Evaluation of NOAA-21 VIIRS Thermal Emissive Bands Calibration Stability and Biases Using CrIS Observed and Gap-Filled Spectra

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the NOAA-21 satellite was successfully launched in late 2022, as a follow-on to the VIIRS onboard the Sumi-NPP (S-NPP) and NOAA-20 satellites. VIIRS has 14 Reflective Solar Bands, 7 Thermal Emissive Bands (TEB), and 1 Day-Night Band. There are 7 TEB on VIIRS, including 3 mid-wave infrared (MWIR, I4 and M12-M13) and 4 longwave infrared (LWIR, I5 and M14-M16) bands, with center wavelengths ranging from 3.7 to 11.9 µm. Similar to NOAA-20 and S-NPP VIIRS, NOAA-21 VIIRS TEB Sensor Data Records (SDR) are valuable for monitoring severe weather events and deriving a wide variety of Environmental Data Records, such as sea surface temperature (SST), land/ice surface temperature, active fires, and cloud properties.

The Cross-Track Infrared Sounder (CrIS) and VIIRS are onboard the same satellites. CrIS full spectra SDRs are well calibrated radiometrically and spectrally. Co-located VIIRS-CrIS observations are consistently available across wide ranges of scene temperatures and scan angles. CrIS measurements have been routinely used as a reference for evaluating VIIRS TEB calibration stability and biases since the S-NPP and NOAA-20 era. However, there exist gaps in the spectral coverage of CrIS, different from IASI. Traditionally, only VIIRS bands I5, M13, and M15-M16 are within the CrIS spectral coverage and can be inter-compared with CrIS. During the NOAA-21 VIIRS post-launch test (intensive cal/val) period, we further improved VIIRS-CrIS inter-comparison by incorporating CrIS gap-filled spectra, derived using a CrIS gap-filling algorithm proposed by the Global Space-Based Inter-Calibration System (GSICS) IR subgroup. Therefore, in addition to the VIIRS I5, M13, and M15-M16 bands, the calibration stability, biases, and/or anomalies of I4, M12, and M14 were also analyzed in this study.

This study presents NOAA-21 VIIRS-CrIS inter-comparison results with the NOAA operational VIIRS and CrIS SDRs. NOAA-21 VIIRS LWIR (I5 and M14-M16) SDRs have been generally stable during nominal operations since the beginning of the mission. NOAA-21 VIIRS LWIRs agree well with co-located CrIS data, with daily averaged biases within 0.1 K. The early on-orbit VIIRS cold focal plane temperature switch and VIIRS/CrIS calibration updates introduce small changes in daily BT bias time series (less than 0.01 K). Larger than expected scan angle and scene temperature dependent biases were observed in VIIRS LWIR SDRs early in the mission. In particular, VIIRS M15 exhibits larger biases, up to ~0.5 K at 220 K scene temperatures near beginning of scan. We derived improved LWIR calibration parameters using the pitch maneuver data. After the LWIR calibration update was deployed in the operations since June 7, 2023, the LWIR scan angle and scene temperature dependent biases were reduced to ~±0.15K.
NOAA-21 VIIRS MWIR (I4 and M12-M13) SDRs have been stable since the beginning of the mission. M13 spectral response function is shifted toward shorter wavelength compared to NOAA-20 and S-NPP. The gap-filled spectra improve NOAA-21 M13 spectra coverage from 96.7% to 99.8%, and were used to estimate M13 calibration bias more accurately. Moreover, the gap-filled spectra also improve our understanding of the scan-angle and scene temperature dependent biases in the NOAA-21 MWIRs.

We also evaluated the NOAA-21 VIIRS TEB calibration anomalies during blackbody warm-up/cool-down (WUCD) events using the CrIS observed and gap-filled spectra. Similar to NOAA-21, NOAA-21 WUCD anomalies are ~ 0.05 K (when the latest operational calibration parameters were used), which introduce spurious spikes in the daily SST time series. To support SST retrievals, we have developed NOAA-21 WUCD bias correction coefficients, and plan to deliver for operational deployment before the end this hurricane season. Preliminary results indicate that residual WUCD biases are expected to be ~0.01 K for the SST bands (M12 and M15-M16), comparable to NOAA-20 and S-NPP.

Acknowledgement: Dr. Changyong Cao (NOAA/STAR VIIRS SDR team lead) and NOAA STAR VIIRS radiance team

Disclaimer: The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the author(s) and do not necessarily reflect those of NOAA or the Department of Commerce.