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Table 1: Summary of variables and definitions
Variables | Abbreviation | Definition |
Cloud mask / Cloud fraction | CMA/ | 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/ | CTP/ | 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/ | LWP/ | 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
Term | Definition |
|---|---|
Brokered product | The C3S Climate Data Store (CDS) provides both datasets produced within the C3S and so-called brokered products. The latter are existing products (data) produced under an independent programme or project which are made available through the CDS. |
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 document provides a description of the product validation results for the Essential Climate Variable (ECV) Cloud Properties. This CDR comprises inputs from two 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 (ii) those produced under this contract for the Climate Data Store, specifically those coming from processing of the Sea and Land Surface Temperature Radiometers (SLSTR).
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Table 2-1: Summary of the TCDR accuracy and stability of the Cloud Properties dataset (together with the GCOS requirements) extracted from table 7-2 in [D1]. Green shaded cell indicate compliance with the requirements, yellow cells nearly compliance and red cell no compliance.Anchor table2_1 table2_1
Product name | GCOS targets | Cloud CCI dataset | Comments | |
Cloud Fractional Cover (CFC) | Accuracy | 5 % | -5.1 % | Level-2 validation against CALIOP |
Stability (per decade) | 3 % | -0.52 % | Level – 3C (L3C) comparisons to MODIS C6.1 | |
Cloud Top Height (CTH)/ Pressure (CTP) | Accuracy (low/mid/high) | 0.5/0.7/ 1.6 km | 0.12 km (liquid cloud) | Level-2 validation against CALIOP |
Stability (per decade) | 15 hPa | 0.45 hPa | Level-3C (L3C) comparisons to MODIS C6.1 | |
Cloud Optical Thickness (COT) | Accuracy | 10 % | No validation (n/v) | No validation possible due to a lack of reliable reference data. Used to estimate LWP and IWP, that are validated. |
Stability (per decade) | 2 % | -0.03 % (liquid cloud) | L3C comparisons to MODIS C6.1 | |
Liquid Water Path (LWP) | Accuracy | 25 % | -2.4% | Level-2 validation against AMSR-E |
Stability (per decade) | 5 % | -0.06 %1 | L3C comparisons to MODIS C6.1 | |
Ice Water Path (IWP) | Accuracy | 25 % | -39.9 % | Level-2 validation against DARDAR |
Stability (per decade) | 5 % | -0.04 %2 | L3C comparisons to MODIS C6.1 | |
Cloud Effective Radius (CER) | Accuracy | 10 % | n/v | No validation possible due to a lack of reliable reference data. Variable is based on LWP and IWP. |
Stability (per decade) | 1μm | -0.96 μm (liquid) | L3C comparisons to MODIS C6.1 |
1 Value obtained from the difference between CDR LWP trend (0.99g/m2/decade) and MODIS LWP trend (8.06g/m2/decade) divided by mean MODIS C6.1 LWP (123g/m²).
2 Value obtained from the difference between CDR IWP trend (-2.27g/m2/decade) and MODIS IWP trend (7.82g/m2/decade) divided by mean MODIS C6.1 IWP (208g/m²).
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Table 2-2: Bias of TCDR and ICDR cloud properties estimate in comparison with MODIS. (Dataset till June 2022)Anchor table2_2 table2_2
Parameters | TCDR (2003-2011) bias | ICDR (2017-2022) bias | ICDR (2019-2022) bias | ICDR merged bias (SLSTR-A and B) |
CFC | -8.1% | -6% | -6% | -6% |
CTP | -25 hPa | -15 hPa | -13 hPa | -13 hPa |
LWP | -17.3 g/m2 | -9.0 g/m2 | -1.1 g/m2 | 0.67 g/m2 |
IWP | -28.8 g/m2 | -25 g/m2 | -40 g/m2 | -25 g/m2 |
Figure 2-1: CFC, CTP, LWP and IWP from SLSTR (ICDR dataset) for March 2017, (areas that have no data are shown in white). Anchor figure2_1 figure2_1 
Figure 2-2. CFC, CTP, LWP and IWP from MODIS dataset for March 2017 (areas that have no data are shown in white).Anchor figure2_2 figure2_2
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| Cloud Fractional Cover | Cloud Top Pressure | Ice Water Path | Liquid Water Path | ||
|---|---|---|---|---|---|
| Percentiles | p2.5 p97.5 | -1% 3% | -7.9 hPa 6.93 hPa | -16.4 g/m² 11.3 g/m² | -4.65 g/m² 4.48 g/m² |
| Sentinel-3A: | |||||
| 01/2017 - 12/2020 | 36/48 | 08/48 | 40/48 | 18/48 | |
| 01/2017 - 12/2021 | 12/60 | 41/60 | 20/60 | 39/60 | |
| 01/2017 - 06/2022 | 12/66 | 41/66 | 26/66 | 42/66 | |
Sentinel-3B: | |||||
| 10/2018 - 12/2021 | 07/39 | 26/39 | 26/39 | 26/39 | |
| 10/2018 - 06/2022 | 07/45 | 26/45 | 32/45 | 33/45 | |
| Sentinel-3A+B: | |||||
| 10/2018 - 06/2022 | 02/45 | 42/45 | 06/45 | 27/45 | |
Percentiles were calculated based on the comparison of the TCDR using the Advanced Along Track Scanning Radiometer (AATSR) instrument against MODIS Collection 6.1 (Terra satellite only) as reference dataset for the variables Cloud Fractional Cover (CFC), Cloud Top Pressure (CTP), Ice Water Path (IWP) and Liquid Water Path (LWP). Percentiles were based on the time from 2002-2012 with monthly means and applied to the ICDR from 01/2017 (10/2018) to 06/2022 for Sentinel-3A (Sentinel-3B and merged product Sentinel-3A+B) based on measurements of the Sea and Land Surface Temperature Radiometer (SLSTR).
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