Extension of AVHRR-based climate data records: exploring ways to simulate AVHRR radiances from Suomi NPP VIIRS data

Abstract

The long series of multispectral measurements from the Advanced Very High Resolution Radiometer (AVHRR), which began in 1979, is now approaching its end, with the last remaining AVHRR sensor currently operating aboard EUMETSAT’s Metop-C satellite. Several climate data records (CDRs) built on AVHRR data now face the end of their observational record. However, since many modern imagers contain AVHRR-heritage spectral channels, the potential for an extension of these AVHRR-based climate data records exists. This study investigates the possibility of simulating original National Oceanic and Atmospheric Administration-19 (NOAA-19) AVHRR channels from the Suomi National Polar-orbiting Platform (NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) radiances using collocated AVHRR–VIIRS datasets from 2012–2013. Spectral band adjustments (SBAs) were derived using linear regression and neural networks (NNs). The NN approach produced the best results, and separating daytime from nighttime 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 achieved only through 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 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 Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations–Cloud-Aerosol Lidar with Orthogonal Polarization (CALIPSO–CALIOP) cloud products and agreed well with the 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 CLARAA3.5. 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 Second Generation (SG) METimage sensors, with the first satellite of the latter scheduled for launch in August 2025. The successful simulation of AVHRR radiances from METimage and VIIRS data enables the CLARA CDR extension for several decades.