Impact studies of the relative benefit of current observing systems to data assimilation and forecast skill at short and medium range are important to estimate the value of specific data types and to optimize cost-effective use of observation resources. This information can be used to improve utilization of current observation systems, to locate target areas where additional observations could provide the most benefit and to determine priorities for new observing systems. For several years, many different techniques have been used by NWP centers to evaluate the impact of observations on NWP forecast quality. The most popular one is the Observing System Experiment (OSE), which compares analyses and forecasts of different NWP runs, with and without a specific observing system. New diagnostics rating the impact of any observation subset on the analyses and forecast, based on the adjoint of the data assimilation mathematical operators and of a short range forecast model, have recently been developed. In the last years, significant developments have taken place in the Global Observing System, such as high-infrared sounders (AIRS,IASI), GPS radio-electrical signals (received at land stations or on a satellite for radio-occultation soundings), radar reflectivities and radar Doppler winds. Design studies on current and future satellite missions can be performed using simulated data instead of real data and OSSEs (Observing System Simulating Experiments) instead of OSEs. Conventional observing systems are also being adapted through regional programs like the EUMETNET Composite Observing System (EUCOS). Targeting strategies are being used or considered for increased use by means of such programs as, THORPEX, MEDEX or Eurorisk-PREVIEW and field experiments such as COPS, ETReC , AMMA or IPY

This session will accept papers from all these techniques related to the impact of observations on NWP analyses and forecasts as well as targeting strategies including, but not limited to, the following

- Global and regional observing system experiments (OSEs)
- Design studies of future observing systems (OSSEs)
- Data targeting strategies and the development of operational Data Targeting Systems
- The use of targeted data in field experiments (COPS, ETReC, IPY, etc.)
- Strengths and weaknesses of current sensitivity computation methods
- Adaptive, targeted NWP assimilation techniques
- Assessment of the impact of targeted observations on NWP models through OSE�s.

Of particular importance to this session will be presentations on studies documenting the impact of innovative observing systems on meso-scale NWP models, not necessarily operational. Impact studies related to atmospheric chemistry are also welcome, as are studies documenting the interactions between weather, atmospheric chemistry and surface processes (sea surface temperature, ice, snow, vegetation, ...).