Comparison of Microwave Measurements with Radiative Transfer Model Simulations
Microwave radiometer antenna temperature (Ta) and/or brightness temperature (Tb) inter-calibration is important for measurement long-term time series integrity. In addition, Ta and/or Tb measurement monitoring is important for diagnosing instrument performance degradation. One method used to carry out these tasks is the Simultaneous Nadir Overpass (SNO) method for microwave sounders, or the Simultaneous Conical Overpass (SCO) method for microwave imagers. The SNO and SCO methods are based on the fact that many polar-orbiting satellites revolve around the Earth at slightly different periods, which causes them to occasionally view the same location at nearly the same time. There are some limitations to this method though.
The SNO/SCO events from sun-synchronous polar-orbiting satellites occur at high latitudes, where the observed Ta and/or Tb values are normally low. Also, satellites in the same orbit with a half orbit lag – e.g., NOAA-20 and S-NPP Advanced Technology Microwave Sounder (ATMS) - will fail to produce SNO events. The low-inclination non-sun-synchronous satellites, e.g., the Megha-Tropiques (M-T) satellite, yield numerous collocations in the tropical region with sun-synchronous polar-orbiting satellites, e.g., the S-NPP satellite and offer more opportunities for direct time and space collocations. Nevertheless, inter-comparing similar instruments only reveals the relative differences between the instruments, and cannot be used to identify the absolute errors in the measurements.
One method of monitoring that is much less bound by such limitations is the Radiative Transfer Model (RTM) Background Simulation (BS) method. This method compares microwave radiometer Ta and/or Tb values with radiative transfer model simulated values. In order to implement this method in clear-sky regions, atmospheric sounding data that includes pressure, temperature, water vapor mixing ratio, and ozone mixing ratio are needed to establish representative boundary conditions for the RTM. There are diverse atmospheric sounding sources that can be used to support this method. These include soundings generated from traditional radiosondes, NWP model output, and Global Navigation Satellite System (GNSS) Radio-Occultation (RO). Once simulated or background (B) Ta and/or Tb values are computed, they can be subtracted from observed (O) Ta and/or Tb values to yield O-B Ta biases.
Application of this method to the S-NPP ATMS using National Center for Environmental Prediction (NCEP) Global Forecast System (GFS) 6-hr forecast outputs can be found in a paper by Weng et. al. . A study by Moradi et. al.  evaluated the radiometric accuracy of observations from the S-NPP ATMS and Sondeur Atmospherique du Profil d' Humidité Intropicale par Radiométrie (SAPHIR) onboard Megha-Tropiques through intercalibration and validation versus in situ radiosonde and GNSS-RO observations. The RTM-BS method implemented by Iacovazzi et. al.  harnessed the Constellation Observing System for Meteorology, Ionosphere, and Climate/Formosa Satellite Mission 3 (COSMIC-1/FORMOSAT3, or COSMIC for brevity) GNSS-RO sounding data to establish boundary conditions for the Community Radiative Transfer Model (CRTM). Simulated Ta from the CRTM was used to estimate monthly-average O-B Ta bias for each operational Advance Microwave Sounding Unit – A (AMSU-A) and ATMS instrument commissioned after January 1, 2000 except for Metop-C AMSU-A – i.e., NOAA-18, NOAA-19, Metop-A and Metop-B AMSU-A and S-NPP and NOAA-20 ATMS. Furthermore, using the CRTM-simulated Ta values as a transfer standard, the monthly-average O-B Ta bias values for each instrument can be the foundation to inter-compare Ta observations from different microwave radiometer makes and models by computing the double differences.
Author: Robbie Iacovazzi, NOAA/NESDIS/STAR, 5830 University Research Court, College Park, MD, USA, 20740-3823; Robert.Iacovazzi@noaa.gov.
- Weng, F., X. Zou, X. Wang, S. Yang, and M. D. Goldberg, 2012: Introduction to Suomi national polar-orbiting partnership advanced technology microwave sounder for numerical weather prediction and tropical cyclone applications, J. Geophys. Res., 117, D19112, doi:10.1029/2012JD018144.
- Moradi,I., R. R. Ferraro, P. Eriksson and F. Weng, 2015: Intercalibration and Validation of Observations From ATMS and SAPHIR Microwave Sounders, in IEEE Transactions on Geoscience and Remote Sensing, vol. 53, no. 11, pp. 5915-5925, doi: 10.1109/TGRS.2015.2427165.
- Iacovazzi, R.; Lin, L.; Sun, N.; Liu, Q., 2020: NOAA Operational Microwave Sounding Radiometer Data Quality Monitoring and Anomaly Assessment Using COSMIC GNSS Radio-Occultation Soundings. Remote Sens. 12, 828.
- 14 Apr 2020