Validation of AMSU-A measurements from two different calibrations in the lower stratosphere using COSMIC radio occultation data

GPS radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission were used to validate the measurements of the advanced microwave sounding unit-A (AMSU-A) in the lower stratosphere from different satellites. AMSU-A observations from two different calibrations—the pre-launch operational and post-launch simultaneous nadir overpass (SNO) calibrations—were compared to microwave brightness temperatures (Tb) simulated from COSMIC data. Observations from three satellites (NOAA-15, -16, and -18) were used in the comparison. The results showed that AMSU-A Tb measurements from both calibrations and from all three NOAA satellites were underestimated in the lower stratosphere, and that the biases were larger in polar winters, especially over the southern high latitudes. In comparison to operational calibration, the SNO-calibrated AMSU-A data produced much smaller biases relative to the COSMIC data. The improvement due to SNO calibration was quantified by a Ratio index, which measured the bias changes from operational to SNO calibrations relative to the biases between the operational-calibrated AMSU-A data and the COSMIC data. The Ratio values were 70 % for NOAA-15 and >80 % for NOAA-18 and -16, indicating that the SNO calibration method significantly reduced AMSU-A biases and effectively improved AMSU-A data quality.