The improvement of NWP in high-impact weather situations for the benefit of society, economy and environment is in the focus of international research activities and the basis for the international programme THORPEX. This session will focus on the design and development of new flexible observing methods and technologies for data targeting, sensitivity computation methods and adaptive assimilation techniques as some of the important challenges to improve forecasts for high-impact weather events. The session will accept papers from a wide range of topics including, but not limited to, the following:

· 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.

For several decades, many different techniques have been used by Numerical weather prediction (NWP) centres to evaluate the impact of some observations on the quality of the NWP forecasts. The most popular one is the Observing System Experiment (OSE), where data assimilation and forecasting systems are run with/ without one particular observing system (or one particular observation set): resulting analyses and forecasts with and without the observation set are then inter-compared. This activity is sometimes achieved when developing or tuning an operational data assimilation system, to check the efficiency of the assimilation algorithm on one particular data type. It is often achieved as a pre-operational test between introducing a new type of data into operations. Indeed many new observations have emerged during the last few years, which can now be used in NWP: high resolution infra-red satellite sounders, GPS radio-electrical signals (received in a land station, or on a satellite for radio-occultation), radar Doppler winds or reflectivities. The same type of studies can be performed for observing systems which are being designed and are not yet existing: simulated data are then used instead of real data, and OSSEs (Observing System Simulation Experiments) are performed instead of OSEs. As OSEs and OSSEs are generally expensive in computer time, several cheaper alternatives are now currently used in the data assimilation community.

This session will gather presentations on all these techniques related to the observation impact on NWP analyses and forecasts. For more than 10 years, WMO has regularly documented the impact of operational observing systems, especially at the global and synoptic scales, less at meso-scal. See for example http://www.wmo.int/pages/prog/www/BAS/CBS-meetings.html
The presentations are especially looked for among the 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 well as the studies documenting the interactions between the weather, the atmospheric chemistry and the surface processes (sea surface temperature, ice, snow, vegetation...).