Other algorithm studies
Read our other algorithm studies for current, future and multi-missions.
16 February 2023
29 September 2021
Atmospheric aerosols are small solid or liquid particles suspended in the air. They are not only important for climate, but also because they influence local meteorology, due to their effects on cloud formation and associated albedo, and degrade air quality, leading to millions of premature deaths each year
To retrieve information on atmospheric aerosols from satellite-based sensors, the whole atmosphere-Earth system needs to be taken into account, i.e. upwelling radiation from the Earth surface due to (sub-) surface reflection and reflection by atmospheric constituents: gases, aerosols and clouds. AOD retrieval requires a performant cloud filtering, an effective separation of atmospheric and surface contributions to the reflection observed at the top of the atmosphere (TOA), and good prior information on particle properties.
Retrieval from a dual-view sensor like the Copernicus Sentinel-3 Sea and Land Surface Temperature (SLSTR) is of particular interest, as the limited multi-viewing and the lack of blue channels lead to a heterogeneous spatial distribution of the geometry information content and, hence, the capability to separate aerosol and surface scattering with or without prior surface knowledge. SLSTR differs in many ways from its Along Track Scanning Radiometers (A)ATSR predecessors: the dual-view orientation was rotated by 180 deg, the swath widths of both nadir and oblique views were extended, and a new Short Wave Infra Red (SWIR) channel was added at 2.25µm.
In the frame of the European Commission (EC) Copernicus programme, EUMETSAT has responsibility for the development, maintenance, validation and evolution of the operational Copernicus Sentinel-3 Near Real Time (NRT &mdash < 3h) Aerosol processor. In order to characterise and fully understand the benefits, but also the challenges, of retrieving aerosol information from SLSTR, this study was placed with the Finnish Meteorological Institute (FMI). The original FMI dual view algorithm (ADV) was developed for the Along Track Scanning Radiometers (ATSR) and here the adaptation of ADV to SLSTR is called SDV.
The objective of this study was to characterise andquantify the capabilities of the Sentinel-3 SLSTR measurement and associated physics information for the purpose of a mature AOD product globally, compliant with scientific NRT requirements. For that purpose, the idea was to use a reference algorithm with demonstrative capabilities for aerosol dual-view radiometers and with a suitable retrieval configuration with respect to SLSTR. To this end, the mature SDV algorithm was exercised with SLSTR data and the resulting retrieval parameter, mainly AOD at 550nm (AOD550), was investigated in-depth using state-of-the-art radiative transfer simulations combining aerosols and surfaces, reference measurements from the AERONET network, and by comparison with the most commonly used satellite AOD product, i.e. the MODIS-Terra C6.1 merged Dark Target Deep Blue AOD product (the MODIS-Terra observations are close in time to the SLSTR observations). Retrieval errors were characterised as a function of dual-view geometry, surface type, wind speed and white cap parameterisations over oceans.
The project kicked-off in 2019 and concluded in March 2021. FMI and EUMETSAT investigated the SLSTR aerosol retrieval challenges in various geophysics conditions by exploiting a state-of-the-art SLSTR simulated database, based on extensive Bidirectional Reflectance Distribution Function (BRDF) surface treatment and atmospheric radiative transfer calculations (see Figure 1). For BRFF, the European Space Agency (ESA) ADAM/GlobCover data and the libradtran/DISORT radiative transfer model was used. It included all surface types, aerosol conditions, and extensive viewing geometry array (solar, viewing, and azimuth) to cover the comprehensive range of SLSTR scattering angles.
The main points investigated were:
The main lessons learned with respect to SLSTR were:
Details of the analysis are available in the final reports (see Documents tab below), and have been of great benefit to the ongoing evolutions of the Copernicus Sentinel-3 SLSTR NRT aerosol product from EUMETSAT.