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.
GRASP (Generalized Retrieval of Aerosol 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 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 cloud-mask algorithm. The data at polar regions at latitudes higher than 60 degrees north and south were not processed.
The current MERIS/GRASP aerosol and surface data were processed with the GRASP code version v0.7.3. 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: biomass burning aerosol (spherical particles, refractive index of 1.45-0.031i); urban aerosol (spherical particles, refractive index of 1.45-0.031i); oceanic aerosol (spherical particles, refractive index of 1.37-0.0001i); desert dust (non-spherical particles, with spectrally dependent complex refractive index, with real part of 1.56 and imaginary of 0.0032, 0.0028, 0.0023, 0.0019, 0.0013, 0.0012, 0.0010 for 413, 443, 490, 510, 560, 665, 760 and 870 nm, correspondingly). 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. The further details of the retrieval settings can be provided by request.
The raw results files from GRASP contains full year of data of small regions (10×4 pixels). For the convenience of the users, the daily global files were created as well as their aggregates in time and space. Only the most reliable and demanded data were exported for final public distribution. Data format of this public archives is NetCDF.
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 to 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 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 stored under level 1.5. These data are not publicly available, but can be provided by a special request. This archive ensure good surface retrieval but quality of aerosol products is not guaranteed.
- We remove unphysical values like water surface model over land and the other way around.
- As atmospheric composition should not change drastically, we apply an outlier detection algorithm to each cell of 10×10 pixels. This helps removing peaks. Our algorithm consists on calculate the average (mean) and the standard deviation (std) of the AOD865 of the cell and we iteratively remove the farthest pixel the average if the cell does not fulfill following condition: std<0.05 and std/mean<0.05. We apply this algorithm until we the set of pixels fulfill the conditions. In case we have 22 or less pixels, we remove all of them (we cannot apply the statistics).
This results 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 0: raw results from grasp
Level 1: daily files from the output
Level 1.5: data softly screened. All surface pixels are good but we cannot guarantee the quality of aerosol information
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).
Only levels 2 and 3 are publicly available since from 0 to 1.5 are considered internal files results of intermediate processings. Climatological monthly aggregation is the aggregation of all Januarys, Februaries, … in the archive as seasonal climatology is all winters, all springs, …. .
The access to the internal archive could be provided under request. Please, contact us for further information.