National POES System (NPOESS) is scheduled to fly during the 2007-2010 period. The impact of future instruments need to be assessed with experiments using simulated observations. These experiments are known as Observing System Simulation Experiments (OSSEs). OSSE will provide a guideline for selection and design of the instrument.

This is a collaboration among NCEP/EMC, NASA/DAO, Simpson weather associates, and NOAA/NESDIS. Through this collaboration, operational data assimilation systems will be ready to handle new data in time for the launch. This process involves the evaluation of the operational load, the development of the data base and data-processing, and a quality control system. All of this development will accelerate the operational use of data from the future instruments.

The first "nature run" (true atmosphere for the OSSEs) was provided by the European Centre for Medium- Range Weather Forecasts (ECMWF) for February 1993. The representativeness of the nature run was evaluated and found to be suitable to conduct OSSEs after some adjustment for low level cloud. Alternative nature run has been also prepared by NASA/DAO including summer and being evaluated.

Data impact of existing instruments in real and simulated data are compared for calibration. The impact test showed satisfactory similarity on TOVS 1B data and RAOB wind between real and simulated analysis. It is found that RAOB temperature may require some bias correction. In addition to random error large scale error is being tested as well as various representativeness error.

The initial results showed that the impact of RAOB winds is found slightly weaker in simulation and the impact of RAOB temperature is slightly stronger in the simulation. However, with large scale error added, impact of RAOB wind become stronger in simulation. Impact of T1B is slightly larger in simulation. One of the reasons for the larger impact of T1B in simulation is the lack of measurement error in the simulated data. Under-estimation of the cloud effect in the simulation is another possible reason for the large impact in simulation. However, localized SST anomaly in late February caused large impact of 1B data in real analysis. These facts need to be considered when we asses the data impact of new instruments.