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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 from there 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 | 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. |
Near-real-time (NRT) | Data which is provided within a short time window (often taken to be three hours, but there is no fixed definition) of the measurement. NRT data is often supplanted by a subsequent data stream, which is subject to more rigorous checking of data quality. |
Radiative transfer | The mathematical modelling of the interaction of electromagnetic radiation with some medium – in this case solar and thermal-infrared radiation passing through the Earth’s atmosphere. |
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
This Algorithm Theoretical Basis Document (ATBD) is associated with the CDS catalogue entry: Cloud properties global gridded monthly and daily data from 1982 to present derived from satellite observations. The ATBD describes the algorithms used to generate the Climate Data Record (CDR) on Cloud Properties brokered from the European Space Agency Cloud Climate Change Initiative (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.
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Table 1‑1: The specifications of the (A)ATSR instruments.Anchor #table1-1 #table1-1
| Capability | (A)ATSR Specifications |
Swath | Nadir view | 512 km |
Oblique view | 512 km | |
Global Coverage Revisit Times | 3-4 day (mean) | |
Spatial Sampling interval at Sub-satellite point (km) | VIS-SWIR | 1 km |
IR | 1 km | |
Spectral channel centre (µm) | VIS | 0.554 (Ch1); 0.659 (Ch2) 0.865 (Ch3) |
SWIR | 1.61 (Ch4) | |
MWIR/TIR | 3.70 (Ch5); 10.85 (Ch6); 12.00 (Ch7) |
Figure 1‑2: Time coverage of ATSR-2, AATSR and SLSTR at the time of writing.Anchor figure1-2 figure1-2
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Table 1‑2: 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.Anchor table1-2 table1-2
Platform | Altitude (km) | Inclination | Period (min) | Repeat Cycle (days) | Ground-track deviation | Local Time at Descending node |
ERS-2 | 780 | 98.5° | 100 | 35 | 10:30 | |
ENVISAT | 799.8 | 98.55° | 100.59 | 35 | 10:00 | |
Sentinel-3 | 814.5 | 98.65° | 100.99 | 27 | ±1 km | 10:00 |
The SLSTR instrument is described in detail by the Sentinel-3 SLSTR User Guide (see References), but an overview of its specifications is given in Table 1-3.
Table 1‑3: The specifications of the SLSTR instruments (taken from the SLSTR User Guide (see References)).Anchor table1-3 table1-3
| Capability | SLSTR Specifications |
Swath | Nadir view | 1,400 km |
Oblique view | 740 km | |
Global Coverage Revisit Times (nadir view) | 1 satellite | 1 day (mean) |
2 satellites | 0.5 day (mean) | |
Spatial Sampling interval at Sub-satellite point (km) | VIS-SWIR | 0.5 km |
IR-Fire | 1 km | |
Spectral channel centre (µm) | VIS | 0.554 (S1); 0.659 (S2) 0.868 (S3) |
SWIR | 1.374 (S4); 1.613 (S5); 2.25 (S6) | |
MWIR/TIR | 3.742 (S7); 10.85 (S8); 12.02 (S9) | |
Fire ½ | 3.742 (F1); 10.85 (F2) | |
Radiometric Resolution | VIS (Albedo =0.5%) | Signal-to-Noise Ratio (SNR) > 20 |
SWIR (Albedo =0.5%) | Signal-to-Noise Ratio (SNR) > 20 | |
MWIR (T =270K) | NEΔT < 80 mK | |
TIR (T=270K) | NEΔT < 50 mK | |
Fire 1 (<500 K) | NEΔT < 1 K | |
Fire 2 (<400 K) | NEΔT < 0.5 K | |
Radiometric Accuracy | VIS-SWIR (Albedo = 2-100%) | < 2% (Beginning of Life) |
<5% (End of Life) | ||
MWIR –TIR | < 0.2 K (0.1 K goal) | |
Fire (< 500 K) | < 3 K |
2. Input and auxiliary data
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Table 2‑1: (A)ATSR and SLSTR channels used to produce the Cloud_cci cloud properties v3.0 TCDR and SLSTR v3.x ICDRAnchor table2-1 table2-1
(A)ATSR Channel number | SLSTR Band name | Nominal wavelength |
2 | S2 | 0.67 μm |
3 | S3 | 0.87 μm |
4 | S5 | 1.6 μm |
6 | S8 | 10.8 μm |
7 | S9 | 12.0 μm |
| Info | ||
|---|---|---|
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1 A key objective of the cloud_cci project under which the (A)ATSR CDR was produced was the production of consistent records of cloud properties from a range of satellite sensors that provided similar measurements (including AVHRR, (A)ATSR and the Moderate Resolution Imaging Spectroradiometer (MODIS)). In order to achieve this, a set of common channels, shared by all included instruments, was used in retrieving cloud properties – these correspond to the channels provided by AVHRR. |
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Table 2‑2: Auxiliary data used in generating the Cloud_cci TCDR and SLSTR ICDR cloud properties products.Anchor table2-2 table2-2
Dataset | Description |
ECMWF ERA-Interim | ECMWF reanalysis products provide pressure, temperature, humidity and ozone profiles, as well a priori surface temperature, sea ice extent and near-surface wind speed for ocean surface reflectance calculation. |
MODIS MCD43A12 V006 | The MODIS BRDF product provides land-surface reflectance. |
IREMIS UW Baseline Fit | The Global Infrared Land Surface Emissivity (IREMIS) University of Wisconsin-Madison Baseline Fit to the MODIS MOD11 emissivity product provides land-surface emissivity. |
RTTOV | The standard coefficient and database files provided with RTTOV v 12.1 are used, where not superseded by other auxiliary data (as is the case with the emissivity and surface reflectance atlases) |
Further details of the input and auxiliary data used are provided in the Cloud_cci CC4CL ATBD [D2], Section 3.
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Table 3-1: Symbols used in the forward model expressionsAnchor table3-1 table3-1
| The solar-zenith, satellite-zenith and relative azimuth angles, respectively. Used to denote the angular dependence of terms in the forward model expressions. | ||||||||||
| Used to denote the direction of radiative propagation. Eg.
denotes the downwelling transmission of the solar beam. | ||||||||||
| The above-cloud (ac) and below-cloud (bc) transmittances of the atmosphere for each instrument channel, as calculated by RTTOV in the pre-processing stage of the analysis. The additional “d” subscript denotes diffuse rather than direct-beam transmission. | ||||||||||
|
and
direct and diffuse transmittance of the cloud layer for each instrument channel. The term
is the transmission of diffuse irradiance into a particular viewing direction;
for equal illumination and viewing angles. Values of these parameters are calculated offline for a range of COT and CER values, which are tabulated in lookup-tables (LUTs). | ||||||||||
| These quantities are the bi-directional, hemispherical and bi-hemispherical reflectances of the cloud layer, for each instrument channel. As with the cloud transmittances, these parameters are tabulated offline and provided as LUTs to the retrieval. | ||||||||||
| The bi-directional, hemispherical (at the illumination and viewing angles) and bi-hemispherical reflectances of the surface, for each channel. ρ_bd and ρ_db are both hemispherical reflectances, with the former denoting the diffuse reflectance of the incoming solar-beam, while the later denotes the reflectance of diffuse downwelling into the viewing direction. | ||||||||||
| The atmospheric radiance (from thermal emission), for each instrument channel, from the atmosphere above and below the cloud layer respectively.
is also a function of surface temperature,
. See McGarragh et al 2017
in the forward model. | ||||||||||
| The Planck function, as a function of cloud-top temperature (
), for each instrument channel. | ||||||||||
| The effective cloud emissivity (as a function of the cloud properties), for each instrument channel. This is also provided by LUTs of offline calculations. |
As mentioned in Table 3-1, the values for cloud transmission, reflectance and emissivity are calculated offline for a range of cloud properties and tabulated as a function of cloud optical depth (COT) and cloud effective radius (CER). These calculations are done using the Discrete Ordinates (DISORT) radiative transfer code (see Stamnes et al. (1988)). The cloud-top pressure (CTP) is a function of the position of the cloud layer within the atmosphere.
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Table 4‑1: The data variables included in the Cloud_cci TCDR and SLSTR ICDR monthly cloud properties files brokered to, or produced for, the CDS.Anchor table4-1 table4-1
Property | Unit | Variable name | Comment |
Latitude | Degrees North | lat | Values correspond to grid centres (-89.75°, -89.25°, -88.75°, …, 89.75°). |
Longitude | Degrees East | lon | Values correspond to grid centres (-179.75°, -179.25°, -178.75°, …, 179.75°). |
Cloud pixel count | - | pixel_count pixel_count_day | The number of level 2 cloud pixels included in the averaging. Totals are given for all pixels and daylight pixels. |
Liquid cloud pixel count | - | liquid_count liquid_count_day | The number of level2 pixels, classified as liquid water cloud, included in the averaging. Totals are given for all pixels and daylight pixels. |
Ice cloud pixel count | - | ice_count ice_count_day | The number of level2 pixels, classified as ice water cloud, included in the averaging. Totals are given for all pixels and daylight pixels. |
Cloud fractional cover | - | cfc cfc_unc | The cloud fractional-area cover, defined as the fraction of level 2 pixels within each level 3 grid cell flagged as cloudy. |
Cloud-top pressure | hPa | ctp ctp_std ctp_unc ctp_cor | The pressure at the cloud top, as determined from thermal infrared emission. This value is directly related to cth and ctt by ECMWF ERA profiles. |
Cloud-top height | km | cth cth_std cth_unc cth_cor | The height of the cloud top above mean sea level, determined from thermal infrared emission. This value is directly related to ctp and ctt by ECMWF ERA profiles. |
Cloud-top temperature | K | ctt ctt_std ctt_unc ctt_cor | The temperature at cloud top, determined from thermal infrared emission. This value is directly related to ctp and cth by ECMWF ERA profiles. |
Cloud effective radius | µm | cer cer_std cer_unc cer_cor | The effective radius of cloud droplets or ice crystals. |
Cloud optical thickness | - | cot cot_std cot_unc cot_cor | The column optical thickness at a wavelength of 500 nm. |
Liquid water path | gm-2 | lwp lwp_std lwp_unc lwp_cor | The column mass of liquid cloud water. |
Ice water path | gm-2 | iwp iwp_std iwp_unc iwp_cor | The column mass of ice cloud water. |
Table 4‑2: The data variables included in the Cloud_cci TCDR and SLSTR ICDR daily cloud properties files brokered to, or produced for, the CDS.Anchor table4-2 table4-2
Property | Unit | Variable name | Comment |
Latitude | Degrees North | lat | Values correspond to grid centres (-89.95°, -89.85°, -89.75°, …, 89.95°). |
Longitude | Degrees East | lon | Values correspond to grid centres (-179.95°, -179.85°, -179.75°, …, 179.95°). |
Cloud pixel count | - | pixel_count_day pixel_count_night | The number of level 2 cloud pixels included in the averaging. Totals are given for daylight and night pixels separately. |
Liquid cloud pixel count | - | liquid_count_day liquid_count_night | The number of level2 pixels, classified as liquid water cloud, included in the averaging. Totals are given for daylight and night pixels separately. |
Ice cloud pixel count | - | ice_count_day ice_count_night | The number of level2 pixels, classified as ice water cloud, included in the averaging. Totals are given for daylight and night pixels separately. |
Cloud fractional cover | - | cfc_day cfc_night cfc_day_unc cfc_night_unc | The cloud fractional-area cover, defined as the fraction of level 2 pixels within each level 3 grid cell flagged as cloudy. |
Cloud-top pressure | hPa | ctp_day ctp_night ctp_day_std ctp_night_std ctp_day_unc ctp_night_unc ctp_day_cor ctp_night_cor | The pressure at the cloud top, as determined from thermal infrared emission. This value is directly related to cth and ctt by ECMWF ERA profiles. |
Cloud-top height | km | cth_day cth_night cth_day_unc cth_night_unc | The height of the cloud top above mean sea level, determined from thermal infrared emission. This value is directly related to ctp and ctt by ECMWF ERA profiles. |
Cloud-top temperature | K | ctt_day ctt_night ctt_day_unc ctt_night_unc | The temperature at cloud top, determined from thermal infrared emission. This value is directly related to ctp and cth by ECMWF ERA profiles. |
Cloud effective radius | µm | cer_day cer_day_std cer_day_unc cer_day_cor | The effective radius of cloud droplets or ice crystals. |
Cloud optical thickness | - | cot_day cot_day_unc | The column optical thickness at a wavelength of 500 nm. |
Liquid water path | gm-2 | lwp_day lwp_day_unc | The column mass of liquid cloud water. |
Ice water path | gm-2 | iwp_day iwp_day_unc | The column mass of ice cloud water. |
Table 4‑3: List of global attributes included in the Cloud_cci TCDR and SLSTR ICDR cloud properties files brokered to, or produced for, the CDS.Anchor table4-3 table4-3
Attribute name | Description |
title | Descriptive title of the file contents |
Project | “Climate Change Initiative-European Space Agency” |
product_version | The version number of the product |
conventions | Lists the naming and meta data conventions used |
standard_name_vocabulary | Defines the standard name convention used |
Institution | The source institution of the data |
Source | The source (level 1) data used in the product |
geospatial_lon_resolution | The grid spacing in the longitude axis |
geospatial_lat_resolution | The grid spacing in the latitude axis |
geospatial_lon_min | The minimum longitude covered by the product |
geospatial_lon_max | The maximum longitude covered by the product |
geospatial_lat_min | The minimum latitude covered by the product |
geospatial_lat_max | The maximum latitude covered by the product |
spatial_resolution | Alternative description of the spatial grid used. |
geospatial_vertical_min | Definition of vertical grid used (0 indicated no vertical grid) |
geospatial_vertical_max | Definition of vertical grid used (0 indicated no vertical grid) |
Platform | Satellite platform from which observations originated |
Sensor | Satellite sensor which made the observations used |
creator_email | |
creator_url | |
date_created | ISO date and time string of processing time of the particular file |
creator_name | Alternative for institution |
time_coverage_duration | ISO time string defining temporal coverage of the product |
time_coverage_resolution | ISO time string defining temporal resolution of the product |
references | Link to further information about the product |
history | Brief description of provenance of the product |
summary | Brief description of product contents |
keywords | List of keywords (for data discovery) |
comment | Any further comments on the product not covered by other fields |
license | License conditions of the product |
cdm_data_type | Common Data Model Data Type used in the NetCDF file |
keywords_vocabulary | The standard list from which the keywords have been extracted |
naming_authority | ID string of the institution naming product (and contents) |
tracking_id | An ISO Universally Unique Identifier (UUID) number for the file |
id | A human readable identifier of the product |
time_coverage_start | ISO time string of the start of the product’s temporal coverage |
time_coverage_end | ISO time string of the end of the product’s temporal coverage |
inputfilelist | List of the primary input files used to create the product |
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