from the equatorial upper troposphere would enhance the aerosol loading at the off equatorial station SV, where the amplitude of QBOU is normally negligible. This leads to the mixing up of tropospheric air, resulting in the entrainment of significant amount of tropospheric aerosols to the stratosphere, leading to an increase in the abundance of stratospheric aerosols, which could then be modulated by the stratospheric QBOU. The analysis revealed discernible signatures of QBO in AOD at the equatorial locations over Asia and Africa, due to the significant contribution of upper tropospheric/lower stratospheric aerosols to the column AOD.

Figure 1. The wavelet spectra of the time series of monthly mean AOD (at 500 nm) along with that of zonal wind U at 50 hPa for each of the stations

As cloud scavenging and rainout are effective removal mechanisms of the atmospheric aerosols, such oscillations in Outgoing Longwave Radiation (OLR, cloudiness) and rainfall would modulate the AOD variations. The wavelet spectra of the time series of the monthly mean, NCEP derived OLR for all the stations considered in this study are shown in Figure 2 (left panel). Presence of QBO as well as QTO (oscillations of 3-5 years)) are clearly discernible, with weaker amplitudes at the off-equatorial station of SV. This is because, in the tropics easterly (westerly) QBOU causes cooler (warmer) and higher (lower) tropopause, and thereby enhanced (reduced) convection. Analysis revealed an in-phase relation between QBOU and OLR at the equatorial stations, and an out-of-phase relation at the off-equatorial stations. Since increased cloudiness would, in general, be closely associated with increased precipitation, we examined the wavelet spectra of the monthly mean rainfall at TVM and ATP (where the data were available) in Figure 2 (right panel). As the rainfall is a highly

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efficient aerosol removal mechanism through wet deposition the positive phase of QBO in rainfall would lead to increased wet removal, hence reduced aerosol loading and lower AOD.

Figure 2. The wavelet spectra of the time series of the monthly mean OLR (NCEP derived) for the 4 stations (left panel) and that of precipitation for the Indian stations of TVM and ATP (right panel)

Summary and Conclusions

Long-term time series of monthly mean AOD at four stations in Indian sector, Arabian sector, and African sector are examined for signatures of long period oscillation associated with atmospheric wave activity. All stations depicted significant signatures of QBO in AOD at 500 nm, which were well associated with the QBO in the stratospheric (50 hPa)

zonal wind U. While the QBOAOD and QBOU, were out-of-phase at the equatorial stations they were in in-phase at the off equatorial and subtropical stations. Analysis of OLR

showed an in-phase (opposite phase) relationship at the equatorial (off-equatorial) stations between the corresponding periodicity in QBOU, implying that the easterly phase of QBOU favors more convection (cloudiness) and hence low OLR. At all the stations, QBOOLR had an opposite phase relationship with the QBOAOD. It was observed over the Indian stations

QBOAOD was out-of-phase with the QBO in rainfall. It is attributed to the increased wet removal by increased precipitation.

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E–O–3

Aerosol Single Scattering Albedo over Delhi using Ultraviolet Irradiance Measurements at the Surface

Sachchidanand Singh, Kirti Soni, T. Bano, S. Nath, and R.S. Tanwar

Radio & Atmospheric Sciences Division, National Physical Laboratory, New Delhi – 110012.

E-mail : ssingh@nplindia.org

Introduction

Aerosol single scattering albedo (SSA), which is the ratio of aerosol scattering to extinction coefficient, is a very important parameter in determining the net radiation flux at any level in the atmosphere including the surface and hence in deciding the aerosol radiation forcing. There are several methods described in the literature by which SSA can be determined at the surface and in the atmosphere (Herman et al., 1975; Bodhaine, 1995; Nakajima et al., 1996; Devaux et al.,1998; Dubovik et al., 2000; Petters et al., 2003; and others). In the present case SSA has been determined over Delhi using an UV multifilter rotating shadowband radiometer (UVMFR-SR) along with a radiative transfer model.

Observation and Methodology

The UVMFR-SR was operated at the National Physical Laboratory, New Delhi, at four UV wavelengths centered at 299.8, 304.5, 311.5 and 317.5 nm to obtain the diffuse as well as total (Direct + Diffuse) irradiance. The direct irradiance was obtained by subtracting

Figure 1. Cloud free (left) and Cloud affected (right) total irradiance spectra as observed with UVMFR on 24 and 27 September, 2009 respectively

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the diffused irradiance from the total irradiance. Thus the direct to diffuse ratio was obtained at all the four wavelengths. As the irradiance measurements are often affected due to the clouds, only the cloud free irradiance measurements (Figure 1) have been used for finding SSA. The SSA is being estimated using an iterative process using the tropospheric ultraviolet radiative transfer model (TUV). The model uses a discrete ordinate method to determine radiative transfer through the atmosphere. The important model inputs include the AOD, total ozone column, ground albedo, SSA, date and location (lat/long) to obtain the direct to diffuse ratio (DDR) of irradiance at different wavelengths. Starting from a reasonable guess of SSA the DDR values are obtained. The TUV is iterated by varying SSA until the output matched that of the measured DDR values. The SSA using this method can be estimated only during the cloud free days by carefully looking at the actual irradiance spectra. Figure1 shows one such cloud free spectrum on 24th Sept 2009 and a cloud affected one on 27th September.

Results

In the present study, SSA using the method described above has been estimated for two clear sky days 31st March and 24th September, 2009. The value of SSA on 24th September at around 13hrs was found to be 0.68 at 299.8nm, 0.73 at 311.5nm and 0.74 at 317.5nm. The SSA values are found to increase with the wavelength in the UV range 299.8 to 317.5 nm. The SSA values are also found to increase during the day from 10:00

to 15:00 hrs at almost all the three wavelengths at which SSA was estimated. Figure 2 shows a plot of SSA variation during the day on 24 September, 2009.

References

Bodhaine, Barry, A., (1995), Aerosol absorption measurements at Barrow, Mauna Loa and the south pole, J. Geophys. Res.,100, (D5) 8967-8975.

Devaux, C., A. Vermeules, J. L. Deuze, P. Dubuisson, M. Herman, and R. Santer, and M. Verbrugghe (1998), Retrieval of aerosol Single Scattering Albedo from ground based measurements application to observational data, J. Geophys. Res., 103, 8753-8761.

Dubovik, O., S. Smirnov, B. N. Holben, M. D. King, Y. J. Kaufman, T. F. Eck, and I. Slutsker (2000), Accuracy assessment of aerosol optical properties retrieved from AERONET Sun and sky radiance measurements, J. Geophys. Res., 105(D8), 9791– 9806.

Herman, B.M., R.S. Browning, and J. J. Deluisi (1975), Determination of the effective imaginary term of the complex refractive index of atmosphere dust by remote sensing the; diffuse-to-direct radiation method,

J. Atmos. Sci. 32, 918-925.

Nakajima, T.; Tonna, G.; Rao, R.; Boi, P.; Kaufman, Y. & Holben, B (1996). Use of sky brightness measurements from ground for remote sensing of particulate polydispersions Appl. Opt., OSA, 35, 2672-2686.

Petters, J.L., V.K. Saxena, J.R. Slusser, B.N Wenny, and S. Madronich (2003), Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model, J. Geophys. Res., 108,(D9) 4288, doi:10.1029/2002JD002360.

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E–O–4

Variation of Aerosol Optical Depth over Indo-Gangetic Basin using AERONET, MODIS and MISR

Sarvan Kumar1, Sanjay Kumar1, M. K. Srivastava2, A. K. Singh1*

1Atmospheric Research Lab., Department of Physics,Banaras Hindu University, Varanasi-221005

2Department of Geophysics, Banaras Hindu University,Varanasi-221005

* Email : abhay_s@rediffmail.com

ABSTRACT : The Indo-Gangetic basin extends 2000 km E-W and about 400 km N-S and is bounded by Himalayas in the north. This basin is unequally found to be affected by high aerosol optical depth throughout the year. Himalayas restricts the movement of aerosols toward north and as a result dynamic nature of aerosol is seen over the Indo-Gangetic basin. High AOD in this region has detrimental effect on the health of more than 460 million people living in this part of India besides adversely affecting clouds formation and monsoonal rainfall pattern. We have used ground based Kanpur and Gandhi college Aerosol Robotic Network (AERONET) station and Multi-angle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) Terra level-3 AOD product for year 2007-2008 to study the variability of aerosol over the Indo-Gangatic plain. An increase in both satellite-derived as well as ground observed aerosol loading over the 2007-2008 time periods has been found over major cities located in the IG basin. We have computed the spatial correlation between MISR, MODIS and AERONET AOD during the 2007–2008. The correlation coefficients between AERONET and MISR are found 0.84 and 0.77 for Kanpur and Gandhi College respectively, whereas the correlation coefficients between AERONET and MODIS are 0.71 and 0.68 for Kanpur and Gandhi College respectively.

Keywords: Aerosols, AERONET, MISR, MODIS, Indo-Gangetic Basin.

Introduction

Aerosols are tiny particles suspended in the air. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray.

Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, aerosols made by human activities currently account for about 10 percent of the total amount of aerosols in our atmosphere. Most of that 10 percent is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agricultural regions, and overgrazed grasslands.

The Indo-Gangetic basin is one of the largest basins in the world. It is bounded by Himalaya in the north, the Aravalli mountain in the west, the Vindhyans and Chhotanagpur Plateau in the south and the Brahmputra ridge in east. It is traversed by Ganga river and its major tributaries. This basin is densely populated (460 million) primarily due to presence of numerous small and big rivers and fertile soil that make this region highly productive. Large scale uncontrolled urbanization and industrial development in this region have cause high pollution levels in air, water, and land. Satellite data from the Multiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging

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Spectroradiometer (MODIS) are available with good spatial and temporal coverage and are being widely used to understand atmospheric processes and climate variability (Kaufman et al., 2002; Prasad et al., 2007).

We have carried out a study of monthly average level 3 MODIS and MISR and monthly average level 2 AERONET over Kanpur and Gandhi College (Ballia) for the year 2007- 2008. Intercomparision and validation of satellite products from different instruments is necessary to build a long term database for climatological studies and potentially to improve upon the accuracy and coverage achievable with a single sensor. In this paper, we have shown the variation of AERONET, MISR and MODIS AOD and comparison between AERONET with MISR and MODIS for year 2007-2008 over the IG basin, India.

Data used

Aerosol optical depth data have been obtained using level-3 MODIS (Moderate Resolution Imaging Spectroradiometer) gridded atmosphere monthly global product

Figure 1. Variation of monthly average AERONET (500nm), MISR (555nm) and MODIS (550nm) AOD over Kanpur and Gandhi College

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MOD08_M3 (http://daac.gsfc.nasa.gov) at spatial resolution of 10x10. The MISR (Multiangle Imaging Spectroradiometer) level 3 AOD data, which is available (http:// www.misr.jpl.nasa.gov) at spatial resolution of 0.50x0.50, have been taken for year 2007- 2008. We have also used ground based Kanpur and Gandhi College level 2 AOD data from Aerosol Robotic Network (AERONET) (http://aeronet.gsfc.nasa.gov).

Variability of AERONET, MODIS and MISR AOD over the IG basin (Kanpur and Gandhi College)

Figure 1 shows variation of monthly average AERONET (500nm), MISR (555nm) and MODIS (550nm) AOD over Kanpur and Gandhi College. Increase in aerosol loading over cities Kanpur and Gandhi College is evident from the positive slope in AOD based on 2 years of satellite (MISR and MODIS) data and this trend is also reflected in ground-based AERONET data. The increasing trend of slop shows the increasing AOD value in year 2007-2008. The variation in AOD is found between 0.27-1.4 during year 2007-2008.

Correlation of AERONET with MODIS and MISR derived AOD (2007-2008)

We computed correlation coefficients using monthly average AERONET (500nm) AOD data with monthly average level 3 MODIS (MOD08_M3) and MISR (MIL3MAE.004) data. The correlation plots for 2007-2008 are shown in Figure 2. The correlation coefficient for AERONET and MODIS for ground stations Kanpur and Gandhi College is found 0.71 and 0.68, also for AERONET-MISR it is 0.84 and 0.77 respectively. We have also tried to study the correlation coefficient between MISR and MODIS which is found 0.81 and 0.73 for Kanpur and Gandhi College respectively.

Figure 2. Correlation plot of AERONET with MISR and MODIS

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Conclusion

High AOD is observed over the Ganga basin throughout the years 2007-2008, is alarming as this basin is one of the most productive basins of Indian subcontinent having population of more than 460 million. AOD is found to be increasing rapidly that may cause adverse effect to the agricultural crops and also to the human health. We have found that the correlation coefficient for AERONET and MODIS for ground stations Kanpur and Gandhi College is found 0.71 and 0.68, also for AERONET-MISR it is 0.84 and 0.77 respectively. The high correlation with MISR than MODIS shows that MISR provides more reliable AOD data. Increased aerosol loading may likely affect the rainfall which is responsible for the observed drought conditions over the Indian subcontinent. Detailed analysis of AOD, crop yields and rainfall data are required to understand the impact of increasing aerosol loading over the Indian subcontinent.

Acknowledgement

The work is financially supported by ISRO, Bangalore. We are thankful to MODIS (http://modis.gsfc.nasa.gov) and MISR (Jet Propulsion Laboratory http://www-misr.jpl. nasa.gov/ and NASA Langley Research NASA, IIT Kanpur and Gandhi College for AOD data (http://aeronet.gsfc.nasa.gov/).

Anup K. Prasad, Ramesh P. Singh, Comparison of MISR-MODIS aerosol optical depth over the Indo-Gangetic basin during the winter and summer seasons (2000-2005),Remote Sensing of Environment ,107 (2007) 109–119

Anup K. Prasad R.P. Singh and Ashbindu Singh, Variability of Aerosol Optical Depth Over Indian Subcontinent Using MODIS Data, Journal of Indian Society of Remote Sensing V. 32, No. 4 (December, 2004 Issue)

Anup K. Prasad, Ramesh P. Singh, Ashbindu Singh, and Menas Kafatos, Seasonal Variability of Aerosol Optical Depth over Indian Subcontinent (2005 IEEE).

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E–O–5

Long-range Transport of Dust Aerosols over Indian Region – A Study using Satellite Data and Mesoscale Model

Shailesh Kumar Kharol1, K.V.S. Badarinath1, D. G. Kaskaoutis2, Anu Rani Sharma1, V. Ramaswamy3 and H. D. Kambezidis2

1Atmospheric Science Section, National Remote Sensing Centre, Dept. of Space-Govt. of India, Balanagar, Hyderabad – 500 625,India.

2Atmospheric Research Team,Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Lofos Nymphon, P.O.Box 20048, GR-11810 Athens, Greece

3National Institute of Oceanography, Goa, India

Email: badrinath_kvs@nrsc.gov.in

Introduction

Deserts in western Asia produce large amounts of mineral dust particles that enter the atmosphere. Dust, which is a common aerosol type over the deserts, emitted by wind erosion in arid and semiarid areas, is considered to be one of the major sources of tropospheric aerosol loading. Desert dust can be transported by the mean wind to thousands of kilometers away from the source regions; this transport plays an important role in the regional and global radiative balance both at the top of the atmosphere (TOA) and at the surface. The present study addresses an intense dust storm event over the Persian Gulf and the Arabian Sea (AS) region and its transport over the Indian subcontinent using multi-satellite observations and ground-based measurements at a central urban location, Hyderabad, India.

Datasets and Methodology

In the present study, data from the Indian geostationary satellite KALPANA-1 VHRR are used to analyze the spread of mineral-dust particles over the Persian Gulf, AS and northwestern part of India from a strong dust outbreak occurred during the period of 19- 24 February 2008 with stronger intensity on 22 February. Vertically-resolved attenuated backscatter during daytime and nighttime from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data are also used as an additional tool to monitor the dust vertical distribution over the Indian region. Furthermore, satellite data from MODIS and OMI sensors are used to understand the spatial distribution of aerosols during the dust event. The Mesoscale Model 5 (MM5) is also used in order to provide the synoptic meteorological systems that have controlled the long-range transport of dust. Ground based measurements on Aerosol Optical Depth (AOD), solar radiation and vertical profile of aerosols were performed using MICROTOPS II sun photometer, MFRSR and Portable Micro pulse Lidar over Hyderabad focusing on examining the variations caused by the dust presence.

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Results and Discussion

A cold front, originating from the Turkish highlands, passed over the southern coasts of Saudi Arabia, Iran and Pakistan on 21st February 2008 and triggered an intense dust storm on 22nd and 23rd February over south Asia and AS. Figure 1 shows the Terra- MODIS visible (true color) image on 22 and 23 February (a) as well as the False Color Composites (FCC) of both Terra and Aqua on 22 February (b) above south Asia. An

Figure 1. (a) True visible Terra-MODIS image on 22nd and 23rd February 2008 above south Asia; (b) False Color Composite (FCC) of Terra/Aqua – MODIS on 22nd February 2008 above AS; (c) Aerosol backscatter profile on 22nd February, 2008 at Hyderabad, India

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intense dust load can be clearly seen in both figures mainly over western and northern AS and Pakistan. The thick dust plume on 22 February seems to attenuate also moving northeasterly on 23 February (Fig. 1a). Winter dust storms are relatively rare compared to the large occurrence of dust storms during summer in this region. However, in 2008, a number of dust storms were initiated due to an extremely dry winter, which has lower soil moisture over the entire south Asia. Once the dust is raised its direction of transport depends on the prevailing wind flow, which is constrained by the synoptic pressure systems. Over AS, dust is normally transported southwest due to the northeast trade winds, which blow towards the ITCZ lying in the southern Hemisphere during Austral summer. However, if a low-pressure system develops over AS or Bay of Bengal, then the dust is preferentially transported towards it. In the present case, a low pressure system formed over Indian Ocean, mainly located between 800E to 900E longitude and -50S to 10N latitude region resulted intense dust storm over Persian Gulf region due to pressure gradient variation (figure not shown). The strong westerlies transport the dust from the Arabian Peninsula towards northern AS, Pakistan and western India. The Ozone Monitoring Instrument (OMI) Aerosol Index (AI) is also examined to provide an independent assessment of dust presence and plume location. During the dust event drastic increase in Terra/ Aqua-MODIS AOD550 and AURA–OMI AI were observed. CALIPSO observations on vertical profile of aerosols are in qualitative agreement with values of MODIS-AOD550 and OMI-AI. In order to understand the long-range transport of dust aerosols and their impact on ground reaching solar irradiance measurements were carried out over tropical urban region of Hyderabad, India. Figure – 1c shows the boundary layer LIDAR operating at 532 nm derived aerosol backscatter profile on 22nd February, 2008. Despite the boundary- layer aerosols, an elevated aerosol layer (~3 km) is observed above them suggesting long- range transport of dust aerosols. The plume-like structure of increased aerosol concentration is visible above the boundary layer aerosols extending up to 3 km. Further, we have also analysed the variation in ground reaching total solar irradiance (400 – 1100 nm) derived from Multi-filter Rotating Shadow band Radiometer (MFRSR) on 22nd and 23rd February, 2008. The reduction in solar irradiance started after 15:00 hrs on 22nd February, 2008 and observed up to 14:00 hrs on 23rd February, 2008. The observed reduction in ground reaching solar irradiance is ~6% on 23rd February, 2008 compared to 22nd February, 2008.

Conclusions

In the present study a wintertime intense dust storm occurred in south Asia (including Persian Gulf, Arabian Sea and western India), was investigated via remote-sensing observations and ground-based measurements. Results of the present study indicate the fact that low pressure systems over oceans (i.e., Bay of Bengal/Indian Ocean) cause changes in weather patterns leading to dust storm over Gulf and Arabian Peninsula. The shamal or north-west winds are caused by a wave of high pressure that funnels through the Gulf between Saudi Arabia and Iran, and are the most hazardous weather conditions in the region have linkages with low pressure systems over Indian Ocean/Bay of Bengal.

Acknowledgements

The authors thank the Director of NRSC & Dy. Director (RS&GIS-AA), NRSC for necessary help at various stages and ISRO-GBP for financial support.

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E–O–6

Characteristics of Spectral Aerosol Optical Depths over Nainital-Temporal Variations and the Role of Local and Regional Meteorology

U. C. Dumka1, 2

1Tata institute of Fundamental Research, National Balloon Facility, Hyderabad 500 062, India

2Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263129, India

E-mail: ucdumka@gmail.com

Introduction

Atmospheric aerosol plays an important role in the radiation budget of the Earth’s atmospheric system by exerting direct and indirect radiative forcing of climate. Due to the variety of aerosol sources, their short atmospheric life time and the dynamic processes that may alter them after generation and diverse aerosol types with varying physical, chemical and optical properties, which are not uniformly distributed around the globe, aerosol radiative forcing exhibits large temporal and spatial differences. Hence the accurate assessment of the aerosol impact on radiative balance is a challenging task. Therefore to understand the effect of aerosols on the Earth’s atmosphere system, it is essential to study and understand their physical, chemical as well as optical properties at several locations. With this, extensive measurements of spectral aerosol optical depths were carried out during the period January 2002 to December 2005 at the high altitude location, Manora Peak, Nainital.

Experimental Details and Data Base

The experimental site (29.37°N; 79.45°E) Manora Peak, Nainital is located in the lower part of Central Himalayas at an altitude of ~1950 m above mean sea level and hence is above the planetary boundary layer for most of the time [Pant et al., 2006]. The geographical location and topography of the observational site over the Indian subcontinent is given in Figure 1(a) and more details are available in earlier papers [e g. Pant et al., 2006]. Regular measurements of spectral aerosol optical depths made at Manora Peak Nainital with a ten channel Multi wavelength Solar Radiometer (MWR) for the period of January 2002 to December 2005 are used in the present study. From the MWR data, AODs were estimated at ten wavelength bands centered at 0.38, 0.40, 0.45, 0.50, 0.60, 0.65, 0.75, 0.85, 0.935 and 1.025 μm (full width half maximum bandwidth of 0.005 μm) following the conventional Langley plot technique [Shaw et al., 1973; Sagar et al., 2004]. More details of the MWR, observations and data analysis and error budget are given in Saha et al., [2005].

Results and Discussion

Figure 1(b) shows the monthly variation of aerosol optical depth at four representative wavelengths (0.38, 0.50, 0.75 and 1.025 μm; two in visible region and two in near infrared region) respectively.

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The gap in the plots corresponds to the absence of data. Generally AODs show a gradual increase from a very low value in November, December, January and February to a peak towards April to June. A rapid decrease follows with the onset of monsoon and the AODs reach the annual minimum again by November/December. The variations are generally consistent over the years and almost similar at all the wavelengths, even though the magnitude of variation is different at different wavelength. Further details about the variation of AODs are presented elsewhere [Dumka et al., 2008]. Also a well pronounced seasonal pattern is observed consistently in all the years with the mean AOD increasing sharply in summer month from their very low values (<0.1 at 0.50 μm) in winter months. This is followed by gradual decrease during monsoon and post monsoon seasons. The spectral variation of AOD is important in assessing the radiative impact as well as it provides the useful information on the aerosol size distribution. The spectral dependence of AODs contains information about the physical characteristics of aerosols and that can be inferred from the Ångström relationship as [Ångström, 1961]

where a is the wavelength exponent and is the Ångström coefficient. a is a measure of the relative dominance of small particles, while is a measure of the aerosol loading and is more associated with the large particles.

FIGURE 1: - (a) The geographical location of the study area over the Indian subcontinent. Color code in the figure shows the altitudes in meters above the mean sea level. (b) The monthly mean variation spectral aerosol optical depth during January 2002 to December 2005

The, a and values were computed for the individual AOD spectra, by evolving a least squares fit to above Equation in log-log scale. The high values of a with low during the winter months clearly indicates the dominance of fine/sub micron size aerosols where as during summer months with low a and high aerosol loading (i.e. high value of ) indicates the dominance of coarse mode aerosols.

The observed temporal feature of AOD at Manora Peak Nainital has indicated systematic variation on monthly and seasonal time scales. It shows a pronounced peak in the month of May/June and minimum during December/January during which the AOD values are comparable to those reported for the Antarctic region. The observed variations of aerosol are examined in the light of regional and synoptic meteorological properties. Sagar et al.,

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[2004] and Dumka et al., [2008] have shown that the sharp increase in the AOD from the middle of March is mostly attributed due to the transport of windblown dust from the desert areas and west Asia. In addition to the advection by air mass; the increased solar heating of the land mass over the lower plains adjacent to the site during the summer season would result in increased convective mixing and elevation of the atmospheric boundary layer aerosols. This would also contribute to the increase in AOD over the observational site during the summer. In addition to this, it is obvious that the long range transport of aerosols form the arid regions is also important in contributing to the observed changes in the monthly mean variation of AOD. With a view to examining and delineating this, a 5 day isentropic air mass back trajectories are computed using HYSPLIT model at an altitude of 3500 m AMSL. Carefully examination of trajectory analysis also supports to the contribution by the windblown dust aerosols in the enhanced the AODs at Nainital during March to June when the meteorological conditions are favorable [Dumka, 2008].

Conclusions

The main conclusions of our studies are as follows:

1.The AODS at 0.50 mm are very low (= 0.1) in winter and increased rather steeply to reach high values (~0.5) in summer. The monthly mean AODs varies significantly (by more than a factor of four to six) from January to June period.

2.The seasonal variation in AODs while examined in conjunction with synoptic scale wind fields and rainfall have revealed that the long range transport of dust due west contribute significantly in the enhancing the aerosol loading particularly in the coarse mode, during summer.

3.The Ångström wavelength exponent (a) are found to be high during the winter months and low values during the summer months while the Ångström turbidity parameter ( ) shows the low values during the winter and high values during summer months.

Acknowledgements

The authors wish to thank the technical staff of Aryabhatta Research Institute of Observational Sciences for providing valuable help during the observations. This study is part of the project funded by ISRO, under Geosphere Biosphere Program of the Department of Space, Government of India.

References

1.Ångström, A., Tellus, 13, 214-223, 1961.

2.Pant et al., J. Geophys. Res., Vol., 111, D17206, doi: 10.1029/2005JD006768, 2006.

3.Shaw, G.E., et al., J. Appl. Meteorol., 12, 374-380, 1973.

4.Sagar, R., et al., J. Geophys. Res., 109, 10.1029/2003JD003954, 2004.

5.Saha, A. et al., J. App. Meteorol, 43(6), 902–914, 2005.

6.Dumka, U. C., et al., JAMC, 2008

7.Dumka, Ph. D., Thesis, K. U., Nainital, 2008.

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E–O–7

Vertical and Spatial Variation of CCN and Submicron Aerosol Size Distribution over Indian Continental Tropical Convergence Zone during Monsoon Season of year 2008

V. Patidar, J. Jaidevi, S.N. Tripathi* and Tarun Gupta

Department of Civil Engineering, Indian Institute of Technology Kanpur,208016, Kanpur, India

*Corresponding Author Email: snt@iitk.ac.in

Introduction

Atmospheric aerosols affect the global energy budget by scattering and absorbing sunlight (direct effects) and by changing the cloud microphysical structure, cloud lifetime, and cloud cover (indirect effects). An increase in aerosol concentration would lead to smaller cloud droplet size and higher cloud albedo i.e. brighter clouds [Twomey, 1977]. The smaller cloud droplet size resulting from increased aerosol concentration also inhibits precipitation, leading to an increased cloud lifetime and coverage, popularly referred in the literature as second indirect aerosol effect [Albrecht 1989]. These indirect effects [Twomey, 1977; Albrecht, 1989] continue to be the source of greatest uncertainty in climate predictions [IPCC, 2001]. The concentration of Cloud Condensation Nuclei particles (CCN), especially, in the lower troposphere, has a profound influence on the microphysical processes in clouds, and consequently on many aspects of weather and climate. These interactions have been summarized in a number of recent reviews, addressing in particular the effects of aerosols on climate [Lohmann and Feichter, 2005] and on cloud processes and precipitation [McFiggans et al., 2006]. There have been no experiments carried out in the past to study the spatial and vertical variation of CCN in the Indian-Continental Tropical Convergence Zone (I-CTCZ).

Method

Extensive airborne measurements of vertical and spatial profiles of cloud condensation nuclei (CCN) concentration at constant supersaturation of 0.6% using DMT’s CCN counter and submicron aerosol size distributions using Scanning Mobility Particle Sizer (SMPS, TSI 3936) were made onboard a Pressurized twin turbo prop executive aircraft Super King Air B200, during monsoon season from September 4–15, 2008 for the first time over the Indian CTCZ region.

Results

The vertical profiles of CCN concentrations for different regions show distinct layers which can be confirmed by respective temperature profiles. Vertical profiles show that most CCN particles are concentrated at lower altitude, (1.2 km) near boundary layer and CCN concentration decreased with increasing altitude. It suggests that the main source of

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CCN is the surface anthropogenic aerosol loading. The comparison of vertical profiles of CCN concentrations at different locations shows remarkable differences at different altitudes. The spatial measurements around the cloud boundaries show an apparent decrease of CCN above clouds.

The aerosol size distributions provide characteristic signatures of CCN for different regions, atmospheric conditions as well as regional sources. Consistent and noticeable differences in observed aerosol size distributions between cloudy and clear sky conditions suggest that a greater amount of gas-to-particle conversion occurs above the clouds, presumably through in-cloud aqueous phase oxidation processes, leading to appearance of nucleation mode (~20 nm) in the size distribution. The size distributions at higher altitude (>3 km), above the clouds, exhibited very high number concentrations of very fine particles (diameter < 0.04 ìm). These and more results will be presented in the proposed presentation

Acknowledgement

This work is financially supported by DST ICRP and ISRO GBP and MT programmes. We also thankful to the entire NRSC team for making the aircraft available for the experiment.

Reference

Albrecht, B. A. (1989), Aerosols, cloud microphysics and fractional cloudiness, Science, 245, 1227– 1230.

Intergovernmental Panel on Climate Change (2001), Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J. T. Houghton et al., 881 pp., Cambridge Univ. Press, New York.

McFiggans et al., (2006), the effect of physical and chemical aerosol properties on warm cloud droplet activation, Atmos. Chem. Phys., 6, 2593–2649.

Twomey, S. (1977), The influence of pollution on the shortwave albedo of clouds, J. Atmos. Sci., 34, 1149– 1152.

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E–O–8

Performance of a Lidar System during Morning Observations

K Raghunath1 and N Harikrishnan2

1 National Atmospheric Research Laboratory, Gadanki- 517 112,

2Department of Optoelectronics, University of Kerala,Trivandrum

1e mail: kraghunath@narl.gov.in

2e mail : harikrishh@gmail.com

Introduction

The primary aim of this work is to demonstrate the feasibility of a lidar system for daytime measurements in the lower altitudes up to 30 km and then later to extend the profiling to upper layers. The extension of lidar operation to daytime helps us to study the properties of aerosols that may be affected in presence of the sun. The clouds have different properties during daytime; they are less dense and thus the changes due to solar radiation over cloud, help us to study about various cloud activities. The lidar profiling of atmosphere during daytime helps us to compare the properties of various atmospheric constituents that gets changed in the presence of solar radiation.

A lot of factors have to be considered before doing the atmospheric profiling during daytime. Some of the main factors include the solar irradiance during the measurement time, solar zenith angle, the wavelength to be used for profiling, the power of laser, the filter to be used etc. Signal to Noise Ratio (SNR) is one parameter to assess lidar performance. One of the suitable methods to calculate the Signal-to-Ratio (SNR) is to take observations from the minimal background noise conditions and then during adverse condition. The paper discusses various SNR improvement techniques and brings out the most effective technique.

Methods for suppressing the solar background noise

The received signal from the lidar during day will contain signal at the backscattered wavelength of the laser along with the counts due to the radiation from sun. The solar background noise counts are less during the early hours of a day and gets immense as the day progresses; thus these noise strength has to be suppressed in order to take observations during daytime. Some of the methods for reducing the background solar noise are:

1.Reduce optical system bandwidth by using spectral filters like an optimized Fabry- Perot etalon in the receiver [1],[2]

2.Changing aperture size of telescope

3.Reducing field of view of the receiver by using adjustable iris

4.Using a solar blind wavelength: 266nm / 355 nm in UV region

5.Narrow line width operation of laser source on injection seeding

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The full length presentation gives SNR improvements with each of the methods for suppressing the noise, but here the effects of method 1 is discussed

Expected Photon counts due to solar radiation

The solar radiation vary with time; by taking the measured/reference solar irradiance values, signal counts due to solar radiation at a particular time can be estimated. The expected solar counts at various instants at surface due to solar radiation (taken from a reference data) for NARL lidar receiver by using 35cm diameter telescope, an IF of bandwidth 1 nm and field of view 1 mRad are as shown in the Figure 1.

Figure 1. Expected noise counts due to solar radiation on using an 1 nm Interference filter

From the Figure 2, the noise counts due to solar radiation are expected to have an order of 102 to 103 during early morning time and then the counts increase much more as the time passes on to about105 to 108. Thus, we were able to conduct lidar observations during the early morning period, when solar zenith angle is about 10 degrees, on an experimental basis when the noise counts due to solar radiation are very less.

SNR estimation from the lidar data

NARL lidar system was operated on successive days implementing the methods as mentioned above. The lidar system employs 30W laser system with 350mm telescope aperture as a receiver and 1nm IF bandwidth with PMT as detector. By comparing the SNR with and without using an appropriate method we can estimate the improvement in SNR of the signal as given Equation 1. The received signal is approximated to Poisson distribution for detection

(1)

Where N( r) is received signal at range r

Pbkg is received power due to solar background

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SNRs have been estimated using the first three methods i.e using Fabry Perot interferometer ( with 2 pm bandwidth and 16pm FSR) [1][2], changing the aperture size and changing the field of view as mentioned above. The schematic is shown in Figure 2.

Figure 2. Schematic diagram for measuring SNR using various techniques

Figure 3. SNR (dB) of the early morning profile (24-11-09 at 6.38 am)

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The rest of the methods need significant hardware augmentation and hence not attempted. Of all the three methods, using narrow pass band filter (etalon) has improved SNRs considerably and is reported here. The SNR profile with and without etalon is shown in Figure 3 and improvement in SNR at a particular altitude of 10km is shown in Figure 4.

Figure 4. SNR improvement in dB on using an etalon (24-11-09) at an altitude of 10km

Conclusion

Thus, the day profiling of the atmosphere was done during morning hours to study the feasibility of operating a high power monostatic biaxial lidar system in presence of the solar radiation. The improvement in the SNR was estimated by changing the configurations such as on using etalon as the filter, reducing aperture area, field iris etc and the SNR was found to be satisfactory during day profiling. Changing to a narrow pass band filter in the form of etalon is the most effective method for reducing the sky background noise. Its effectiveness during late morning hours need to be studied.

Acknowledgements

The efforts of NARL contract staff in operating the lidar system is hereby acknowledged.

References

Dengxin Hua, Masaru Uchida, and Takao Kobayashi, Ultraviolet Rayleigh–Mie lidar for daytime-temperature profiling of the troposphere. Applied Optics,Vol. 44, No 7 (2005)

Jack A. McKay, Single and tandem Fabry–Perot etalons as solar background filters for lidar, Aplied Optics, Vol 38, No 27, (1999).

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