Inter-satellite calibration of the NOAA AMSU-A measurements over tropical oceans

Time series of monthly mean brightness temperatures over the tropical ocean (20°S to 20°N) measured under clear-sky condition by two Advance Microwave Sounding Unit-A (AMSU-A) radiometers onboard the polar-orbiting NOAA-18 and NOAA-19 satellites are investigated for the time period from 2009 to 2012. To demonstrate the characteristics of the data over this test site, time series of monthly mean ocean brightness temperatures are constructed separately for the ascending and descending passes. These time series provide a useful pattern of annual variation of the AMSU-A measurements for inter-calibration of microwave radiometers onboard multiple satellites to estimate the Intersatellite/intersensor Calibration Biases (ICBs) at individual channels. Differences (∆T = TN18 − TN19) of monthly mean brightness temperatures (as denoted by TN18 and TN19, respectively) between the NOAA-18 and NOAA-19 near-nadir measurements are used to estimate the ICBs at individual channels. It is found that the ∆T values distribute around individual ICBs with near zero slopes, except channel 8 which shows a noticeable anomalous trend in the ∆T time series. This abnormal behavior coincides with the degradation of the NOAA-19 noise equivalent delta temperatures (NEΔTs) which have greatly exceeded the AMSU-A specification since early 2010. This instrument problem is clearly detected in this study. Diurnal variations of the time series of data were corrected with a diurnal cycle of the measurements, which are modeled with a second-order Fourier series, from four AMSU-A radiometers onboard NOAA-15, -16, -18 and METOP-A, respectively. These four satellites have different local equator crossing times across the diurnal period. The results presented in this study will be useful for data users to have a better understanding of the measurements before using these AMSU-A data for weather forecasting or to create data sets for climate applications.