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History of modifications

List of datasets covered by this document

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Acronyms

Scope of the document

This Algorithm Theoretical Basis Document (ATBD) describes the generation of the Climate Data Record (CDR) on cloud properties brokered from ESA's Cloud_cci programme and its extension with an interim-CDR (ICDR) derived from the Sea and Land Surface Temperature Radiometer (SLSTR) on the Sentinel-3 platform. This extension of the ESA Cloud_cci dataset is generated specifically for C3S by RAL Space, but is derived using the same algorithm and processing chain. Therefore, this document refers to several Cloud_cci documents (see _Related documents), namely the ESA Cloud_cci ATBDs [D1, D2], the ESA Cloud_cci Product User Guide (PUG) [D3], and the ESA Cloud_cci Error characterization report [D4]. These documents describe the data processing chain and the algorithms used to generate the cloud property products. The assessment described in this document is carried out within the scope of C3S and the intellectual property rights of the products themselves remain with the Cloud_cci, in the case of the CDR, or lie with STFC RAL Space, in the case of the ICDR. This document is not part of the official Cloud_cci documentation but produced solely in the scope of the brokering to the CDS.

Executive summary

The CDR on cloud properties is a product of the CC4CL dataset produced by ESA's Cloud_cci project and brokered to C3S, and the SLSTR ICDR is an extension to the brokered product. Accordingly, this document largely refers to the ESA Cloud_cci documentation.

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The provided cloud products are cloud fractional cover, cloud top level (consisting of cloud top temperature, pressure, and height), and cloud physical properties (consisting of cloud optical thickness, effective radius, and water path for both the liquid and the ice phase). Note that the brokered service within Copernicus provides only a subset of the original CCI cloud properties dataset, thus the mentioned products do not cover the entire range on cloud products contained in the original dataset provided by Cloud_cci.

1. Instruments

The brokered Cloud_cci cloud dataset is derived from the ATSR-2 and AATSR instruments, which are described in Cloud_cci ATBD [D1], Section 2.2.

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2. Input and auxiliary data

This section summarises the required input data used in the retrieval algorithms.

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2.1 Fundamental climate data record

The input data for the Cloud_cci dataset is the so-called AATSR-multi-mission level 1b data record, and this forms the fundamental climate data record for this product. The processing of measured radiances to level 1b is described in the ESA AATSR Detailed Processing Model Level 1b [D8] and ENVISAT-style products for ATSR-1 and ATSR-2 data [D9] documents.

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SLSTR level-1 data is provided in NetCDF-4 formatted files, which are segregated into 3-minute frames. The level-1 data provide TOA reflectance (for visible and SWIR channels) and brightness temperature (for TIR channels), pixel viewing geometry, measurement time and geolocation, as well as other auxiliary information (see the SLSTR User Guide [D10] for further details).

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2.2 Specific input and auxiliary data

The specific spectral channels used in producing the (A)ASTR and SLSTR cloud properties datasets are detailed in Table 2-1, which were chosen to match the so-called heritage-channels provided by the long-running AVHRR instrument series. The cloud retrieval only makes use of the nadir view of the (A)ATSR and SLSTR instruments.

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Further details of the input and auxiliary data used are provided in the Cloud_cci CC4CL ATBD [D2], Section 3.

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3. Algorithms

This section describes the algorithms used to derive the final cloud properties products.

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3.1 Retrieval of swath-based cloud properties (level-2 data)

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3.1.1 Cloud fractional cover

The algorithm used to retrieve the level-2 data on the cloud fractional cover is briefly summarised in Cloud_cci ATBD [D1] Section 3.1 and the CC4CL ATBD [D2] Section 2.1. A comprehensive description can be found in Sus et al. 2017 [D6].

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3.1.2 Cloud physical properties

The algorithm used to retrieve the level-2 data on the cloud physical properties is briefly summarised in Cloud_cci ATBD [D1], Section 3.3. Detailed descriptions can be found in the CC4CL ATBD [D2] and McGarragh et al 2017 [D7].

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3.2 Error budget estimates

3.2.1 Cloud fractional cover

Error budget estimates of cloud fractional cover as well as assumptions and limitations associated with the retrieval algorithms are found in the Cloud_cci Comprehensive Error Characterisation Report [D4].

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3.2.2 Cloud physical properties

Error budget estimates of cloud physical properties as well as assumptions and limitations associated with the retrieval algorithms are found in the Cloud_cci CC4CL ATBD [D2] Section 2 and the Comprehensive Error Characterisation Report [D4].

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3.3 Generation of final products (level-3 data)

The generation of the final level-3 products (the daily and monthly means) is outlined in Cloud_cci ATBD [D1], Section 4. Note that within the scope of this project, the provided dataset constitutes only a subset of the original Cloud_cci cloud properties dataset. The Cloud_cci PUG [D3] Annex C and the Cloud_cci Comprehensive Error Characterisation Report [D4] Section 6.1 describe the propagation of uncertainty into L3 products.

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3.3.1 Cloud fractional cover

Details on the level-3 cloud fractional cover retrieval are given in Cloud_cci ATBD [D1] Section 4.2.1.

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3.3.2 Cloud physical properties

Details on the level-3 cloud physical properties retrieval are given in Cloud_cci ATBD [D1] Section 4.2.2.

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4. Output data

This section summarises information on the output files.

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4.1 File format

Cloud_cci TCDR and SLSTR ICDR products are provided to the CDS in NetCDF (version 4), which are compliant with the conventions CF 1.8 and the NASA Global Change Master Directory (GCMD) Science Keywords vocabulary. Filenames follow the structure:

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Data are provided as monthly and daily means of cloud properties, as described in Table 4-1 and 4-2, on a regular latitude-longitude grid. For monthly products this grid has a spacing of 0.5° in both dimensions (thus grid centres lie at -89.75°, -89.25°, -88.75°, …, 89.75° in latitude and -179.75°, -179.25°, -178.75°, …, 179.75° in longitude). Daily products have a grid spacing of 0.1° in both dimensions.

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4.2 File contents

The data in the monthly cloud products are given in Table 4‑1, while those for the daily products are given in Table 4‑2. Most data fields have four related variables, which are the mean value of the data itself (for example, the mean cloud-top pressure is found in the ctp variable), followed by the standard deviation of the data, denoted by _std, the propagated uncertainty, denoted by _unc, and the spatially correlated uncertainty, denoted by _cor. The definition and details of the calculation of these terms is given in the Cloud_cci Comprehensive Error Characterisation Report [D4].

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References References

References

References are listed in ESA Cloud_cci ATBD [D1], Section 6; ESA Cloud_cci CC4CL ATBD [D2]; ESA Cloud_cci PUG [D3], Section 6; in ESA Cloud_cci CECR [D4], Section 11; in ESA Cloud_cci ATBD (MLEV) [D5], Section 4; in AATSR Level 1b Detailed Processing Model [D8], Section 3; in ENVISAT-style Products for ATSR-1 and ATSR-2 data [D9], Section 8; and Sus et al. 2017 and McGarragh et al. 2017 [D6, D7].

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