Assessment of Direct Radiative Effects of Aerosols using Assimilated Datasets over the Indian Region

Abstract

Uncertainties in estimation of radiative effects of aerosols, pose the primary challenge for the accurate climate impact assessment of aerosols. Especially, over the Indian region which is characterized by significant spatio-temporal heterogeneities in aerosol properties, improvement in the estimation of aerosol radiative effects at regional scale is pertinent to the accurate investigation of the effects of aerosols on the Indian summer monsoon and the regional climate. One of the most effective means of acquiring this target is to use spatially homogeneous and temporally continuous datasets of critical aerosol properties, such as spectral aerosol optical depth (AOD) and single scattering albedo (SSA), which are the most important parameters for estimating aerosol radiative effects. However, observations do not provide the above; the space-borne observations though provide wide spatial coverage, are temporally snap shots. In addition, the satellite retrievals of aerosols suffer from substantial uncertainties and biases which primarily emanate from assumptions made in the retrieval procedure, cloud contamination as well as the possible sensor degradation over extended periods. On the other hand, the ground-based measurements provide more accurate and temporally continuous data, but are spatially near-point observations. Realizing the need for spatially homogeneous and temporally continuous datasets on one hand and the near-non-existence of such data over the south Asian region, construction of quality-enhanced gridded aerosol products by synthesizing thelong-term space-borne and ground-based data, has been taken up as a prime objective of the present thesis. On this background, this thesis presents spatially homogeneous gridded datasets of Aerosol Optical Depth (AOD) and Absorption Aerosol Optical Depth (AAOD), generated for the first time over this region. These data products are developed by assimilating the highly accurate aerosol measurements from the dense networks of 44 (for AOD) and 34 (for AAOD) ground-based observatories of Aerosol Radiative Forcing NETwork (ARFINET) and AErosol RObotic NETwork (AERONET) spread across the Indian region, with satellite-retrieved AOD and AAOD, following statistical assimilation schemes. The satellite data used for AOD assimilation includes AODs retrieved from MODerate Imaging Spectroradiometer (MODIS) and Multiangle Imaging SpectroRadiometer (MISR) over the same domain. For AAOD assimilation, the AAODs estimated from ground-based Black Carbon (BC) mass concentration measurements from the network of 34 ARFINET observatories and satellite-based (Kalpana-1, INSAT-3A) infrared (IR) radiance measurements, are blended with gridded AAODs (500 nm) derived from Ozone Monitoring Instrument (OMI)-retrieved AAODs (at 354 nm and 388 nm). The merged datasets are demonstrating improved confirmation with the independent ground-based measurements (which are not used in assimilation) vis-a-vis respective satellite products. As ensured by assimilation techniques employed, the uncertainties in merged AODs and AAODs are significantly smaller than those in corresponding satellite products. These assimilated products also exhibit all important, large-scale spatial and temporal features which are already reported for this region. Nonetheless, themerged AODs and AAODs are significantly different in magnitude, from the respective satellite products.

In the view of these benefits shown by merged products over their satellite counterparts, I have estimated the Direct Radiative Effect (DRE) of aerosols in clear sky conditions over the Indian subcontinent by incorporating the merged AOD and SSA into a widely used radiative transfer model (SBDART). The results show stronger atmospheric warming and surface cooling over Indo-Gangetic region as compared to the rest of the parts, which is in line with regional distribution shown by DRE estimated using satellite-retrieved aerosol products. Nonetheless the DRE corresponding to assimilated products exhibit substantially higher atmospheric warming vis-a-vis DRE estimated using satellite products. The uncertainty analysis for DRE estimates indicates that the DRE estimated employing merged datasets exhibit substantially reduced uncertainties as compared to those in DRE estimated using satellite products. This is primarily due to smaller uncertainties in assimilated products as compared to their satellite counterparts. On the background of this, the assimilated AOD and AAOD products presented here provide an excellent opportunity to improve the climate impact assessment of aerosols and to unravel the mystery of aerosol-monsoon link in a more accurate manner.