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List of datasets covered by this document
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Acronyms
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Scope of the document
This document provides a description of the product validation methodology for the global Cloud Properties Climate Data Record (CDR). These products are brokered to (in case of (A)ATSR) or produced for the Climate Data Store (in the case of SLSTR) by the Copernicus Climate Change Services (C3S).
ESA’s Cloud_cci ATSR2-AATSR version 3.0 (Level-3C) dataset, produced by STFC RAL Space from the second Along-Track Scanning Radiometer (ATSR-2) on board the second European Remote Sensing Satellite (ERS-2) spanned 1995-2003, the Advanced ATSR (AATSR) on board ENVISAT spanned 2002-2012. In addition the SLSTR on board of Sentinel-3 spans from 2017 to present and forms the ICDR. The validation for the ICDR is over the period from January 2017 to December 2019. The retrieval algorithm is presented in [D2] and the validation methodology refers to the Cloud_cci Product Validation and Intercomparison Report [D1]. The same methodology is applied to SLSTR dataset.
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The TCDR and ICDR provide level-3 data, monthly means on a regular global latitude-longitude grid (with a resolution of 0.5 0.5) and daily data (with a resolution of 0.10.1) and includes these products: Cloud Fractional Cover (CFC), Cloud Phase (water/ice), Cloud Optical Thickness (COT), Cloud particle Effective Radius (CER), Liquid/Ice Water Path (LWP/IWP), and Cloud Top Pressure (CTP), Height (CTH) and Temperature (CTT).
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1. Validated products
The Cloud_cci ATSR2-AATSR Cloud Properties CDR, version 3.0, is brokered to the CDS from the ESA Cloud_cci project and produced by STFC RAL. The SLSTR ICDR, version 3.x, is supplied to the CDS via the same route and uses the same processing software and infrastructure as the TCDR.
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The Cloud_cci dataset can be downloaded here: https://climate.esa.int/en/projects/cloud/data/. The SLSTR based ICDR extends the coverage, with a five year gap, from 2017 onwards and is only available through Copernicus Climate Data Store (CDS).
Poulsen et al. (2019) [D3] is the paper describing the TCDR dataset that includes cloud products as well as Surface Radiation Budget and Earth Radiation Budget products.
The Product Validation and Intercomparison Report [D1] is the document with validation and intercomparison of cloud properties against other satellite and ground datasets.
2. Description of validating datasets
The Cloud Properties TCDR from ATSR2 and AATSR instruments are validated against CALIOP for cloud fractional cover, and cloud top height.
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More details on the datasets used for the validation are described in [D1] section 2.4.2 and Annex A.
3. Description of product validation methodology
The validation strategy is described in [D1] section 2.4. We use the bias, i.e. mean difference between Cloud_cci and the reference data, as the metric for accuracy. The bias corrected root mean squared error (bc-RMSE) is used to express the precision of CDR compared to a reference data record. The stability is the variation of the bias over a multi-annual time period.
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ICDR data are only validated against MODIS dataset following the same methodology described in section 4.1 of [D1].
3.1 Cloud Fractional Cover (CFC)
Cloud Fractional Cover (CFC) from TCDR is validated against SYNOP in [D1] section 3.2, validated against CALIOP in [D1] section 3.1.1 and compared with MODIS in [D1] section 4.1.1. The validation methodology can be found in the corresponding sections.
ICDR CFC is compared against MODIS using the same methodology described in [D1] section 4.4.1.
3.2 Cloud Top Height (CTH)
Cloud Top Height is validated against the CALIOP data and the validation methodology is described in [D1] section 3.1.3.
ICDR CTH is indirectly validated trough CTP comparison with MODIS.
3.3 Cloud Top Pressure (CTP), Cloud Optical Thickness (COT) and Cloud Effective Radius (CER)
For the TCDR these parameters are compared against MODIS Collection 6.1. Cloud Top Pressure (CTP) is compared in [D1] section 4.1.2, Cloud Effective Radius (CER) is compared in [D1] section 4.1.5 and 4.1.6 and the Cloud Optical Thickness (COT), is compared in [D1] section 4.1.3 and 4.1.4 for the liquid and ice cloud phase. The comparison methodology is described in the corresponding section introduction, 4.1 [D1]. The ICDR uses the same methodology as the TCDR.
3.4 Cloud Liquid Water Path (LWP) and cloud Ice Water Path (IWP)
For the TCDR, Cloud Liquid Water Path (LWP) is validated in [D1] section 3.1.4 against AMSR-E products and compared with MODIS Collection 6.1 in [D1] section 4.1.7. The validation and comparison methodology is described in the mentioned sections.
For the TCDR, Cloud Ice Water Path (IWP) is validated against DARDAR IWP product in [D1] section 3.1.5 and compared with MODIS Collection 6.1 in section 4.1.8. The validation and comparison methodology can be found in the corresponding sections.
For the ICDR, data re compared with MODIS only, according to the methodology described in [D1] section 4.1.
4. Summary of validation results
The validation results for TCDR are provided in [D1] section 7, together with recommendation for use. Here is an extract from table 7-2 in [D1]:
Comments from [D1] | |||
Cloud fractional cover | Accuracy |
| Level-2 validation against CALIOP |
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| Values taken from Table 4-2 and Table 4-13 (L3C comparisons to MODIS C6.1) | |
Cloud top height/ pressure | Accuracy | 0.12km (liquid cloud) | Level-2 validation against CALIOP |
Stability (per decade) |
| Values taken from Table 4-4 and Table 4-15 (L3C comparisons to MODIS C6.1) | |
Liquid cloud optical depth | Accuracy | n/v | No validation possible due to a lack of reliable reference data. through LWP and IWP validation |
Stability (per decade) |
| Values taken from Table 4-5 and Table 4-16 divided by mean MODIS C6.1 Terra COTliq (13) (L3C comparisons to MODIS C6.1) | |
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| No validation possible due to a lack of reliable reference data. through LWP and IWP validation |
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| Values taken from Table 4-6 and Table 4-17 divided by mean MODIS C6.1 Terra COTice (10) (L3C comparisons to MODIS C6.1) | |
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| Level-2 validation against AMSR-E (Figure 3-1) |
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| Values taken from Table 4-9 and Table 4-20 divided by mean MODIS C6 LWP (123g/m²) (L3C comparisons to MODIS C6) | |
Ice water path | Accuracy | -39.9% | Level-2 validation against DARDAR |
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| Values taken from Table 4-10 and Table 4-21 divided by mean MODIS C6 IWP (208g/m²) (L3C comparisons to MODIS C6) | |
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| No validation possible due to a lack of reliable reference data. through LWP and IWP validation |
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| Values taken from Table 4-7 and Table 4-18 (L3C comparisons to MODIS C6) | |
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| No validation possible due to a lack of reliable reference data. through LWP and IWP validation |
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| Values taken from Table 4-8 and Table 4-19 (L3C comparisons to MODIS C6) |
Intercomparison (using the monthly mean data from January 2017 to December 2019) of ICDR products with MODIS present biases consistent with values found through the TCDR MODIS comparison (D1, section 4.1) for cloud fractional cover (- 5 %), cloud top pressure (-15 hPa) and liquid water path (-15.0 g/m2), but higher values for ice water path, with global average bias of -31 g/m3 for SLSTR-A and -46 g/m3 for SLSTR-B (TCDR IWP bias with MODIS was -29 g/m3).
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