Analysis of off-nadir bias patterns in NOAA/NESDIS/STAR version 2.0 recalibrated AMSU-A products

NOAA/NESDIS/Center for Satellite Applications and Research (STAR) has been recalibrating the Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit-A (AMSU-A) using the simultaneous nadir overpass (SNO) method to generate long-term global atmospheric temperature climate data records. STAR version 2.0 MSU/AMSU-A products have recently been released to the public. Among the products, AMSU-A observations were newly recalibrated using the SNO method, however, independent from the MSU recalibration. Theoretically, SNO matchups for deriving calibration coefficients only contain intersatellite biases information at nadir. The off-nadir biases (ONBs) in the STAR recalibrated datasets are of particular concern to product users. MSU datasets have been extensively examined in previous studies [Zou et al., 2006; Zou et al., 2009; Zou and Wang, 2010]. The purpose of this study is to analyze ONB patterns in AMSU-A products, in which observations from limb position 8 – 23 are used. ONBs may be caused by a variety of factors, such as instrument antenna pattern, limb effect, diurnal effect, and residual warm target contamination. In this study, we will compare ONB patterns in NOAA pre-launch and SNO calibrated radiances, as well as before and after different correction procedures. Preliminary results indicated that ONBs in the STAR version 2.0 AMSU-A recalibrated radiance products are reasonably small. Figure 1 shows global ocean mean ONBs time series in the SNO recalibrated and limb-corrected NOAA-15 AMSU-A channel 5 radiances (mid-tropospheric temperature). The ONBs are less than 0.2 K for all limb positions. Moreover, trends introduced by OBNs are negligible (< 0.01 K/decade). Figure 2 shows intersatellite bias time series for a representative satellite pair: NOAA-16 and NOAA-15 (channel 5). Standard deviations in intersatellite bias time series are less than 0.03 K for all limb positions. However, seasonal variations were observed in both ONBs and intersatellite biases, which may be caused by the current limb correction scheme. We will analyze ONB patterns for other channels and AMSU-A instruments and address any issues indentified. Major References: Zou, C.-Z., et al. (2006), J. Geophys. Res., 111. Zou, C.-Z., et al. (2009), J. Clim., 22(7), 1661-1681. Zou, C.-Z., and W. Wang (2010), J. Atmos. Oceanic Technol., in press.