CM-SAF Visiting Scientist Activity CM_VS14_01 Report: Characterisation of SSM/T-2 radiances using ERA-Interim and other reanalyses

Title
CM-SAF Visiting Scientist Activity CM_VS14_01 Report: Characterisation of SSM/T-2 radiances using ERA-Interim and other reanalyses
Report
Date Published
08/2015
Series/Collection
ERA Report Series
Document Number
21
Author
S. Kobayashi
P. Poli
V. John
Event Series/Collection
ERA Report
Abstract Preceding the Advanced Microwave Sounding Unit-B (AMSU-B), the Microwave Humidity Sounder (MHS), and the Advanced Technology Microwave Sounder (ATMS), operational observations of microwave humidity sounders began in the early 1990s, with the Special Sensor Microwave Water Vapor Profiler (SSM/T-2). The SSM/T-2 radiances have not been used so far in global reanalyses. In this study, SSM/T-2 data are compared to several reanalyses (ERA-Interim, ERA-20C, and JRA-55), using clear-sky radiative transfer computations. The study brings new light to the problem of the “unspecified” polarisation state of SSM/T-2, concluding that the antenna was oriented towards horizontal (not vertical) polarisation in the limit of nadir viewing. The report identifies several issues that need to be taken into account when producing fundamental climate data records from the SSM/T-2 measurements, or assimilating them into future reanalyses, such as geolocation errors, unphysical values at quasi-periodic positions, stability issues, and inter-satellite biases between satellites. The results indicate that ERA-Interim matches SSM/T-2 183GHz observations within 2–3K of standard deviation. This study also reports on a second set of computations, carried out after correcting some of the problems identified in the first computations. This results in an improved fit to ERA-Interim, within ±1K in the mean for observations from the 150GHz channel. This highlights the importance of accurate reference data and radiative transfer models for error characterisation, and the necessity of an iterative process in such calculations in order to enhance understanding of the error characteristics.
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