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Béatrice Pouponneau
,
Franck Ayrault
,
Thierry Bergot
, and
Alain Joly

Abstract

A case study of an eastern Atlantic cyclogenesis event is conducted. The focus is on the impact of aircraft data on both the analysis and forecast of the event. The case takes place between 1 and 3 February 1994 and involves a phase of explosive deepening.

Several new techniques are brought to bear on this problem. An automatic tracking algorithm of vorticity maxima allows the construction of the cyclogenesis scenario in a relatively objective way. It also provides a clear depiction of some of the problems met in a sample of test forecasts. The origin of these problems is determined by employing the adjoint technique in order to point out which parts of the initial conditions are important in explaining the divergences between forecasts.

The cyclogenesis is shown to result from successive baroclinic interactions of a surface vorticity maxima with two upper-level structures in a finite-length baroclinic zone. The largest impact on the forecast is shown to result not from a direct influence of the upper-level observations, but rather from an indirect effect via the observation selection algorithm. While the cyclone development clearly involves upper-level–low-level interaction, the most detrimental difference in initial conditions originates at low levels.

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