Atmospheric observations consist of a mixture of in situ, visual, and remotely sensed observations. These provide an extensive database for research and numerical weather prediction. However, significant data deficiencies still exist, and new observing systems are continually being proposed. Observing system experiments (OSE's) are conducted to assess the usefulness of different types of existing atmospheric observations. Observing system simulation experiments (OSSE's) are conducted to evaluate the potential impact of proposed observing systems, as well as to determine tradeoffs in their design, and to evaluate data assimilation methodology. This paper contains a review of the development of the global atmospheric observing system, a description of the principal types of data, an overview of OSE and OSSE methodology, and results from recent experiments to evaluate the relative utility of the principal atmospheric observing systems and the potential for new observing systems. These experiments show the critical contributions being made by both conventional and space-based observations, and indicate considerable potential for future satellite observing systems to improve data assimilation.
