A Justification of the Modulation Approximation to the 3D Full Water Wave Problem

Abstract: We consider small amplitude wave packet-like solutions to the 3D inviscid incompressible irrotational infinite depth water wave problem neglecting surface tension. Formal multiscale calculations suggest that the modulation of such a solution is described by a profile traveling at group velocity and governed by a hyperbolic cubic nonlinear Schr\"odinger equation. In this paper we show that, given wave packet initial data, the corresponding solution exists and retains the form of a wave packet on natural NLS time scales. Moreover, we give rigorous error estimates between the true and formal solutions on the appropriate time scale in Sobolev spaces using the energy method. The proof proceeds by directly applying modulational analysis to the formulation of the 3D water wave problem developed by Sijue Wu.