The role of model and initial condition error in numerical weather forecasting investigated with an observing system simulation experiment

Abstract

A series of experiments that explore the roles of model and initial condition error in numerical weather prediction are performed using an observing system simulation experiment (OSSE) framework developed at the National Aeronautics and Space Administration Global Modeling and Assimilation Office (NASA/GMAO). The use of an OSSE allows the analysis and forecast errors to be explicitly calculated, and different hypothetical observing networks can be tested with ease. In these experiments, both a full global OSSE framework and an ‘identical twin’ OSSE setup are used to compare the behaviour of the data assimilation system (DAS) and evolution of forecast skill with and without model error. The initial condition error is manipulated by varying the distribution and quality of the observing network and the magnitude of observation errors. The results show that model error has a strong impact on both the quality of the analysis field and the evolution of forecast skill, including both systematic and unsystematic model error components. With a realistic observing network, the analysis state retains a significant quantity of error due to systematic model error. If errors of the analysis state are minimised, model error acts to rapidly degrade forecast skill during the first 24–48 hours of forward integration. In the presence of model error, the impact of observation errors on forecast skill is small, but in the absence of model error, observation errors cause a substantial degradation of the skill of medium-range forecasts.

• data assimilation
• OSSE
• numerical weather prediction
• model error
• initial condition error

7. Acknowledgments

The OSSE was conducted with the assistance of Ricardo Todling, Meta Sienkiewicz, King-Sheng Tai and Joseph Stassi at the GMAO. Erik Andersson provided the ECMWF NR through arrangements made by Michiko Masutani. Support for this project was encouraged by Michele Rienecker and provided by GMAO core funding. The authors also thank three anonymous reviewers for their helpful comments.