Reply on RC1

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Abstract
Radiances from the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA-19 satellite were successfully simulated from Suomi-NPP Visible Infrared Imaging Radiometer Suite (VIIRS) radiances using collocated AVHRR/VIIRS datasets from 2012–2013. Spectral Band Adjustment Factors (SBAFs) were derived using linear regression and neural networks (NNs). The NN approach produced the best results, and separating daytime from night-time conditions when simulating AVHRR channel 3B at 3.7 µm was key. Furthermore, daytime radiance corrections in this channel must depend on actual surface and cloud reflectances to be realistic, which was only achieved by the NN approach. The cloud mask, cloud top height, and cloud phase products were produced from the simulated AVHRR radiances using the same retrieval methods for NOAA-19 data used to compile the CLARA-A3 climate data record (CDR). CLARA-A3 is the third edition of the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF) CDR with cloud parameters, surface albedo, surface radiation, and Top of Atmosphere (TOA) radiation products from AVHRR. Products were validated using CALIPSO cloud products and agreed well with original CLARA-A3 products, with the best results provided by the NN simulation approach. The NN-based approach best reproduced the corresponding products for cloud optical thickness (COT), cloud effective radius (CRE), liquid water path (LWP), and ice water path (IWP). The CLARA-A3 CDR will be complemented and extended with VIIRS-based products to cover the period 1979–2024 (46 years). This edition will be known as CLARA-A3.5. Future extensions and editions can follow a similar approach by applying the same radiance simulation method to collocated data from the Metop-C AVHRR and the Metop-SG METimage sensors, the first version of the latter scheduled for launch in August 2025. Successful simulation of AVHRR radiances from METimage data enables the extension of the CLARA CDR for several decades using observations from VIIRS and METimage.

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