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| Titel |
Sensitivity to observations applied to FASTEX cases |
| VerfasserIn |
A. Doerenbecher, T. Bergot |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 8, no. 6 ; Nr. 8, no. 6, S.467-481 |
| Datensatznummer |
250005890
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| Publikation (Nr.) |
 copernicus.org/npg-8-467-2001.pdf |
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| Zusammenfassung |
| The concept of
targeted observations was implemented 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 disappointing results: the efficiency of
the additional observations deployed over sensitive areas did not turn out
to remain consistent from one case to another. The influence of targeted
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 nature of the assimilation algorithm, as well as the
deployment of the conventional network of observations (redundancy issues
between targeted and conventional network) in the definition 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 observations (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 influence of the 3D-Var assimilation scheme on
the sensitive structures to be sampled, we present here some detailed
results on a FASTEX targeting case. We focus on the dropsondes 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 particularly
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 stretches up to 250 hPa. However, the
maximum values can be found at approximately 700 hPa in both sensitivity
fields. The studied case shows that the efficiency of observations depends
not only on the sensitivity but also on the deviations between the
observations and the background field. An example of the use of this
diagnosis for comparing the relative efficiency of different kinds of
observations is also presented. This work points out that it is very
complicated to optimize the efficiency of adaptive observations, and that
the assimilation of an entire set of observations (both conventional and
adaptive network) needs to be considered. |
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