The Development of AMSU-A Fundamental CDR's

Current passive microwave sounders, such as the Advanced Sounding Unit-A (AMSU-A) on board of the NOAA POES satellites and EUMETSAT MetOp-A, have been designed for operational weather prediction. Their measurements are not calibrated with sufficient stability for climate applications for derived hydrological products. A properly calibrated Fundamental Climate Data Record (FCDR) needs to be developed to enable the utilization of these data for Thematic CDR (TCDR) and to extend their application into the JPSS era (e.g., POES/AMSU to NPP/ATMS to JPSS/ATMS). With the support of the NOAA Scientific Data Stewardship (SDS) Project, effort is underway at NOAA/NESDIS and its Cooperative Institute for Climate and Satellites (CICS) to develop more than 10 years of AMSU-A window channels FCDR. This includes channels 1 – 4 and 15, i.e. 23.8, 31.4, 50.3, 52.8, and 89 GHz. Presently, seven satellites carry AMSU-A radiometer: NOAA-15 to -19, Aqua, and MetOp-A. This project aims at developing a set of consistent and inter-calibrated AMSU-A radiance CDR from this set of sensors. The plan is to take multiple paths for the FCDR generation in order to achieve a final set of product with high confidence. The calibration approaches include Simultaneous Nadir Overpass (SNO), utilizing radiative transfer model such as Community Radiative Transfer Model (CRTM), among others. AMSU-A window channel measurements are subject to the impact of several biases such as scan asymmetry, antenna pattern effect and satellite drift etc. Among these bias sources, scan asymmetry represents the greatest post-launch calibration challenge and severely impacts the quality of the hydrological products that are retrieved using AMSU-A data. The current operation model is to implement a relatively simple asymmetry correction algorithm several months after satellite launch and no further adjustment is ever performed even though there is evidence that the asymmetry characteristics vary over time. Another source that contributes to data inconsistency is operational changes such as yaw and time corrections. The first stage of the project was to acquire data (AMSU-A level 1b and ERA-interim) and metadata (operational changes etc.) which has been successfully completed. The current focus is on the characterization of the scan asymmetry for each of the seven AMSU-A sensors using CRTM and ERA-interim data. This is initially carried out over ocean under clear sky conditions. The complete project also includes the creation of AMSU-B/MHS FCDR and water cycle TCDR such as precipitation, precipitable water, cloud water, cloud ice etc. These tasks will be carried out as this three-year project progresses.