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General definitions
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.
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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.Anchor table1_1 table1_1
Altitude | Inclination | Period | Repeat Cycle | Ground-track deviation | Local Time at Descending node |
814.5 km | 98.65° | 100.99 min | 27 days | ±1 km | 10:00 |
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]).Anchor table1_2 table1_2
Capability | SLSTR Specifications | |
Swath | Nadir view | 1,400 km |
Oblique view | 740 km | |
Global Coverage Revisit Times | 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(265 – 310 K) | < 0.2 K (0.1 K goal) | |
Fire (< 500 K) | < 3 K |
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(A)ATSR Channel number | SLSTR Band name | Nominal wavelength |
2 | S2 | 0.67 m |
3 | S3 | 0.87 m |
4 | S5 | 1.6 m |
6 | S8 | 10.8 m |
7 | S9 | 12.0 m |
In addition to (A)ATSR or SLSTR level 1 data, the CC4CL retrieval scheme also relies on a range of auxiliary datasets, which are detailed in Table 2-2.
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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 MCD43A1 The product used is MCD43A1 V006. See the MODIS user guide at https://www.umb.edu/spectralmass/terra_aqua_modis/v006 (accessed 7-May-2020). 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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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].
Both daily and monthly data files have common global attributes, which are defined in Table 4‑3. All data arrays (aside from the lat/lon arrays defining the geographic grid) in the monthly products are 720×360 element arrays, while those in the daily files are 3600×1800 elements. In the case of daily files, data are separated into daylight and night data, based on a solar zenith angle cut-off. This separation is done for two main reasons:
- In locations where the daylight (descending half of the orbit) and night sections (ascending half of the orbit) overlap, data values separated in time by approximately 12 hours would be combined if a simple daily average was taken. Not only can such an average not be considered a valid daily mean, but will result in inconsistent results in adjacent sections of the orbit track which do and do-not overlap
- Cloud optical depth and effective radius rely on information for the shortwave (A)ATSR or SLSTR channels, and thus are not retrieved for night pixels. Thus, in order to provide a consistent set of cloud properties for each retrieval, only daylight cloud top pressure, height and temperature as well as water path should be included in the averaging where effective radius and optical depth are present.
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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 | 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 | 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 | 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 | 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 | 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 | 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 | 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 | The effective radius of cloud droplets or ice crystals. |
Cloud optical thickness | - | cot | The column optical thickness at a wavelength of 500 nm. |
Liquid water path | gm-2 | lwp | The column mass of liquid cloud water. |
Ice water path | gm-2 | iwp | The column mass of ice cloud water. |
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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 | 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 | 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 | 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 | 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 | 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 | 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 | 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 | The effective radius of cloud droplets or ice crystals. |
Cloud optical thickness | - | cot_day | The column optical thickness at a wavelength of 500 nm. |
Liquid water path | gm-2 | lwp_day | The column mass of liquid cloud water. |
Ice water path | gm-2 | iwp_day | The column mass of ice cloud water. |
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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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