|Title||Scaling of GNSS radio occultation impact with observation number using an ensemble of data assimilations|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Harnisch, F, Healy, S, Bauer, P, English, SJ|
An Ensemble of Data Assimilations (EDA) approach is used to estimate how the impact of Global Navigation Satellite System (GNSS) Radio Occultation (RO) measurements scales as a function of observation number in the ECMWF numerical weather prediction system. The EDA provides an estimate of the theoretical analysis and short-range forecast error statistics, based on the ensemble spread, which is the standard deviation of the ensemble members about the ensemble mean. This study is based on computing how the ensemble spread of various parameters changes as a function of the number of simulated GNSS RO observations. The impact from 2000 up to 128000 globally distributed simulated GNSS RO profiles per day is investigated. It is shown that 2000 simulated GNSS RO measurements have a impact similar to real measurements in the EDA, and that the EDA based impact of real data can be related to the impact in observing system experiments. The dependence of the ensemble statistics on observation errors statistics assumed when assimilating the data, rather than the actual observation errors, is emphasized. There is no evidence of saturation of forecast impact even with 128000 GNSS RO profiles per day. However, this result is a well known consequence of always improving the theoretical analysis and short-range forecast error statistics when adding new observations that are assumed to have uncorrelated observation errors. In general, it is found that 16000 GNSS RO profiles per day have around half the impact of 128000 profiles, based on the reduction of ensemble spread values where the GNSS RO measurements have the largest impact.