Coherent structures in random shell models for passive scalar advection

A study of anomalous scaling in models of passive scalar advection in terms of singular coherent structures is proposed. The stochastic dynamical syst em considered is a shell model reformulation of Kraichnan model. We extend the method introduced by Daumont, Dombre, and Gilson (e-print archive chao- dyn/9905017) to the calculation of self-similar instantons and we show how such objects, being the most singular events, are appropriate to capture as ymptotic scaling properties of the scalar field. Preliminary results concer ning the statistical weight of fluctuations around these optimal configurat ions are also presented. [S1063-651X(99)51312-3].