  We study the dynamics of Bose-Einstein condensates in accelerated
two-dimensional optical square lattices by numerically solving the
Gross-Pitaevskii equation. We consider the regime with negligible mean-field
interactions and examine in detail the pulses of atom clouds ejected from the
condensate due to Landau-Zener tunnelling. The pulses exhibit patterned
structures that can be understood from the momentum-space dynamics of the
condensate. Aside from conceiving realization of a pulsed two-dimensional atom
laser, we demonstrate that, by exploring the band structure of the lattice,
Landau-Zener tunnelling and Bragg reflection of the condensate inside the
optical lattice can provide means for manipulation of coherent atom waves.
