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Contributors: G.E. Thomas (UKRI-STFC RAL Space) and R. Shevchuk (Brockmann Consult)
Issued by: STFC RAL Space (UKRI-STFC) / Gareth Thomas and Brockmann Consult / Roman Shevchuk
Date: 26/05/2023
Ref: C3S2_D312a_Lot1.2.1.4-v4.0_2023056_202407_PUGS_ECV_CCICloudPropertiesCLD_SLSTR_v1.2_final
Official reference number service contract: 2021/C3S2_312a_Lot1_DWD/SC1
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List of tables
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Table 1-1: GCOS targets for cloud properties ECVs Table 1-2: Cloud properties included in Cloud_cci TCDR and SLSTR ICDR monthly products Table 2-1: GCOS targets for cloud ECVs Table 2-2: Cloud properties included in Cloud_cci TCDR and SLSTR ICDR daily products Table 2-3: Summary of gaps and anomalies for each satellite |
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Figure 1-1: Overview of data producers, satellites, time coverages and grids for the ICDRs. |
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General definitions
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Table 1: Summary of variables and definitions
Variables | Abbreviation | Definition |
Cloud mask / Cloud fraction | CMA/ CFC | A binary cloud mask per pixel (L2) and |
therefrom derived monthly total cloud fractional coverage (L3C) | ||
Cloud optical thickness | COT | The line integral of the absorption extinction coefficient (at 0.55μm wavelength) along the vertical in cloudy pixels. |
Cloud effective radius | CER | The area-weighted radius of the cloud droplet and crystal particles, respectively. |
Cloud top pressure/ height/ temperature | CTP/ CTH/ CTT | The air pressure [hPa] / height [m] / temperature [K] of the uppermost cloud layer that could be identified by the retrieval system. |
Cloud liquid water path/ Ice water path | LWP/ IWP | The vertical integrated liquid/ice water content of existing cloud layers; derived from CER and COT. LWP and IWP together represent the cloud water path (CWP) |
Table 2: Definition of processing levels
Processing level | Definition |
Level-1b | The full-resolution geolocated radiometric measurements (for each view and each channel), rebinned onto a regular spatial grid. |
Level-2 (L2) | Retrieved cloud variables at full input data resolution, thus with the same resolution and location as the sensor measurements (Level-1b). |
Level-3C (L3C) | Cloud properties of Level-2 orbits of one single sensor combined (averaged) on a global spatial grid. Both daily and monthly products provided through C3S are Level-3C. |
Table 3: Definition of various technical terms used in the document
Jargon | Definition |
Brokered product
TCDR | A Thematic Climate Data Record is a consistently processed time series of a geophysical variable. The time series should be of sufficient length and quality. |
ICDR | An Interim Climate Data Record (ICDR) denotes an extension of TCDR, processed with a processing system as consistent as possible to the generation of TCDR. |
Brokered product | The C3S Climate Data Store (CDS) provides both data produced specifically for C3S and so-called brokered products. The latter are existing products produced under an independent programme or project which are made available through the CDS. |
Climate Data Store (CDS) | The front-end and delivery mechanism for data made available through C3S. It is a platform that provides access to a wide range of climate data, including satellite and in-situ observations, reanalysis and other relevant datasets. |
Retrieval | A numerical data analysis scheme which uses some form of mathematical inversion to derive physical properties from some form of measurement. In this case, the derivation of cloud properties from satellite measured radiances. |
Forward model | A deterministic model which predicts the measurements made of a system, given its physical properties. The forward model is the function which is mathematically inverted by a retrieval scheme. In this case, the forward model predicts the radiances measured by a satellite instrument as a function of atmospheric and surface state, and cloud properties. |
TCDR
It is a consistently-processed time series of a geophysical variable of sufficient length and quality.
ICDR
An Interim Climate Data Record (ICDR) denotes an extension of TCDR, processed with a processing system as consistent as possible to the generation of TCDR.
CDR
A Climate Data Record (CDR) is defined as a time series of measurements with sufficient length, consistency, and continuity to determine climate variability and change.
Scope of the document
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This Product User Guide and Specification (PUGS) document is associated with the Climate Data Store (CDS) catalogue entry: Cloud properties global This Product User Guide and Specification (PUGS) document is associated with the Climate Data Store (CDS) catalogue entry: Cloud properties global gridded monthly and daily data from 1982 1979 to present derived from satellite observations. This Climate Data Record (CDR) comprises inputs from two three sources: (i) brokered products from the Cloud Climate Change Initiative (ESA’s Cloud_cci), namely those coming from processing of the Advanced Along-Track Scanning Radiometer (A)ATSR ) data and (data (also named TCDR (Thematic Climate Data Record)), and two Interim Climate Data Records (ICDR) extensions namely (ii) those produced under this contract for the CDS, specifically those coming from processing of the Sea and Land Surface Temperature Radiometers (SLSTR) . Both CDRs are produced by Rutherford Appleton Laboratory (RAL) Space using the Optimal Retrieval of Aerosol and Cloud (ORAC) iii) those produced under this contract for the CDS, specifically those coming from processing of the SLSTR by Brockmann Consult (BC).
Both of the ICDRs mentioned above are produced using the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm, using the Community Cloud for Climate (CC4CL) processor under the ESA Cloud_cci project. One ICDR was produced by Rutherford Appleton Laboratory (RAL) Space for the period 01/2017 to 06/2022 and the other ICDR by Brockmann Consult (BC) for the period 07/2022 to 12/2023.
This document provides information on how to use these products and it . It is not part of the official CCI documentation, but produced only in the scope of the brokering to the CDS.
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The ESA Climate Change Initiative (CCI) Cloud Properties Climate Data Record (CDR) is a brokered product from the ESA Cloud_cci project, while the extension Interim CDR (ICDR) produced from the Sea and Land Surface Temperature Radiometers (SLSTR) is produced specifically for C3S. The product is generated by RAL Space, using the Community Cloud for Climate (CC4CL) processor, based on the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm. The ICDR product was generated by RAL Space up to 06/2022 and by BC for the period 07/2022 to 12/2023.
The Cloud_cci dataset comprises 17 years (1995-2012) of satellite-based measurements derived from the Along Track Scanning Radiometers (ATSR-2 and AATSR) onboard the ESA second European Research Satellite (ERS-2) and ENVISAT. This CDR TCDR is partnered with the ICDR produced from the Sentinel-3 Sea and Land Surface Temperature Radiometers (SLSTR), beginning in 2017 (Sentinel-3A) and 2018 (Sentinel-3B), when data from each platform became available. In addition to the individual data from each platform, a merged product is provided from 10/2018 onwards.
The TCDR and ICDRs provide both The CDR provides level-3 data (monthly means) and level-2 (daily means) on a regular two grids: (1) regular equal angle global latitude-longitude grid (with a resolution of 0.5°× 0.5°) as well as level-2 data (daily means) on a regular and (2) regular equal area global latitude-longitude grid (with the second option solely for the merged product from 07/2022 onwards). An overview of the various data producers, satellites, grids and time coverages for the ICDRs is shown in section 1, Figure 1-1.
Both grids have a resolution of 0.1°×5°× 0.1°) and includes 5° for monthly means and 0.1°× 0.1° for the daily means. Both grids include these products: cloud fractional cover (CFC), cloud phase (water/ice), cloud optical thickness (COT), cloud particle effective radius (CER), cloud liquid/ice water path (LWP/IWP), and cloud-top pressure (CTP), height (CTH) and temperature (CTT).
These products are brokered to (in the case of (A)ATSR) or produced for the Climate Data Store (in the case of SLSTR) by the Copernicus Climate Change Services (C3S). The respective data products (daily and monthly means) are first described in terms of their input data and a brief overview of the algorithms is provided; their target requirements in the scope of C3S and achieved performances are given; relevant information for usage is provided. The latter comprises geographical grid specifications, the data format, naming conventions, and the acknowledgement policy.
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The Cloud_cci global monthly cloud properties product, version 3.0, is brokered to the CDS by the C3S from STFC RAL Space. The SLSTR ICDR until 06/2022, versions 3.1.1 and 4.0, is supplied to the CDS via the same route and uses the same processing software and infrastructure as the CDRTCDR. The SLSTR ICDR, version 4.0 from 07/2022 to 12/2023 is processed on BC infrastructure using the same processing software for Level-2, the additional grid version (equal area) was processed using the Calvalus aggregator for Level-3 step [D6]. Although the format of the brokered data products differs from the official CCI products, the intellectual property rights remain with the Cloud_cci team. In detail, the official daily and monthly files from Cloud_cci contain more than 180 variables, but the specified C3S format splits them into separate parts per ECV. That way, each file has a reduced data amount as well as an increased usability. The landing page for the official CCI products is https://climate.esa.int/en/projects/cloud/. Detailed information on the cloud products and their use are outlined in the CCI Product User Guide [D3].
The product provides estimates The product provides estimates of monthly mean global cloud properties over land and ocean from the Along Track Scanning Radiometer ((A)ATSR) series of satellite sensors, on a 0.5°×0.5° latitude-longitude grid from mid-1995 until early 2012. The SLSTR based ICDR extends the coverage, with a five-year gap, from 2017 onwards and is only available through the CDS. Version 3.x 1.1 of the ICDR provides data from a single SLSTR (flying on board either Sentinel-3a or -3b), while version 4.0 combines data from both SLSTR instruments into a single combined product (starting with the launch of the Sentinel-3b in late 2018). The processing of version 3.1.1 and 4.0 are is identical – the sole difference is whether the two instruments are treated separately or treated as a single data-stream.
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All monthly means are provided on an equal angle projection. In addition, the product from 07/2022 onwards is provided on an equal area projection as well (see Figure 1-1 for an overview).
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1.1
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The Cloud_The Cloud_cci product brokered to the CDS consists of cloud properties derived from the (A)ATSR instruments; specifically:
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The analysis algorithm applied in producing the Cloud_cci products is the Optimal Retrieval of Aerosol and Clouds (ORAC), while the overall data processing system used in the project is referred to as Community Cloud for Climate (CC4CL). The algorithm, processing chain and all input and auxiliary data are detailed described in the C3S ATBD [D1], Cloud_cci ATBD [D2], Sus et al. (2018) and McGarragh et al. (2018).
The brokered CDR TCDR is extended, with a five-year gap, from 2017 with an ICDR derived from the SLSTR instruments, which fly onboard the Copernicus operational Sentinel-3 satellites. From Since late 2018, a second Sentinel-3 platform is available, which doubles the spatial coverage of SLSTR and allows for nearly complete global coverage twice a day. The SLSTR product is produced using the same processing chain as the (A)ATSR CDR.TCDR.
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The target requirements for cloud observations are laid-out in the Target Requirements and Gap Analysis Document (TRGAD) [D5], but are summarised here. These requirements originate from the WMO Global Climate Observing System (GCOS) initiative, which defines and lays down targets for the observation of essential climate variables (ECVs). It should be noted that GCOS defines ideal targets for Earth observation systems, which are often not attainable using existing or historical observing systems. Table 1-1 provides an overview of the GCOS requirements for cloud properties.
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Quantity | Cloud_cci variable | Targets |
Cloud amount (Cloud Fractional Cover) | CFC |
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Cloud top pressure | CTP |
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Cloud top temperature | CTT |
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Cloud Optical Depth | COT |
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Cloud Liquid Water Path / Ice Water Path | LWP, IWP |
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Cloud particle Effective |
radius | CER |
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Cloud_cci and SLSTR ICDR products are provided 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-CLOUD-INST_ORAC_EA_PLATFORM{}_YYYYMM{_}_fvVVV_fvV.V.nc,
where INST and PLATFORM refer to the instrument and platform from which data originates (either ATSR2 and ERS2, or AATSR and ENVISAT for the
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TCDR, or SLSTR and Sentinel-3a or -3b for the ICDR), EA is added for the additional equal area grid and left out for the equal angle grid, and YYYYMM provides the year and month covered by the monthly mean product, and
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V.V denotes the product version
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Data are provided as monthly means of cloud properties, as described in Table 1-2, 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).
The list of cloud properties, the variable names used within the NetCDF data files and their units are given in Table 1-2.
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Property | Unit | Variable name | Comment |
Cloud fractional cover | - | cfc | |
Cloud-top pressure | hPa | ctp | |
Cloud-top height | km | cth | |
Cloud-top temperature | K | ctt | |
Cloud optical thickness | - | cot | Only retrieved on day-light side of orbit |
Cloud effective radius | µm | cer | Only retrieved on day-light side of orbit |
Cloud liquid water path | gm-2 | lwp | Only retrieved on day-light side of orbit |
Cloud ice water path | gm-2 | iwp | Only retrieved on day-light side of orbit |
For lists and details of known issues and limitations of the data, the user is referred to the referenced ATBDs and papers in Section 1.1, as well as the TRGAD [D5]. In particular, it should be noted that the monthly mean is computed using data from sun-synchronous polar-orbiting satellites, with a relatively narrow swath. Thus, it cannot be considered a representation of the mean properties of clouds across the whole month for two reasons:
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1 At high latitudes temporal sampling becomes somewhat better, as adjacent orbits overlap. |
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The ORAC retrieval algorithm is an implementation of Optimal Estimation (Rodgers, 2000) and as such provides uncertainty estimates on all retrieved parameters, propagated from the uncertainty in the input satellite measurements (as well as any forward modelling error included in the uncertainty calculations). The propagation of the L2 uncertainty into L3 products, such as those brokered to the CDS, is not straight forward. Users should refer to the Cloud_cci Comprehensive Error Characterisation Report [D4].
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3. Correlated uncertainty in value. If a the L2 pixels are assumed to be correlated with each other, the uncertainty in the resulting average can be calculated using:
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where σ2std is the unweighted standard deviation about the mean and c is the correlation between the L2 pixels.
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The Cloud_cci global daily cloud properties product, version 3.0, is brokered to the CDS by the C3S from STFC RAL Space. The SLSTR ICDR, versions 3.1.1 and 4.0 are supplied to the CDS via the same route and uses the same processing software and infrastructure as the CDRthe TCDR. The SLSTR ICDR, version 4.0 from 07/2022 to 12/2023 is processed on BC infrastructure using the same processing software for Level-2, the additional grid version (equal area) was processed using Calvalus aggregator for Level-3 step. Although the format of the brokered data products differs from the official Cloud_cci products, the intellectual property rights remain with the Cloud_cci team. In detail, the official daily and monthly files from Cloud_cci contain more than 180 variables, but the specified C3S format splits them into separated parts per ECV. That way, each file has a reduced data amount as well as an increased usability. The landing page for the official CCI products is https://climate.esa.int/en/projects/cloud/.
The product provides estimates of daily mean global cloud properties over land and ocean from the Along Track Scanning Radiometer ((A)ATSR) series of satellite sensors, on a 0.1°×0.1° latitude-longitude grid from mid-1995 until early 2012. The SLSTR based ICDR extends the coverage, with a five-year gap, from 2017 onwards and is only available through the CDS. Version 3.x of the ICDR provides data from a single SLSTR (flying on board either Sentinel-3a or -3b), while version 4.0 combines data from both SLSTR instruments into a single combined product (starting with the launch of the Sentinel-3b in late 2018). The processing of version 3.x and 4.0 are identical – the sole difference is whether the two instruments are treated separately or treated as a single data-stream.
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All monthly means are provided on an equal angle projection. In addition, the product from 07/2022 onwards is provided on an equal area projection as well (see Figure 1-1 for an overview).
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The Cloud_cci product brokered to the CDS consists of cloud properties derived from the (A)ATSR instruments; specifically:
- ATSR-2, which flew on the second European Research Satellite (ERS-2) from June 1995 until January 2008. ERS-2 lost global coverage in June 2003, due to failure of the onboard data storage; Cloud_cci data do not extent extend beyond this date (thus providing approximately one-year of overlap with the succeeding AATSR product.
- Advanced-ATSR (AATSR), which flew on ENVISAT from May 2002 until the satellite’s failure in April 2012.
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The analysis algorithm applied in producing the Cloud_cci products is the Optimal Retrieval of Aerosol and Clouds (ORAC), while the overall data processing system used in the project is referred to as Community Cloud for Climate (CC4CL), and these are detailed in the C3S ATBD [D1], Cloud_cci ATBD [D2], Sus et al. (2018) and McGarragh et al. (2018).
The brokered CDR TCDR is extended, with a five-year gap, from 2017 with an ICDR derived from the SLSTR instruments, which fly onboard the Copernicus operational Sentinel-3 satellites. From Since late 2018, a second Sentinel-3 platform is available, which doubles the spatial coverage of SLSTR and allows for nearly complete global coverage twice a day. The SLSTR product is produced using the same processing chain as the (A)ATSR CDR.TCDR.
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The target requirements for cloud observations are laid-out in the Target Requirements and Gap Analysis Document (TRGAD) [D5], but are summarised here. These requirements originate from the WMO Global Climate Observing System (GCOS) initiative, which defines and lays down targets for the observation of essential climate variables (ECVs). It should be noted that GCOS requirements are targets and defines ideal targets for Earth observation systems, which are often not attainable using existing or historical observing systems. Table 2-1 provides an overview of the GCOS requirements for cloud properties.
Table 2-1: GCOS targets for cloud ECVs. Note that the Cloud_cci doesn't doesn’t meet the temporal requirements due to the nature of the satellite observations, but exceeds the spatial resolution and aims to achieve the uncertainty and stability requirements. Anchor table2_-1 table2_-1
Quantity | Cloud_cci variable | Targets |
Cloud amount (Cloud fractional cover) | CFC |
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Cloud top pressure | CTP |
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Cloud top temperature | CTT |
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Cloud Optical Depth | COT |
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Cloud Liquid Water Path / Ice Water Path | LWP, IWP |
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Cloud particle Effective |
Radius | CER |
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Cloud_cci products are provided 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-DAILY-CLOUD-INST_ORAC_EA_PLATFORM{}_YYYYMMDD{_}fvVVV{_}_ fvV.V.nc,
where INST and PLATFORM refer to the instrument and platform from which data originates (either ATSR2 and ERS2, or AATSR and ENVISAT for the CDRTCDR, or SLSTR and Sentinel-3a or -3b for the ICDR), YYYYMMDD EA is added for the additional equal area grid and left out for the equal angle grid, and YYYYMM provides the year , and month and day covered by the daily monthly mean product, and VVV V.V denotes the product version (3.0 for the TCDR, 3.1 for the ICDR).
Data are provided as daily means of cloud properties, as described in Table 2-2, on a regular latitude-longitude grid, with a spacing of 0.1° in both dimensions (thus grid centres lie at -89.95°, -89.85°, -89.75°, …, 89.95° in latitude and -179.95°, -179.85°, -179.75°, …, 179.95° in longitude).
Unlike the monthly product, the daily product separates the variables into day and night observations, stored in different variables within the product. In this context "day" “day” measurements are defined as those where the solar zenith angle is low enough that the solar reflectance can be used in the retrieval scheme. In the case of ORAC this means a solar zenith angle of θ0<80° . There are two main reasons for this approach:
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The list of cloud properties, the variable names used within the NetCDF data files and their units are given in Table 2-2.
Table 2-2: Cloud properties included in Cloud_cci TCDR and SLSTR ICDR daily products .available through the CDS. Anchor table2_-2 table2_-2
Property | Unit | Variable name | Comment |
Cloud fractional cover-day | - | cfc_day | |
Cloud fractional cover-night | - | cfc_night | |
Cloud-top pressure-day | hPa | ctp_day | |
Cloud-top pressure-night | hPa | ctp_night | |
Cloud-top height-day | km | cth_day | |
Cloud-top height-night | km | cth_night | |
Cloud-top temperature-day | K | ctt_day | |
Cloud-top temperature-night | K | ctt_night | |
Cloud optical thickness-day | - | cot_day | |
Cloud effective radius-day | µm | cer_day | |
Cloud liquid water path-day | gm-2 | lwp_day | |
Cloud liquid water path-night | gm-2 | lwp_night | |
Cloud ice water path-day | gm-2 | iwp_day | |
Cloud ice water path-night | gm-2 | iwp_night |
For lists and details of known issues and limitations of the data, the user is referred to the referenced ATBDs and papers in Section 1.1.
Anchor section2.3.1 section2.3.1
2.3.1 Uncertainties
section2.3.1 | |
section2.3.1 |
The ORAC retrieval algorithm is an implementation of Optimal Estimation (Rodgers, 2000) and as such provides uncertainty estimates on all retrieved parameters, propagated from the uncertainty in the input satellite measurements (as well as any forward modelling error included in the uncertainty calculations). The propagation of the L2 uncertainty into L3 products, such as those brokered to the CDS, is not straight forward. Users should refer to the Cloud_cci Comprehensive Error Characterisation Report [D4].
Uncertainty and variability in the monthly daily mean cloud properties is expressed using three separate values, as described by Bennartz (2017, pers. comm.):
1. Standard deviation about the mean. This value is the unweighted standard deviation of the L2 retrieval values about the mean value reported for each L3 pixel.
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where σi are the uncertainties of the L2 N L2 pixels included in the average, and the <> indicates the mean value.
3. Correlated uncertainty in value. If a the L2 pixels are assumed to be correlated with each other, the uncertainty in the resulting average can be calculated using:
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where σ2std is the unweighted standard deviation about the mean and c is the correlation between the L2 pixels.
2.3.2 Gaps and anomalies
A small number of gaps occurs in the daily means of the dataset. These are due to maintenance reasons or outages of instruments. In addition, anomalies appear and lead to a number of days which should be treated with caution. Table 2-3 provides a list of days which are either not available or anomalous.
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Sentinel-3A
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Sentinel-3B
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The original repository for the CCI formatted L3 data can be accessed through the Cloud_cci homepage https://climate.esa.int/en/projects/cloud/data/, as can all relevant documentation from the project. It should be noted that Cloud_cci L3 data is not in the same format as data brokered to the CDS, however the same L2 retrieval output was used to produce both products. In detail, the official daily and monthly files from Cloud_cci contain more than 180 variables, but the specified C3S format split them into separate parts per ECV. That way, each file has a reduced data amount as well as an increased usability.
Cloud_cci data should also be available through the ESA_cci data portal http://cci.esa.int/data, which provides online data mining, analysis and visualization tools for CCI data through the CCI Toolbox.
Only the brokered (A)ATSR based CDR TCDR is available through the ESA_cci data portal (original provider) as well as the CDS (brokered from CCI within the C3S Project), with SLSTR data being supplied only to the CDS.
3.1 Data access through CCI
Within C3S, the distribution will be through the CDS (https://cds.climate.copernicus.eu/) where documentation created for the inclusion of the data in the CDS, such as this PUGS, will be also provided.
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only supplied by the CDS.
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Within C3S, the distribution will be is through the CDS (https://cds.climate.copernicus.eu/) where documentation created for the inclusion of the data in the CDS, such as this PUGS, will be is also provided.
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2 Product ordering process
You need to be registered and logged in to order products in the CDS. A login is provided upon registration, all products in the CDS are delivered free of charge.
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Bennartz, R., 2017: Error propagation in the generation of gridded satellite datasets, DWD Visiting Scientist Report.
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Sus, O., M. Stengel, S. Stapelberg, G. McGarragh, C.A. Poulsen, A.C. Povey, C. Schlundt, G.E. Thomas, M. Christensen, S. Proud, M. Jerg, R.G. Grainger, R. Hollmann, 2018: The Community Cloud retrieval for Climate (CC4Cl) – Part 1: A framework applied to multiple satellite imaging sensors. Atmos. Meas. Tech., 11, 3373-3396, DOI: 10.5194/amt-11-3373-2018.
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This document has been produced with funding by the European Union in the context of the Copernicus Climate Change Service (C3S).The activities leading to these results have been contracted , operated by the European Centre for Medium-Range Weather Forecasts , operator of C3S on behalf on the European Union (Contribution Agreement signed on 22/07/2021). All information in this document is provided “as is” "as is" and no guarantee of warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author’s author's view. |
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