MERIS Data Release

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Introduction

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.

Retrieval specifications:
The inversion were directly for retrieving both aerosol and surface properties. For assuring stable retrieval the aerosol was  assumed as a mixture of for aerosol component mixed externally: 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 the different components were retrieved. However, the retrieved component distribution is not validated and not provided as product. The “optimized” settings (inversion strategy used by GRASP) were used in RT modeling of multiple-scattering in the atmosphere. The full list of the retrieved parameters, as well as, additional characteristics derived from retrieved parameters can be found in the product list provided below. The further details of the retrieval setting can be provided by request.

The post-processing
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.

Then,

  • 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.

Archive Structure

Data is versions as vX.Y, where X represent the processing version and Y represents the post processing version.

Internal archives:
Level 0: Raw GRASP output
Level 1: Internal. Tile output files.
Level 1.5 internal: daily and not -filtered

The access to the internal archive could be provided under request. Please, contact us for further information.

Public files:

Level 2: Data filtered at 10km resolution, sinusoidal projection

  • climatological
    • monthly
    • seasonal
    • special file: all archive average
  • daily
  • monthly
  • seasonal
  • yearly

Level 3: Regrid level 2

  • 0.1 degree:
    • climatological
      • monthly
      • seasonal
      • special file: all archive average
    • daily
    • monthly
    • seasonal
    • yearly
  • 1 degree:
    • climatological
      • monthly
      • seasonal
      • special file: all archive average
    • daily
    • monthly
    • seasonal
    • yearly

File description

The files consist of the following parameters arranged in alphabetical order:

  Parameter   Description
  AOD   Aerosol Optical Depth for 413, 443, 490, 510, 560, 665, 753, 865 nm
  config   Information about the segmenter configuration used to create this result
  Cox_Munk_iso_BRM_’wl’_first_parameter   Surface albedo of water body at 413, 443, 490, 510, 560, 665, 753, 865 nm
  Cox_Munk_iso_BRM_665_second_parameter   Fraction of Fresnel reflection contribution at 665 nm
  Cox_Munk_iso_BRM_665_third_parameter   Mean square ocean surface slope at 665 nm
  crs   Spatial reference system information
  DHR   Directional Hemispherical Reflectance at 413, 443, 490, 510, 560, 665, 753, 865 nm
  Landpercentage   Land percentage
  Latitude   Latitude
  Longitude   Longitude
  MASL   Elevation above sea-level in meters
  NDVI   Normalized Difference Vegetation Index
  Ross_Li_BRDF_’wl’_isotropic_parameter   Ross Li BRDF isotropic parameter at 413, 443, 490, 510, 560, 665, 753, 865 nm  
  Ross_Li_BRDF_665_geometric_parameter   Ross Li BRDF normalized geometric parameter at 665 nm  
  Ross_Li_BRDF_665_volumetric_parameter   Ross Li BRDF normalized volumetric parameter at 665 nm
  x   x-coordinate in projected space
  xs   x-coordinate in segmenting space
  y   y-coordinate in projected space
  ys   y-coordinate in segmenting space

Results and Validation

The global validation has been done for many parameters using AERONET data. The below figures provide general overview of the validation results. More results of the validation (for different sites, etc.) can be obtained by the request.

Plots will be added soon.

Known Bugs