Conclusions and Future Work

The objective of this study is to demonstrate the potential to use GPS RO and AMSU/MSU data to construct consistent climate temperature records. This is done by using the COSMIC synthetic AMSU Tbs as calibration references to inter-calibrate AMSU Tbs from N15, N16, and N18. Our analysis led to the following conclusions.

1. The COSMIC synthetic AMSU Tbs are highly correlated with AMSU Tbs. The mean Tb biases for AMSU-COSMIC pairs over the 60°N to 90°N zone and the 60°S to 90°S zone are very consistent with the AMSU inter-satellite biases from different missions. The variation of AMSU-COSMIC Tb biases is highly coherent with the NOAA satellite orbit change with local time. AMSU-COSMIC Tb biases are in general lower during the southern hemispheric winter in the night and higher in the Northern Hemisphere in the day. Because COSMIC data, unlike MSU/AMSU data, do not contain orbit drift errors and are not affected by on-orbit heating and cooling of the satellite component, they are very useful to identify the AMSU time/location-dependent biases for different missions. The mean AMSU-COSMIC Tb biases for different NOAA missions are all negative. This may be due to the inconsistency between the AMSU operational calibration coefficients generated before launch and the orbital changes on AMSU measurements after launch, and/or positive systematic biases introduced from MWFcimss . We leave it for a future study. 2. The calibration coefficients found from NOAA-NOAA pairs are also consistent with that of NOAA-COSMIC pairs. For example, we can construct the calibration coefficients for new COSMIC-N15 pairs (AMSU Tb for the newly derived N15 is TbCOSMIC_N15_N16) using those from COSMIC-N16 and N15-N16 pairs. We assess the robustness of the slope and offset for the new COSMIC-N15 pairs by comparing TbCOSMiC_N15_N16 to TbCOSMiC_N15 which is constructed from COSMIC-N15 pairs. The tight fit of TbCOSMiC^15 and TbCOSMiC^15^16 (the standard deviation is less than 0.1 K) demonstrates the consistency between the slope and offset (calibration coefficients) found in the N15-COSMIC pairs and those constructed using N16-COSMIC pairs and N15-N16 pairs.

Results here demonstrate that although there are only a few years of GPS RO data from both COSMIC and CHAMP that are collocated with AMSU measurements (from 2001 and beyond), we may still be able to use GPS RO data to inter-calibrate AMSU/MSU data. The calibrated AMSU/MSU data, in turn, can be used to calibrate other overlapping AMSU/MSU data for those years without GPS RO data (e.g., from 1979 to 2001). In the future, multi-month data from GPS RO and AMSU/MSU data will be used to further quantify the time/location-dependent AMSU/MSU inter-satellite biases. The consistency of the calibration coefficients among collocated NOAA Tb pairs from different missions and COSMIC-NOAA pairs for different seasons of different years will also be examined.

Acknowledgements We would like to thank Mr. Hal Woolf from the Cooperative Institution for Meteorological Satellite Studies for providing the fast AMSU Forward Transfer Algorithm package. We would also like to acknowledge the contributions to this work from members of the COSMIC team at UCAR and members of the satellite operational team at the National Space Organization, Taiwan.

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