Observing System Simulation Experiments (OSSEs) are typically designed to use data assimilation ideas to investigate the impacts of prospective observing systems (observation types and deployments). They may also be used to investigate current observational and data assimilation systems by testing the impact of new observations on them Over the last 10 to 15 years, Observing System Simulation Experiments have become widely accepted as the de facto standard for assessing the impact on numerical weather prediction (NWP) applications of new candidate systems. Even though the primary focus of this note is impact assessment for future observing systems, it should be pointed out that OSSEs have applications also in other areas, e.g. research and development in data assimilation methodology. The OSSE proxy of the true atmosphere is called a Nature Run (NR), and when a NR is produced by a free forecast run different from the forecast model used for the data assimilation system (DAS) it is called a full OSSE. OSSEs are very labor intensive projects. It has been realized that the preparation of a NR including evaluation, simulation of observations, and distribution of the data consumes a significant amount of effort. An internationally collaborative effort for full OSSEs, called Joint OSSEs, has been formed over the last three years. In Joint OSSEs a common NR will by used by the various DAS at many institutes. The first Joint OSSE NRs have been produced by the European Center for Medium-Range Weather Forecasts (ECMWF). A NR with a 13 month long period from May 2005-May 2006 with three hourly dumps, T511 horizontal resolution, and 91 vertical levels using daily SST was produced to study global data impacts. The T511NR showed remarkably good cyclone statistics and a realistic tropics. During hurricane season and the US severe storm season, two 35 days long runs with hourly dumps at T799 horizontal resolution and 91 levels (T799NR) have been produced. Ideally, all new instruments should be tested by OSSEs before they are selected for construction and deployment. OSSEs will also be important in influencing the design of the instruments and the configuration of the global observing system. While the instruments are being built, OSSEs will help prepare the DAS for the new instruments. Developing a DAS to assimilate a new type of data is a significant task. However, this effort has traditionally been made only after the data became available. The OSSE effort demands that this same work be completed earlier; this will speed up the actual use of the new data and proper testing. The experience of the OSSEs performed at NCEP also demonstrates that OSSEs often produce unexpected results. Theoretical predictions of the data impact and theoretical backup of the OSSE results are very important as they provide guidance on what to expect. On the other hand, unexpected OSSE results will stimulate further theoretical investigations. When all efforts come together, OSSEs will help with timely and reliable recommendations for future observing systems. At the same time, OSSEs will prepare the operational DAS to promote the prompt and effective use of the new data. Currently various projects are emerging within Joint OSSE. First existing instruments have to be simulated and tested in OSSE system. Calibration of data assimilation system are being conducted. Currently various projects are emerging within Joint OSSE. First existing instruments have to be simulated and tested in OSSE system. Calibration of data assimilation system are being conducted. Various experiments proposed are: 1) Various DWL configuration proposed by both NASA and ESA will be tested by Joint OSSE. 2) Additional Radio Occultation observations 3) UnManned Air Craft System 4) Preparation for data assimilation system toward GOES-R and NPOESS instruments 5) Evaluation of T-PARK project 6) Study of observational error 7) Comparison of various data assimilations system Dr. Anthony Hollingsworth was always an inspiration to OSSEs project and encouraging to the goals.
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