Sensitivity to observations applied to FASTEX cases

The concept of targeted observations was imple- mented during field experiments such as FASTEX, NORPEX or WSRP in order to cope with some predictability problems. The techniques of targeting used at that moment (adjoint- based or ensemble transform methods) lead to quite disap- pointing results: the efficiency of the additional observa- tions deployed over sensitive areas did not turn out to remain consistent from one case to another. The influence of tar- geted observations on the forecasts could sometimes consist of strong improvements, or sometimes strong degradations. It turns out that the latter failure explains why the concept of optimal sampling arose. The efficiency of adaptive sampling appears to depend on the assimilation scheme that deals with the observations. It is then very useful to integrate the na- ture of the assimilation algorithm, as well as the deployment of the conventional network of observations (redundancy is- sues between targeted and conventional network) in the def- inition of the sensitive pattern to be sampled. Therefore, we chose the tool of the sensitivity to observations to allow us to test such an approach. The sensitivity to targeted observa- tions (that utilizes the adjoint of the linearized NWP model and the adjoint of the assimilation operator) seems to be a suitable tool to obtain an insight into the tricky issue of the optimization of the sampling strategies. To understand better the intrinsic patterns and the influ- ence of the 3D-Var assimilation scheme on the sensitive structures to be sampled, we present here some detailed re- sults on a FASTEX targeting case. We focus on the drop- sondes deployed by the Gulfstream IV (jet-aircraft) along its first flight during Intense Observing Period 17 that started on the 17 February 1997. The sensitivity to observation is used as a diagnostic tool for studing targeting from a critical point of view. It is shown that assimilation processes can have an important effect on the classical sensitivity fields, and par- ticularly on their vertical extension. For example, in the studied case, the classical sensitivity fields remain at a lower level than 400 hPa, whereas the sensitivity to observations