Abstract
Abstract. Improving the accuracy of regional aerosol climate impact assessment calls for improvement in the accuracy of regional aerosol radiative effect (ARE) estimation. One of the most important means of achieving this 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 temporal snapshots and suffer from 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 (which is one of the regions where aerosols show large heterogeneity in most of their properties), construction of accurate gridded aerosol products by synthesizing the long-term space-borne and ground-based data has been taken up as an important objective of the South West Asian Aerosol Monsoon Interactions (SWAAMI), a joint Indo-UK field campaign, aiming at characterizing aerosol–monsoon links and their variabilities over the Indian region. In Part 1 of this two-part paper, we present spatially homogeneous gridded datasets of AOD and absorption aerosol optical depth (AAOD), generated for the first time over this region. These data products are developed by merging the highly accurate aerosol measurements from the dense networks of 44 (for AOD) and 34 (for AAOD) ground-based observatories of Aerosol Radiative Forcing over India 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 include AODs retrieved from MODerate Imaging Spectroradiometer (MODIS) and Multiangle Imaging SpectroRadiometer (MISR) over the same domain. For AAOD, the 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, monthly mean) derived from Ozone Monitoring Instrument (OMI)-retrieved AAODs (at 354 and 388 nm). The details of the assimilation methods and the gridded datasets generated are presented in this paper. The merged gridded AOD and AAOD products thus generated are validated against the data from independent ground-based observatories, which were not used for the assimilation process but are representative of different subregions of the complex domain. This validation exercise revealed that the independent ground-based measurements are better confirmed by merged datasets than the respective satellite products. As ensured by assimilation techniques employed, the uncertainties in merged AODs and AAODs are significantly less than those in corresponding satellite products. These merged products also all exhibit important large-scale spatial and temporal features which are already reported for this region. Nonetheless, the merged AODs and AAODs are significantly different in magnitude from the respective satellite products. On the background of above-mentioned quality enhancements demonstrated by merged products, we have employed them for deriving the columnar SSA and analysed its spatiotemporal characteristics. The columnar SSA thus derived has demonstrated distinct seasonal variation over various representative subregions of the study domain. The uncertainties in the derived SSA are observed to be substantially less than those in OMI SSA. On the backdrop of these benefits, the merged datasets are employed for the estimation of regional aerosol radiative effects (direct), the results of which would be presented in a companion paper, Part 2 of this two-part paper.
Highlights
The climate forcing potential of atmospheric aerosols is well accepted by the global scientific community and policy makers (Boucher et al, 2013)
As the grid nodes and locations of ground-based observatories are not collocated, we have interpolated the merged aerosol optical depth (AOD) and absorption AOD (AAOD) from the grid nodes contained by the 3◦ × 3◦ box surrounding the locations of respective ground locations used for validation
The comparisons of collocated merged AODs and AAODs with the respective, independent ground-based estimates are shown by scatter plots in Figs. 4b and 5b
Summary
The climate forcing potential of atmospheric aerosols is well accepted by the global scientific community and policy makers (Boucher et al, 2013). We have used long-term (2001–2013) measurements of AOD at 550 nm from the two widely used space-borne sensors, MODIS and MISR, over the Indian region and the accurate, quality-checked AOD from a network of 44 ground-based Sun photometers – Aerosol Radiative Forcing over India NETwork (ARFINET) and AERONET – for the same period to generate a gridded dataset for AOD using a modified form of a well-established data assimilation technique. We have generated a spatially homogeneous gridded product for AAOD by combining the AAODs estimated using ground-based BC measurements and space-borne infrared radiance measurements (to delineate the dust contribution to AAOD) with AAODs (500 nm) derived from Ozone Monitoring Instrument (OMI) retrievals These merged datasets for AAOD and AAOD are further employed to estimate columnar SSA at 1◦ × 1◦ over the domain.
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