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  • Author or Editor: R. A Anthes x
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X. Zou, Hui Liu, and R. A. Anthes

Abstract

Atmospheric data from National Centers for Environmental Prediction (NCEP) analyses and orbital parameters from 133 real Global Positioning System (GPS) meteorological data soundings are used to compute the “true” bending angle profiles using an accurate 3D ray-tracing procedure. They are then compared with approximate profiles obtained using the spherical symmetry assumption and an efficient 2D ray-tracing model. The average fractional error of the bending angles due to the spherical symmetry assumption is less than 0.15%. The average fractional error due to the use of the 2D ray-tracing model is slightly greater than that due to the spherical symmetry assumption. The vertical error correlations due to the spherical symmetry assumption are sharp between 6 and 8 km and broader above and below this layer. The vertical error correlations associated with the 2D forward model show a nearly diagonal structure below 15 km with high correlation confined to a 2-km layer centered at the observation level.

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L. Cucurull, R. A. Anthes, and L.-L. Tsao

Abstract

Satellite radiance measurements are used daily at numerical weather prediction (NWP) centers around the world, providing a significant positive impact on weather forecast skill. Owing to the existence of systematic errors, either in the observations, instruments, and/or forward models, which can be larger than the signal, the use of infrared or microwave radiances in data assimilation systems requires significant bias corrections. As most bias-correction schemes do not correct for biases that exist in the model forecasts, the model needs to be grounded by an unbiased observing system. These reference measurements, also known as “anchor observations,” prevent a drift of the model to its own climatology and associated biases, thus avoiding a spurious drift of the observation bias corrections.

This paper shows that the assimilation of global positioning system (GPS) radio occultation (RO) observations over a 3-month period in an operational NWP system results in smaller, more accurate bias corrections in infrared and microwave observations, resulting in an overall more effective use of satellite radiances and a larger number of radiance observations that pass quality control. A full version of the NCEP data assimilation system is used to evaluate the results on the bias corrections for the High Resolution Infrared Radiation Sounder-3 (HIRS-3) on NOAA-17 and the Advanced Microwave Sounding Unit-A (AMSU-A) on NOAA-15 in an operational environment.

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