Contributors: G.E. Thomas (UKRI-STFC RAL Space)

Issued by: RAL Space (UKRI-STFC)

Date: 09/10/2020

Ref: C3S_D312b_Lot1.1.2.2-v3.1_202010_ATBD_CCISurfaceRadiationBudget_v1.0

Official reference number service contract: 2018/C3S_312b_Lot1_DWD/SC1

History of modifications

List of datasets covered by this document

Related documents

Acronyms

Scope of the document

This Algorithm Theoretical Basis Document (ATBD) describes the generation of the Climate Data Record (CDR) on Surface Radiation Budget, which is a brokered service from ESA’s Cloud_cci, and its extension with an interim-CDR (ICDR) derived from the Sea and Land Surface Temperature Radiometer (SLSTR). 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 cloud ATBD [D1] and Broadband Radiative Flux Retrieval ATBD [D4], the ESA Cloud_cci Product User Guide (PUG) [D2]. These documents describe the data processing chain and the algorithms used to generate the Surface Radiation Products and the cloud products on which they are based. The assessment described in this document is carried out within the scope of C3S, whereas 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 data brokered to, or produced for the CDS.

Executive summary

The CDR on Surface Radiation Budget is a brokered product of the CC4CL dataset produced by ESA's Cloud_cci project, and the SLSTR ICDR is an extension to the brokered product. Accordingly, this document largely refers to the ESA Cloud_cci documentation, with additional information specific to SLSTR included.

The Cloud_cci record comprises 17 years (1995-2012) of satellite-borne observations derived from measurements of the ATSR series of satellites on board the ESA ERS-2 and ENVISAT satellites. The CDR provided here comprises monthly (0.5° x 0.5° resolution) means of surface radiation on a regular global latitude-longitude grid. The SLSTR ICDR continues this record, with a 5-year gap, with version v3.x providing cloud and radiation budget products from SLSTR from 2017 on.

The Surface Radiation Budget products are surface incoming and reflected shortwave (solar) radiation, surface downwelling and outgoing longwave (thermal-IR) radiation, net-fluxes of shortwave and longwave radiation and total surface radiation budget (shortwave + longwave). Note that the service within Copernicus provides a somewhat different set of products to those provided by the original Cloud_cci products: not all Cloud_cci products are included in the product brokered to the CDS and the net fluxes (providing the Surface Radiation Budget ECVs, and calculated directly from the downwelling and upwelling fluxes) are not included in the original Cloud_cci products.

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 2.3.

ICDR data is derived from the Sea and Land Surface Temperature Radiometer (SLSTR), flown on board the Copernicus Sentinel-3 platform. SLSTR is an improved version of the ATSR instruments, providing additional spectral channels, improved spatial resolution for the visible and shortwave-infrared channels and a considerably wider swath (1,400 km nadir swath, compared to 512 km for AATSR). As part of the Copernicus operational observation system, two SLSTR instruments are kept operational, with a backup instrument also to be in-orbit (although not-yet launched). The two operational instruments were launched on Sentinel-3A on 16^th February 2016 and Sentinel-3B on 25^th April 2018. The two platforms fly in identical, interleaved sun-synchronous orbits, such that the two SLSTR instruments provide nearly global coverage twice daily (with one day and one night overpass). Orbital characteristics of Sentinel-3A are summarised in Table 1-1.

Table 1‑1: The orbital characteristics of the Sentinel-3 satellites. Note that these characteristics are very similar to the ERS-2 and ENVISAT platforms which carried the preceding ATSR instruments.

The SLSTR instrument is described in detail by the Sentinel-3 SLSTR User Guide [D10], but an overview of its specifications is given in Table 1‑2.

Table 1‑2: The specifications of the SLSTR instruments (taken from the SLSTR User Guide [D10]).

2. Input and auxiliary data

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

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 (FCDR) 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.

The input data for the Cloud_cci extension ICDR is the (non near-real-time) ESA Observation mode SLSTR level-1 archive, hosted by the Centre for Environmental Data Analysis (CEDA). At time of writing, no reprocessing of the SLSTR level-1 archive has been undertaken, and improvements in instrument calibration, pixel colocation/geolocation and minor product details have continued to evolve throughout the lifetime of the mission. It is likely that some variation in SRB product quality will be attributable to this inconsistency in the level-1 input data.

SLSTR level-1 data is provided in NetCDF-4 formatted files, which are segregated into 3-minute frames. The level-1 data provides 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).

The Broadband Radiative Flux Retrieval (BRFR) module of the CC4CL processor operates on level 2 Cloud_cci cloud products, as well as level 2 aerosol optical depth products produced for the Aerosol_cci project and C3S 312b Lot2, also using the ORAC retrieval scheme applied to (A)ATSR and SLSTR data. Details of the Aerosol retrieval scheme employed for Aerosol_cci and C3S can be found in the Aerosol_cci ORAC ATBD [D5].

2.2 Specific input and auxiliary data

The specific spectral channels used in producing the (A)ASTR and SLSTR Surface Radiation Budget product 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.

Table 2 1: (A)ATSR and SLSTR channels used to produce the Cloud_cci cloud properties v3.0 TCDR and SLSTR v3.x ICDR

0.67 $\mu$m

0.87 $\mu$m

1.60 $\mu$m

10.8 $\mu$m

12.0 $\mu$m

In addition to (A)ATSR or SLSTR level 1 data, the CC4CL retrieval scheme, and BRFR module, also rely on a range of auxiliary datasets, which are detailed in Table 2-2.

Table 2-2: Auxiliary data used in generating the SLSTR cloud properties and radiation ICDRs.

Further details of the auxiliary data used are provided in the Cloud_cci CC4CL ATBD [D2], Section 3. Full details of input data to the BRFR module are given in Section 2.1 of the BRFR ATBD [D4].

3. Algorithms

This section describes the algorithms used to derive the final Surface Radiation Budget products.

3.1 Retrieval of swath-based surface radiation (level-2 data)

The algorithm used to derive surface (and top-of-atmosphere) radiation is described in Section 2.1 of the Cloud_cci BRFR ATBD [D4].

3.2 Generation of final products (level-3 data)

The generation of the final level-3 products (monthly means) is outlined in Cloud_cci ATBD [D1], Section 5.

3.3 Additional CDS radiative properties

As mentioned above the products brokered to the CDS contain a different set of properties than those included in the original Cloud_cci products. In particular, the CDS products include net-flux (Surface Radiation Budget) values. These are calculated directly from the retrieved up- and down- welling fluxes (and thus do not constitute additional information). The surface net shortwave radiation (sns) is calculated using:

$$sns=sis-srs$$

where sis and srs are the incoming and reflected surface shortwave radiation respectively. Likewise, the surface net longwave radiation (snl) is calculated from:

$$snl=sdl-sol$$

where sdl and sol are the downwelling and outgoing longwave radiation respectively. The total surface radiation budget (srb) is simply the sum of the short- and longwave contributions:

$$srb=(sis+sdl)-(srs+sol)$$

4. Output data

This section summarises information on the output files.

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:

C3S-312bL1-L3C-MONTHLY-SRB-INSTORACPLATFORM{}YYYYMM{_}_fv3.0.nc,

where INST and PLATFORM refer to the instrument and platform from which data originates (either ATSR2 and ERS2, AATSR and ENVISAT for CDR data, or SLSTR and Sentinel-3a or -3b for ICDR data), and YYYYMM provides the year and month covered by the monthly mean product.
Data are provided as monthly means of Surface Radiation Budget, as described in Table 3, on a regular latitude-longitude grid, with 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).

4.2 File contents

All data arrays, described in Table 3, are 720x360 element arrays, on the grid defined by the longitude (lon, with 720 elements) and latitude (lat, with 360 elements) axes. In addition each file contains the global attributes defined in Table 4.

Table 3: The data variables included in the Cloud_cci TCDR and SLSTR ICDR Surface Radiation Budget files brokered to, or produced for, the CDS.

Table 4: List of global attributes included in the Cloud_cci TCDR and SLSTR ICDR Surface Radiation Budget files brokered to, or produced for, the CDS.

References

References are listed in ESA Cloud_cci ATBD [D1], Section 6; in ESA Cloud_cci PUG [D2], Section 6; in ESA Cloud_cci CECR [D3], Section 11; in ESA Cloud_cci BRFR ATBD [D4], 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. and Mcgarragh et al. [D6, D7].

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