Contributors: E. Carboni (UKRI-STFC RAL Space), G. Thomas (UKRI-STFC RAL Space)
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Scope of the document
This document provides a description of the product validation results for the Essential Climate Variable (ECV) Cloud Properties. 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 Service (C3S).
The TCDR is a brokered version of ESA’s Cloud_cci ATSR2-AATSR version 3.0 (Level-3C) dataset, produced by RAL from the second Along-Track Scanning Radiometer (ATSR-2) on board the second European Remote Sensing Satellite (ERS-2) spanned 1995-2003 and the Advanced ATSR (AATSR) on board ENVISAT spanned 2002-2012. In addition, the Sea and Land Surface Temperature Radiometer (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 2018. 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.
Poulsen et al. (2019) [D3] is the paper describing the dataset that includes cloud properties as well as Surface Radiation Budget and Earth Radiation Budget products. This document will mainly refer to the Cloud_cci Product Validation and Intercomparison Report [D1].
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 Radiometer (SLSTR) is produced specifically for C3S. The product is generated by STFC RAL Space, using the Community Cloud for Climate (CC4CL) processor, based on the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm.
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 TCDR is partnered with the ICDR produced from the Sentinel-3A SLSTR, beginning in 2017, and Sentinel-3B SLSTR beginning in October 2018
The TCDR and ICDR provided comprise daily (0.1° x 0.1° resolution), and monthly (0.5° x 0.5° resolution), means of cloud properties on a regular global latitude-longitude grid and includes the following products: Cloud Fractional Cover (CFC), Cloud Optical Thickness (COT), Cloud Effective Radius (CER), Liquid/Ice Water Path (LWP/IWP), and Cloud Top Pressure (CTP), Height (CTH) and Temperature (CTT). Note, that the brokered service within Copernicus provides only a subset of the original CCI cloud properties dataset, thus the mentioned products do not cover the entire range on cloud products contained in the original dataset provided by Cloud_cci.
Table 1: provides a summary of the estimated accuracies of the TCDR together with the GCOS requirements. The validation results are provided in [D1] section 7-2, with a recommendation for use.
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 % | 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 | L3C comparisons to MODIS C6.1 | |
Cloud Optical Thickness (COT) | Accuracy | 10 % | n/v | No validation possible due to a lack of reliable reference data. Through LWP and IWP validation. |
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) | 1m | -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). |
The validation methodology is described in section 2.4 of [D1]. In summary, we use the bias (mean difference) between CDR and 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, which is also known as the standard deviation about the mean. Stability is calculated as the variation of the bias over a multi-annual time period.
The validation results for TCDR products are provided in [D1], section 3 and 4. The evaluation is divided in validation against high quality and satellite-based reference observations (CALIOP, AMSR-E and DARDAR) and an intercomparison to well-established, satellite-based cloud datasets of similar kind (e.g. MODIS Collection 6.1). Table 1 in the Executive summary provides a summary of the resulting TCDR accuracies.
Cloud Fractional Cover (CFC) is validated against CALIOP in [D1], section 3.1.1 and compared with MODIS in [D1], section 4.1.1.
A slight underestimation of cloud occurrences is found in the Cloud_cci data compared to CALIOP, which is primarily due to a lack of sensitivity of passive imager data with respect to optically very thin clouds.
Cloud Top Height is validated against CALIOP in [D1] section 3.1.3.
For liquid clouds only very small biases (0.12km) and bc-RMSE (0.97km) are found.
For ice clouds, the strong underestimation of cloud top height is evident and a common feature for all three Cloud_cci datasets, and mainly caused by high-level, optically thin clouds. Biases are around -3.5 km and bc-RMSE around 2.3 km. Removing the optically very thin cloud layers at the top of the CALIOP profiles, improves the agreement between Cloud_cci and CALIOP significantly.
These parameters are compared against MODIS in the following sections of [D1]: Cloud Top Pressure (CTP) is compared in section 4.1.2. Cloud Effective Radius (CER) is compared in section 4.1.5 and 4.1.6. Cloud Optical Thickness (COT) is compared in section 4.1.3 and 4.1.4 for liquid and ice cloud.
Liquid Water Path (LWP) is validated in [D1] section 3.1.4 against AMSR-E products and compared with MODIS in [D1] section 4.1.7.
Ice Water Path (IWP) is validated against DARDAR IWP product in [D1] section 3.1.5 and compared with MODIS in section 4.1.8.
Validating liquid water path over ocean against AMSR-E gives very convincing results for CDR dataset, with bc-RMSE values of 25 g/m², only small biases (-1.44 g/m²) and high correlations (0.76). Validating ice water path against the combined CALIPSO-CloudSat product DARDAR shows good agreement with correlations of 0.45. There is a general underestimation of IWP by Cloud_cci which in terms of relative bias partly exceeds 50%.
The first 24 months of SLSTR products have been compared against MODIS (Collection 6.1 Terra) following the same methodology described in [D1] section 4.1. We estimate the bias, i.e. mean differences, and the monthly mean global average of C3S and the MODIS data. To compute the monthly mean global average of both datasets we considered only the valid data between -60 and +60 latitude.
Table 2 shows the bias results from this comparison for both TCDR data (from D1) and for ICDR.
Of the cloud properties considered only the IWP has higher bias against MODIS for the ICDR.
Figure 1 and 2 show an example of ICDR monthly products for March 2017 and the equivalent monthly product from MODIS.
Table 2: Bias of TCDR and ICDR cloud properties estimate in comparison with MODIS.
Parameters | TCDR (2003-2011) bias | ICDR (2017-2018) bias |
CFC |
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CTP | -25 hPa | -15 hPa |
LWP | -17.3 g/m2 | -16.0 g/m2 |
IWP | -28.8 g/m2 | -34 g/m2 |
Figure 1: CFC, CTP, LWP and IWP from SLSTR (ICDR dataset) for March 2017.
Figure 2. CFC, CTP, LWP and IWP from MODIS dataset for March 2017
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The main validation results are summarized in [D1], Section 7, including a comparison with GCOS requirements in table 7.2. We report here a summary.
Table 1 provides an overview of the GCOS requirements for the Cloud Properties and the values achieved by TCDR. It should be noted that GCOS requirements are targets and are often not attainable using existing or historical observing systems. The Cloud_cci doesn’t meet the frequency requirement (3h) due to the nature of the satellite observations, but exceeds the spatial resolution (50 km GCOS target). ICDR accuracies (estimate with the first 24 months only) are consistent with TCDR accuracies apart from IWP where we find slightly higher bias in comparison with MODIS (section 2.2).
For nearly all validations for which a trusted reference data source is available, the compliance to GCOS requirements could be shown, e.g. cloud fractional cover stability, cloud top height accuracy and stability, liquid water path accuracy and stability and ice water path stability. Cloud fractional cover accuracy is close to the GCOS target requirements. For effective radius and optical thickness no reliable reference data is available for accuracy compliance analysis.
A general problem is the assessment of the stability. Stability assessments are based on comparisons to MODIS, which in turn however, is not an entirely reliable source itself as it is sometimes characterized by significant trends which may or may not be true.
Recommendation on the usage and know limitations (from [D1] table 7.1):
Cloud Fractional Cover (CFC)
Cloud Top Pressure (CTP)
Cloud effective radius (CER)
Liquid water content (LWP)
Ice water content (IWP)
This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or 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 view. |
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