MERIS (MEdium Resolution Imaging Spectrometer) is an imaging spectrometer operating from Envisat satellite platform in the solar reflective spectral range. Fifteen spectral bands can be selected by ground command. The instrument scans the Earth’s surface by the so called “push-broom” method. Linear CCD arrays provide spatial sampling in the across-track direction, while the satellite’s motion provides scanning in the along-track direction: https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/envisat/instruments/meris.
AATSR (Advanced Along-Track Scanning Radiometer) is an imaging spectrometer operating from Envisat satellite platform in the Thermal, Shortwave and Near infrared and Visible spectral range. Special features of the AATSR instrument included its use of a conical scan to give a dual-view of Earth’s surface and on-board calibration targets making accurate quantitative measurements of radiation from the Earth’s surface. The cone axis of the scan projected downwards and ahead in the along-track direction of the satellite, achieving scanning across the satellite track in two regions.
AATSR was capable of making global measurements with 1 km x 1 km resolution at nadir:
GRASP (Generalized Retrieval of Atmosphere and Surface Properties) algorithm is designed to retrieve complete aerosol and surface properties globally. In order to achieve reliable retrieval from satellites observations even over very reflective desert surfaces, the algorithm was designed as simultaneous statistically optimized inversion of a large group of pixels within one or several images. Such, multi-pixel retrieval regime takes advantage from known limitations on spatial and temporal variability in both aerosol and surface properties. Specifically the variations of the retrieved parameters horizontally from pixel-to-pixel and/or temporary from day-to-day are enforced to be smooth by additional appropriately set a priori constraints. This concept provides satellite retrieval of higher consistency. The details can be found in the articles by Dubovik et al., 2011, 2014, 2021 and in the algorithm documentation.
MERIS/GRASP aerosol and surface date product
This data sets includes the aerosol and surface reflectance retrieval product obtained from the MERIS/Envisat data processed by GRASP code. The MERIS/Envisat data including measurements 8 channels of MERIS absorption window channels in centered at 413 nm, 443 nm, 490 nm, 510 nm, 560 nm, 665 nm, 760 nm and 870 nm were used in GRASP retrieval. The data were inverted at resolution of 10 km in cloud-free conditions as determined by original MERIS cloud-mask algorithm. The data at polar regions at latitudes higher than 70 degrees north and south were omitted.
AATSR/GRASP aerosol and surface date product
This data sets includes the aerosol and surface reflectance retrieval product obtained from the AATSR/Envisat data processed by GRASP code. The AATSR/Envisat data including measurements 4 channels of AATSR absorption window channels in centered at 555, 659, 865 µm and 1600 nm were used in GRASP retrieval. The data were inverted at resolution of 10 km in cloud-free conditions as determined by SST/LST cloud-mask algorithm. The data at polar regions at latitudes higher than 70 degrees north and south were omitted.
ENVISAT (MERIS+AATSR)/GRASP aerosol and surface date product
This data sets includes the aerosol and surface reflectance retrieval product obtained from the synergy of MERIS/Envisat and AATSR/Envisat data processed by GRASP code. The combined MERIS/Envisat and AATSR/Envisat data used in GRASP retrieval included nadir measurements in 8 MERIS channels centered at 413 nm, 443 nm, 490 nm, 510 nm, 560 nm, 665 nm, 760 nm and 870 nm, channels at 560 , 665 and 870 nm that were replaced where available with both nadir and along track direction AATSR observations in channels centered at 555, 659, 865 nm. The data were inverted at resolution of 10 km in cloud-free conditions as determined by the combination of original MERIS and SST/LST cloud-masking algorithms, i.e. only pixels that were considered cloud-free by both algorithms were processed. The data at polar regions at latitudes higher than 70 degrees north and south were omitted.
The current MERIS/GRASP aerosol and surface data were processed with the GRASP code version v1.1.1. The retrieval of aerosol and surface properties is conducted simultaneously. To assure stability of the retrieval, the aerosol are assumed as an external mixture of aerosol components such as:
- Fine moderately absorbing aerosol (spherical particles, refractive index of 1.51-0.05i);
- Fine non-absorbing aerosol (spherical particles, refractive index of 1.395-0.003i);
- Coarse spherical aerosol (spherical particles, refractive index of 1.37-0.0001i);
- Coarse non-spherical aerosol (non-spherical particles, as suggested by Dubovik et al., 2006, with spectrally dependent complex refractive index, with real part of 1.56 and imaginary of 0.0032, 0.0029, 0.0024, 0.0019, 0.0013, 0.0012, 0.0010 and 0.0010 for 413, 443, 490, 510, 560, 665, 760, 870 and 1600 nm, correspondingly).
The details of the aerosol components size distributions could be found in Lopatin et al., 2021. The concentrations of each component are also retrieved. The retrieved components themselves, their spatial and temporal distribution, however, is a not yet validated product and therefore are not provided. The “optimized” settings (inversion strategy used by GRASP) are used in RT modeling of multiple-scattering in the atmosphere. The full list of the retrieved parameters, as well as, additional characteristics derived from the retrieved parameters can be found in the provided below product list. Further details of the retrieval settings can be provided by request.
In order to assure higher quality of the data, some post-processing was applied in order to eliminate the low quality points resulted from cloud contamination, bad surface description near the coast, etc. The post-processing screening follow these steps:
- First, from raw GRASP output we create global daily files. We call them level 1.
- We remove pixels that fulfill one of the following conditions:
- 1 > LandPercentage < 99
- ResidualsRel0 > 0.09
- AOD413 > 10 or AOD865 > 7
- Recalculate NDVI via the formula DHR(870) – DHR(664) ) / ( DHR(870) + DHR(664).
The date passed all above filter files are are exported as level 2.
Level 3 is created as regridding products at 0.1 and 1 degrees resolution in WGS84 projection of data available in level 2. In the regridding process we applied median filter instead of average or any other sophisticated filter.
In summary, the following data sets are prepared:
Level 1: daily files from the output
Level 2: Full resolution data filtered and aggregations (daily, monthly, yearly, seasonal, and climatologically monthly, seasonal and full archive).
Level 3: Regrid at 0.1 and 1 degree of level 2 (including temporal aggregations). Climatological monthly aggregation is the aggregation of all Januarys, Februaries, … in the archive, as well as seasonal climatology for all winters, all springs, etc. are available.
Data format of this public archives is NetCDF.
The access to the internal archive could be provided under request. Please, contact us for further information.