Cloud property retrievals for climate monitoring: implications of differences between SEVIRI on METEOSAT-8 and AVHRR on NOAA-17

In the framework of the Satellite Application Facility on Climate Monitoring (CM-SAF) an algorithm was developed to retrieve Cloud Physical Properties (CPP) from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the Meteosat Second Generation (METEOSAT­8) and the Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration (NOAA) satellites. This paper presents the CPP algorithm and determines if SEVIRI can be used together with AVHRR to build a consistent and accurate dataset of Cloud Optical Thickness (COT) and Cloud Liquid Water Path (CLWP) over Europe for climate research purposes. After quantifying the differences in 0.6 and 1.6 m operational calibrated reflectances of SEVIRI and AVHRR a recalibration procedure is proposed to normalize and absolutely calibrate these reflectances. The effects of recalibration, spatial resolution and viewing geometry differences on the SEVIRI and AVHRR cloud property retrievals are evaluated.

The intercomparison of 0.6 and 1.6 m operationally calibrated reflectances indicates ~6 and ~26% higher reflectances for SEVIRI than for AVHRR. These discrepancies result in retrieval differences between AVHRR and SEVIRI of ~8% for COT and ~60% for CLWP. Due to recalibration these differences reduce to ~5%, while the magnitude of the median COT and CLWP values of AVHRR decrease ~2 and ~60% and the SEVIRI values increase ~10 and ~55%, respectively. The differences in spatial resolution and viewing geometry slightly influence the retrieval precision. Thus, the CPP algorithm can be used to build a consistent and high quality dataset of SEVIRI and AVHRR retrieved cloud properties for climate research purposes, provided the instrument reflectances are recalibrated, preferably guided by the satellite operators.