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Surface pressure information retrieved from GPS radio occultation measurements



Numerical weather prediction (NWP) experiments have been performed to investigate the impact of surface pressure information retrieved from Global Positioning System radio occultation (GPSRO) measurements. The GPSRO measurements are able to reduce the impact of removing all conventional surface pressure observations from the NWP system. The GPSRO measurements can constrain global mean surface pressure errors in analyses and short-range forecasts to the −1 to −1.5 hPa level over a 3-month period, whereas they can be as large as −4 hPa when both GPSRO observations and conventional surface pressure measurements are removed from the NWP system. The large mean errors that arise when both GPSRO and conventional surface pressure measurements are removed are caused by an interaction between the assimilation of the conventional upper-air measurements and the bias correction of radiances with the variational bias correction system (VARBC). In the absence of GPSRO, the VARBC amplifies and spreads errors that are introduced by the conventional upper-air data. However, the GPSRO measurements cannot fully compensate for the loss of the conventional data, particularly in the Northern Hemisphere. It is also shown that the surface pressure results obtained when GPSRO are assimilated are sensitive to biases introduced by other observations, such as aircraft temperature measurements, because these biases can be aliased into surface pressure increments. Experiments designed to compare the amount of information provided by other parts of the global observing system highlight the complementary nature of GPSRO and scatterometer wind retrievals, and show that reasonable surface pressure analyses, with an uncertainty of ∼ 1 hPa, can now be obtained by assimilating just satellite data, without using any conventional data to ‘anchor’ the NWP system. However, the full observing system, including both satellite and conventional observations, remains superior to assimilating just satellite observations.

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