Assimilating satellite data along a slanted path
Bormann, Niels
Over the last two decades satellite radiances have come to have the greatest impact on forecasts compared to other types of observations used in numerical weather prediction (NWP). The satellite data are used to determine the initial conditions at the start of a forecast. However, their assimilation into forecast models has traditionally neglected the fact that, most of the time, satellite instruments view the Earth at an angle and therefore sound an atmospheric column that slants through the atmosphere. Data assimilation systems have instead essentially assumed that satellite instruments sense vertical profiles of the atmosphere, only taking into account the increased path length to determine to which vertical layers the radiances relate. With the upgrade of ECMWF's Integrated Forecasting System (IFS) in November 2016 (IFS Cycle 43r1), ECMWF became the first NWP centre to fully take the slanted viewing geometry into account in its operational system. The change has led to improvements in the assimilation of satellite radiances. This has resulted in improved forecast performance that is statistically significant in the short range, particularly in the stratosphere and at higher latitudes. This article gives an overview of what has changed and how forecasts have improved. The interested reader is referred to Bormann (2017) for further details.
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