Abstract
Cloud-track-wind data, also often called SATOB data, provide an invaluable source of observational information from otherwise sparsely covered parts of the lower and upper troposphere, particularly in the tropics. SATOB data were, for the first time, collected on a global basis during FGGE. During the analysis of these data for the ECMWF Main level III-b production, some questions arose regarding apparent systematic biases in the data in and near jetstreams. In preparation for a reassimilation of parts of the FGGE at ECMWF these questions were adressed again, benefitting from the accumulated experience of global data assimilation at the Centre. Several types of diagnostics, based both on the data assimilation. scheme and independent, collocated observations were extracted and evaluated, based on the Final FGGE level II-b data from December 1978.
The biases were found to be negative, underestimating the windspeed, and quite systematic. All datasets from different producers that could be reliably assessed were found to suffer. The problem was most pronounced in the subtropical jetstreams, where individual wind analyses were found to be seriously deformed by too weak SATOB windspeeds, especially in areas void of other sources of data. Impact was also seen in monthly averages. In a simple calibration experiment, an attempt was made to reduce the biases. The result showed a great impact of the calibration, indicating both the importance of SATOB data and the risks associated with their systematic biases.
