POLDER I-II and POLDER/PARASOL instruments
The POLDER instruments consist of a digital camera with a 274×242-pixel CCD detector array, wide-field telecentric optics and a rotating filter wheel enabling measurements in 9 spectral channels with bandwidths between 20nm and 40nm. Because it acquires a sequence of images every 20 seconds, the instrument can observe ground targets from different view directions. The POLDER I-II instruments onboard ADEOS 1 and 2 are identical while the instrument on the PARASOL platform has been turned 90 degrees to favor multidirectional viewing (maximum of 16 directions compared to 14) over daily global coverage (swath of 2400 km compared to 1600 km). Depending on the altitude of the platforms, the size of the images varies from 2400 x 1800 km2 to 1600 x 2100 km2 with a corresponding ground resolution of 7×6 km2 and 5.3×6.2 km2 at nadir. The PARASOL platform is part of the A-Train and takes advantage of the other instruments in the constellation. The 3 instrument spectral coverage ranges from blue (443µm) through near-infrared (0.91µm) with 3 polarized spectral bands. For POLDER/PARASOL, the bluest polarized channel has been moved from 0.443µm to 0.490µm and a 1.02 µm waveband has been added.
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 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 technical documentation.
PARASOL/GRASP aerosol and surface date product
This data sets includes the aerosol and surface reflectance retrieval product obtained from the POLDER/PARASOL data processed by GRASP code.
The POLDER/PARASOL data including measurements of all angular measurements intensity at 0.443, 0.490, 0.565 , 0,670, 0, 765, 0.865 and a 1.02 µm and Q and U polarization component at 0.490, 0,670 and 0.865 at the instrument native resolution were inverted in cloud-free conditions as determined by original cloud-mask algorithm developed by Laboratoire d’Optique Atmosphérique. The data at polar regions at latitudes higher than 70 degrees north and south were not processed. The observations with less than 10 viewing directions were ignored.
The retrieval were performed using one aerosol component model with 5 size bin size distribution and spectrally dependent complex refractive index. The aerosol vertical distribution was model using exponential profile and scale height was retrieved. 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 raw results files from GRASP contains three months of data of small regions (162×162 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.
- Pixels with AOD443 > 10 are removed.
- The coast is removed so all pixels with land percent is between 1 and 99 are removed. Also, to guarantee a proper coast elimination first pixel into ocean and land is removed.
- We remove unphysical values like water surface model over land and the other way around.
- We apply a criteria over to screen bad pixels (retrieval error is high so we remove completely the pixel):
- ocean → (residual <= 0.13)
- If precondition ndvi < 0.1
- if (dhr670 >= 0.3) then
- if (AOD670<1)
then (residual <= 0.04)
- else (residual <= 0.06)
- if (AOD670<1)
- (dhr670 < 0.3) -> (residual <= 0.07)
- if (dhr670 >= 0.3) then
- # precondition 0.1 <= ndvi < 0.4
- (dhr670 >= 0.25) -> (residual <= 0.075)
- (dhr670 < 0.25) -> (residual <= 0.085)
- # precondition 0.4 <= ndvi < 0.6
- (residual <= 0.1)
- # precondition 0.6 <= ndvi < 1
- (residual <= 0.12)
- If precondition ndvi < 0.1
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.
- the outliers are screened analyzing groups of 20×20 pixels and iteratively we remove the worst pixel (farther to the mean of AOD870) if the group does not fulfil following condition “std of data >= 1.5 AND std / mean of data >= 0.5”. If the result group has less than 60 pixels it is completely removed (noisy area probably due to cloud contamination).
- Some aerosol products can only be calculated when there is some aerosol loading (otherwise it is difficult to measure them). So, we apply extra filters. Angstrom Exponent is only provided if AOD560 is higher than 0.02 over ocean or 0.2 over land. Other more complex products (SSA, Re(m), Im(m), AAOD,Size distribution,SphereFraction) are strongly filtered:
- land: AOD443 >= 0.3 and 0.65 <= SSA <= 1.
- ocean: AOD443 >= 0.02 and 0.65 <= SSA <= 1.
- This results are exported as level 2. Level 2 is also temporally aggregated. The temporal aggregation pixels are removed if AOD443>4.
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.