Abstract
More than one decade of observations from the Advanced Microwave Sounding Unit-A (AMSU-A) onboard the polar-orbiting satellites NOAA-15 to NOAA-19 and European Meteorological Operational satellite program-A (MetOp-A) provided global information on atmospheric temperature profiles, water vapor, cloud, precipitation, etc. These observations were primarily intended for weather related prediction and applications, however, in order to meet the requirements for climate application, further reprocessing must be conducted to first eliminate any potential satellites biases. After the geolocation and cross-scan bias corrections were applied to the dataset, follow-on research focused on the comparison amongst AMSU-A window channels (e.g., 23.8, 31.4, 50.3 and 89.0 GHz) from the six different satellites to remove any inter-satellite inconsistency. Inter-satellite differences can arise from many error sources, such as bias drift, sun-heating-induced instrument variability in brightness temperatures, radiance dependent biases due to inaccurate calibration nonlinearity, etc. The Integrated microwave inter-calibration approach (IMICA) approach was adopted in this study for inter-satellite calibration of AMSU-A window channels after the appropriate standard deviation (STD) thresholds were identified to restrict Simultaneous Nadir Overpass (SNO) data for window channels. This was a critical step towards the development of a set of fundamental and thematic climate data records (CDRs) for hydrological and climatological applications. NOAA-15 served as the main reference satellite for this study. For ensuing studies that expand to beyond 2015, however, it is recommended that a different satellite be adopted as the reference due to concerns over potential degradation of NOAA-15 AMSU-A.
Highlights
Satellite measurements and derived meteorological products from the Polar Operational Environmental Satellite (POES) system have demonstrated their capability in Numerical Weather Prediction (NWP)
Numerous inter-satellite calibration approaches have emerged recently, and can be classified into two broad categories according to their sensor scanning types: simultaneous nadir overpass (SNO) for cross-track scanning sensor pairs like Advanced Microwave Sounding Unit-A (AMSU-A) [5] and simultaneous conical overpass (SCO) for conical scanning sensor pairs like the Special Sensor Microwave Imager (SSM/I) [6]
For the AMSU-A window channels, special consideration must be taken to ensure that the pair of nadir observations is homogenous, as they are much more sensitive to the surface compared to the sounding channels [20]
Summary
Satellite measurements and derived meteorological products from the Polar Operational Environmental Satellite (POES) system have demonstrated their capability in Numerical Weather Prediction (NWP). In order to achieve CDR quality from a satellite time series like AMSU-A, which has spanned seven different satellites since 1998, inter-satellite calibration is perhaps the most critical step, and is commonly accomplished through direct comparisons of collocated observations from pairs of satellite instruments (i.e., NOAA-15 with NOAA-16, etc.). Through these comparisons, systematic calibration transfer functions can be generated to correct the radiometric biases of the monitored sensors in References [2,3,4]. To introduce cross-platform calibration (XCAL), a unified calibration was developed to adapt comparison results from a group of approaches [9]—most of these approaches have been applied to the SSM/I to develop its FCDR [10]
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