On some aspects of the impact of GPSRO observations in global numerical weather prediction

The impact of Radio Occultation observations from Global Positioning System satellites (GPSRO) on global Numerical Weather Prediction (NWP) has been analysed with a recent version of the European Centre for Medium-range Weather Forecasting (ECMWF) Integrated Forecasting System. As in previous studies, the use of GPSRO was found to improve the NWP forecast skill and to drastically decrease model-induced temperature biases in the analysis. The maximum forecast impact is in the lower and middle stratosphere, where the GPSRO observations have the smallest errors, but it is also visible in the troposphere. The tropospheric impact of GPSRO comes in part from direct tropospheric measurements and in part from stratosphere-troposphere interactions: this second mechanism is found to be particularly important during the Northern Hemisphere winter. The forecast impact of GPSRO observations is compared with that of conventional and hyperspectral satellite nadir sounders. It is found that although GPSRO data have a smaller impact than those of either class of nadir sounders, they are still able to account for a considerable fraction (30-70%) of the global forecast error reduction afforded by the use of the full observing system over a system that uses only conventional observations. When forecast verification is performed against radiosonde observations, GPSRO is found to be the most valuable satellite observing system in the lower stratosphere. This is remarkable in view of the relative sparseness of the GPSRO spatial and temporal coverage and an indication of the potential improvements that a denser GPSRO observing network would be able to provide. The forecast impact of GPSRO observations is also evaluated in the context of a data denial assimilation experiment with respect to the full observing system. Results are found to be consistent with those from the reduced baseline observational network and also indicate a statistically significant positive impact on tropospheric synoptic skill scores.