Monitoring NOAA Operational Microwave Sounding Radiometer Data Quality Using CRTM Brightness Temperature Simulations Based on COSMIC GPS Radio-Occultation Atmospheric Sounding Inputs

NOAA operational Advanced Microwave Sounding Unit-A (AMSU-A) and Advanced Technology Microwave Sounder (ATMS) radiometer data are essential to protect life and property and maintain safe and efficient commerce, through their use in numerical weather prediction and climate analysis. Routine monitoring of data quality from these instruments, and understanding and possibly making corrections to eliminate spurious trends in them, is thus important to sustain maximum data effectiveness. One valuable method used to monitor microwave sounder data quality is to trend the brightness temperature (TB) difference (O-B) computed between direct instrument measurements and forward radiative transfer model simulations. This monitoring method typically uses output from a numerical weather prediction (NWP) model, such as the NOAA Global Forecast System, to provide the atmospheric temperature and moisture sounding profiles needed by a radiative transfer model to perform the radiometric simulations. For this study, instead of using NWP-generated soundings, we use COSMIC GPS Radio Occultation (RO) soundings as input to perform Community Radiative Transfer Model (CRTM) simulations of the NOAA operational microwave sounding radiometer measurements.

The data used in this research are all the available NOAA-18, NOAA-19, Metop-A, and Metop-B AMSU-A, and S-NPP and NOAA-20 ATMS observations that are within 50 km of COSMIC GPS-RO atmospheric temperature and moisture sounding profiles. The study is limited to soundings acquired over ocean and equatorward of 60^o latitude during a time span from October 2012 to April 2019. CRTM microwave radiometer TB simulations based on these COSMIC GPS-RO soundings are harnessed to compute a TB O-B value at every single sounding location. For each microwave radiometer - NOAA-18, NOAA-19, Metop-A, and Metop-B AMSU-A, and S-NPP and NOAA-20 ATMS - the TB O-B statistics are computed on monthly time scales to make them more robust for long-term product monitoring. In addition, the CRTM-simulated TBs based on the COSMIC GPS-RO soundings can be used as a transfer standard to inter-compare TBs from different microwave radiometer make and models that have the same bands. For example, monthly TB O-B statistics for NOAA-18 AMSU-A Channels 4-12 and NOAA-20 ATMS Channels 5-13 can be differenced to estimate the “double-difference” TB biases between these two instruments for the corresponding frequency bands.

Results of this study show TB O-B values for each AMSU-A instrument for Channels 4-12, and ATMS instrument for Channels 5-13, have a magnitude of around 1.5 K or less. Similar results are found for the “double-difference” TB biases between similar channels of AMSU-A and ATMS instruments.