Advanced Radiative Transfer Modeling System (ARMS) Updated with New Zeeman Absorption Database

Oxygen (O₂) absorption at microwave frequencies in the upper atmosphere is affected by the Earth’s geomagnetic field and can be split into many sublines. This process is referred to as a Zeeman splitting effect. In this study, an updated Zeeman absorption database is parameterized and implemented in the transmittance module of the Advanced Radiative Transfer Modeling System (ARMS). ARMS is a new generation of satellite observation operator implemented in the CMA Global Forecast System for four-dimensional variational data assimilation (4D-Var). For training the ARMS band transmittance module, the 84-level atmospheric profile dataset provided by the United Kingdom Met Office, the geomagnetic field parameters derived from the International Geomagnetic Reference Field (IGRF-13) and the temperature profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) version 2.0 are utilized as inputs to compute the microwave line-by-line absorption near 60 GHz. This trained ARMS Zeeman transmittance module is then evaluated against 24 days of observations in 2024 from the Special Sensor Microwave Imager/Sounder (SSMIS) on board the Defense Meteorology Satellite Program (DMSP) F-17 satellite. After considering the Zeeman effect in ARMS, the mean observed and simulated brightness temperature bias (O – B) value for channel 19 decreases from 2.69 to 1.30 K. The magnitudes of the mean O – B for channels 20, 21, and 22 are reduced by 8.06, 3.34, and 0.18 K, respectively. Furthermore, the root mean square error (RMSE) for channels 19–21 decreased from 4.652, 12.179, and 6.704 K to 2.529, 3.872, and 2.912 K, respectively. The O – B probability density functions (PDF) for SSMIS upper atmospheric sounding (UAS) channels are all close to normal (Gaussian) distributions.