Time filter

Source Type

Ramachandran S.,Physical Research Laboratory | Srivastava R.,Physical Research Laboratory | Srivastava R.,Indian Center for Climate and Societal Impacts Research
Environmental Sciences: Processes and Impacts | Year: 2013

Aerosol optical properties of external and core-shell mixtures of aerosol species present in the atmosphere are calculated in this study for different relative humidities. Core-shell Mie calculations are performed using the values of radii, refractive indices and densities of aerosol species that act as core and shell, and the core-shell radius ratio. The single scattering albedo (SSA) is higher when the absorbing species (black carbon, BC) is the core, while for a sulfate core SSA does not vary significantly as the BC in the shell dominates the absorption. Absorption gets enhanced in core-shell mixing of absorbing and scattering aerosols when compared to their external mixture. Thus, SSA is significantly lower for a core-shell mixture than their external mixture. SSA is more sensitive to core-shell ratio than mode radius when BC is the core. The extinction coefficient, SSA and asymmetry parameter are higher for external mixing when compared to BC (core)-water soluble aerosol (shell), and water soluble aerosol (core)-BC (shell) mixtures in the relative humidity range of 0 to 90%. Spectral SSA exhibits the behaviour of the species which acts as a shell in core-shell mixing. The asymmetry parameter for an external mixture of water soluble aerosol and BC is higher than BC (core)-water soluble aerosol (shell) mixing and increases as function of relative humidity. The asymmetry parameter for the water soluble aerosol (core)-BC (shell) is independent of relative humidity as BC is hydrophobic. The asymmetry parameter of the core-shell mixture decreases when BC aerosols are involved in mixing, as the asymmetry parameter of BC is lower. Aerosol optical depth (AOD) of core-shell mixtures increases at a higher rate when the relative humidity exceeds 70% in continental clean and urban aerosol models, whereas AOD remains the same when the relative humidity exceeds 50% in maritime aerosol models. The SSA for continental aerosols varies for core-shell mixing of water soluble aerosol (core)-shell (BC) when compared to their external mixture, while the SSA for maritime aerosols does not vary significantly for different mixing scenarios because of the dominance of sea salt aerosols. Thus, these results confirm that aerosol mixing can modify the physical and optical characteristics of aerosols, which vary as a function of relative humidity. These calculations will be useful in parameterising the effect of core-shell vs. external mixing of aerosols in global climate models, and in the evaluation of aerosol radiative effects. © The Royal Society of Chemistry 2013.

Vaid B.H.,Indian Institute of Tropical Meteorology | Vaid B.H.,Indian Center for Climate and Societal Impacts Research | Gnanaseelan C.,Indian Institute of Tropical Meteorology | Jayakumar A.,Indian Institute of Tropical Meteorology
International Journal of Remote Sensing | Year: 2011

National Oceanic and Atmospheric Administration daily sea surface temperature (SST) products based on Advanced Microwave Scanning Radiometer (AMSR) and Advanced Very High Resolution Radiometer (AVHRR) have been used to understand the variability in the tropical Indian Ocean SST. These products are comparable with the deep sea moored buoy observations and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST in the tropical Indian Ocean. However considerable difference is noticed between these satellite SST products and deep sea buoys, especially at the intraseasonal time scale. Further the first Complex Empirical Orthogonal Function (CEOF) mode of TMI and AVHRR SST explains respectively 46.49% and 46.19% of the total variance. The second CEOF mode of TMI and AVHRR SST explains respectively 23.19% and 18.94% of the total SST variance in the tropical Indian Ocean. The AVHRR SST product is important because this daily product has been available since 1985. The analysis shows that AMSR measurements are contributing considerably to the understanding of the tropical Indian Ocean SST variability. Though satellite SST products are able to capture the observed intraseasonal variability reasonably well, more accurate satellite SST products are therefore necessary to understand the climatologically important Indian Ocean region and its air-sea interaction processes. © 2011 Taylor & Francis.

Bhandari S.M.,Indian Center for Climate and Societal Impacts Research | Baba Shaeb K.H.,Regional Remote Sensing Center Central
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives | Year: 2014

We have examined, for the first time, the spatial coherence of oceanic parameters, namely, the ocean surface wind and significant wave height (SWH), using near-instantaneous along-track sampling over vast oceanic tracks provided by Poseidon-2 Radar Altimeter onboard Jason-1. The spatial coherence length scales over the Arabian Sea and the Bay of Bengal regions, derived using Auto correlation analysis, display interesting characteristics in relation to onset and evolution of SW Monsoon over India. The estimated spatial coherence scales are found to be of the order of 100-500 km. Generally, SWH coherence lengths over Arabian Sea are observed to increase from June to July during good monsoon year (2012). This trend is not followed for the poor monsoon year (2002). Similar trend is observed for spatial coherence scales for wind speed. The temporal evolution of analyzed spatial coherence scales of winds and waves over Arabian Sea clearly brings out the distinction between a 'good' (2012) and a 'poor' (2002) monsoon year. The spatial coherence lengths of these parameters over the Bay of Bengal are found to be shorter and do not show any systematic relation to onset and evolution of SW monsoon. Anisotropy in coherence lengths is also analyzed by treating ascending and descending tracks separately. Generally, the descending tracks measurements show higher spatial coherence lengths than the ascending ones. Like the spatial coherence scales, the anisotropy of the coherence scales over the Arabian Sea also mimics the behaviour of the strength of SW Monsoon.

Ramachandran S.,Physical Research Laboratory | Srivastava R.,Indian Center for Climate and Societal Impacts Research
Environmental Science and Pollution Research | Year: 2016

Mixing can influence the optical, physical, and chemical characteristics of aerosols, which in turn can modify their life cycle and radiative effects. Assumptions on the mixing state can lead to uncertain estimates of aerosol radiative effects. To examine the effect of mixing on the aerosol characteristics, and their influence on radiative effects, aerosol mixing states are determined over four environmentally distinct locations (Karachi, Gwangju, Osaka, and Singapore) in Asia, an aerosol hot spot region, using measured spectral aerosol optical properties and optical properties model. Aerosol optical depth (AOD), single scattering albedo (SSA), and asymmetry parameter (g) exhibit spectral, spatial, and temporal variations. Aerosol mixing states exhibit large spatial and temporal variations consistent with aerosol characteristics and aerosol type over each location. External mixing of aerosol species is unable to reproduce measured SSA over Asia, thus providing a strong evidence that aerosols exist in mixed state. Mineral dust (MD) (core)-Black carbon (BC) (shell) is one of the most preferred aerosol mixing states. Over locations influenced by biomass burning aerosols, BC (core)-water soluble (WS, shell) is a preferred mixing state, while dust gets coated by anthropogenic aerosols (BC, WS) over urban regions influenced by dust. MD (core)-sea salt (shell) mixing is found over Gwangju corroborating the observations. Aerosol radiative forcing exhibits large seasonal and spatial variations consistent with features seen in aerosol optical properties and mixing states. TOA forcing is less negative/positive for external mixing scenario because of lower SSA. Aerosol radiative forcing in Karachi is a factor of 2 higher when compared to Gwangju, Osaka, and Singapore. The influence of g on aerosol radiative forcing is insignificant. Results emphasize that rather than prescribing one single aerosol mixing state in global climate models regionally and temporally varying aerosol mixing states should be included for more accurate assessment of aerosol radiative effects © 2016 Springer-Verlag Berlin Heidelberg

Srivastava R.,Indian Center for Climate and Societal Impacts Research | Ramachandran S.,Physical Research Laboratory | Rajesh T.A.,Physical Research Laboratory
Quarterly Journal of the Royal Meteorological Society | Year: 2016

Aerosols in urban regions have a distinct character as they can exhibit significant seasonal and interannual variabilities owing to variations in local emissions and long-range transport. This diversity in aerosol sources can give rise to a complex aerosol mixture over urban regions which can influence resultant aerosol optical properties and their radiative effects. Seasonal variation in aerosol mixing state and aerosol radiative effects over Ahmedabad, an urban region in western India, are deduced from measured aerosol optical properties and a radiative transfer model. Aerosol mixing states for near-surface single scattering albedo (SSA) at 0.55 μ m are different from those obtained for columnar SSA at 0.50 μ m in each season, emphasizing that aerosol mixing state can vary with altitude determining differences in SSA. In winter, mineral dust coated by water-soluble aerosols emerges as the mixing state for near-surface SSA (0.69), while black carbon coated by water-soluble aerosols emerges as the probable mixing state for columnar SSA (0.94). Aerosol radiative forcing at the surface estimated for probable mixing states of columnar SSA follows observations during all seasons except the monsoon. Heating rate is higher for near-surface SSA than for column SSA. Heating rates for near-surface SSA during winter and post-monsoon are 1 K day-1 below 3 km, but ≤0.5 K day-1 for columnar SSA. A secondary peak in heating rate profile is seen between 2 and 4 km during pre-monsoon which is due to presence of absorbing aerosols at these altitudes. External mixing emerges as a probable mixing state during pre-monsoon and monsoon, indicating that mixing depends on aerosol types, their abundance and meteorological conditions. These results can serve as standard regional representatives for aerosol mixing and radiative effects over urban regions. The proposed approach can be applied to other environments to determine extreme bounds of core-shell mixing of aerosols. © 2016 Royal Meteorological Society.

Bhaskar B.V.,Indian Center for Climate and Societal Impacts Research | Mehta V.M.,Indian Center for Climate and Societal Impacts Research | Mehta V.M.,Center for Research on the Changing Earth System
Aerosol and Air Quality Research | Year: 2010

Urban air pollution is rapidly becoming an environmental problem of public concern worldwide. It can influence public health and local/regional weather and climate. In the present study, airborne particulate pollutants data were collected for a period of 4 years (2005-2008) at 13 locations in Ahmedabad, a mega city in Gujarat State in western India. The particulate pollutants data were collected by the Gujarat Pollution Control Board with respirable dust samplers (RDSs). The observed Suspended Particulate Matter (SPM) concentrations varied from 66.0 to 786.0 μg/m3, and concentrations of Particulate Matter of aerodynamic diameters less than 10 microns (PM10) ranged between 17.0 to 327.0 μg/m3. The seasonal- and annual-average concentrations of the two pollutants were mostly above Indian air quality standards and were generally comparable with those observed in most other Indian urban areas. During this study period, there was a continuous decrease of particulate pollutants concentrations within Ahmedabad; however, the concentrations were just above the permissible limits set by the Central Pollution Control Board (CPCB). These particulate pollutants concentrations were compared with meteorological variables such as rainfall, humidity, temperature, and wind speed. Both SPM and PM10 showed significant negative correlations with rainfall. An Air Quality Index (AQI) was calculated for all stations for all months. AQI values varied from 25 to 193.3. AQI was high in summer season and low in monsoon season. AQI values varied from Good (0-50) to Hazardous (300-500). On the basis of the AQI scale, it is found that the atmospheric environment of Ahmedabad is moderately polluted to unhealthy range. Copyright © Taiwan Association for Aerosol Research.

Shaeb K.H.B.,Regional Remote Sensing Center Central | Anand A.,Regional Remote Sensing Center Central | Joshi A.K.,Regional Remote Sensing Center Central | Bhandari S.M.,Indian Center for Climate and Societal Impacts Research
Marine Geodesy | Year: 2015

Significant Wave Height (SWH) measurement data from the AltiKa Radar Altimeter (RA) for the first 13 cycles of satellite coverage are compared with the SWH from Wave Rider Buoys (WRB) located at nine stations along the Indian coast to assess the performance of the altimeter over the coastal region. AltiKa SWH observations within a 30-minute interval and 50 km distance from WRBs are found to be over estimated by 6%, the Root Mean Square Error (RMSE) is 0.36 m, the Scatter Index (SI) is 26%, and the correlation coefficient (r) is 0.91. Relaxing the distance criteria by 50 km leads to increase in RMSE and deterioration of r to 0.89. There is a marked difference in the statistics on the comparison pairs pooled separately for the buoys near west and east coasts, with the latter showing RMSE error 26% more than the former. The method of Cressman weights adopted to correct for the errors arising out of the temporal and spatial differences in altimeter and buoy data comparison pairs resulted in reduction of RMSE by 5% and 25%, respectively, for the 30-minute and 50 km criteria and 4% and 56% for the 30-minute and 100 km criteria. © 2015, Copyright © Taylor & Francis Group, LLC.

Vaidya D.B.,Indian Center for Climate and Societal Impacts Research | Gupta R.,Inter-University Center for Astronomy and Astrophysics
Proceedings of the International Astronomical Union | Year: 2014

The dust grains flowing out of the stars are most likely non-spherical and inhomogeneous; viz. porous, fluffy and composites of many very small particles glued together. We calculate the scattering, absorption and extinction efficiencies of such composite grains. The composite grains consist of host silicate spheroid and inclusions of PAHs. We study the extinction as a function of the volume fraction of the inclusions. In particular, we study the variation in the absorption peak, at the wavelength '4430Å' with the volume fraction of the PAH inclusions. We present the composite grain models and discuss the results. Copyright © International Astronomical Union 2014.

Loading Indian Center for Climate and Societal Impacts Research collaborators
Loading Indian Center for Climate and Societal Impacts Research collaborators