  Starting from an effective action for the order parameter field, we derive a
coupled set of generalized hydrodynamic equations for a Bose condensate in an
optical lattice at finite temperatures. Using the linearized hydrodynamic
equations, we study the microscopic mechanism of the Landau instability due to
the collisional damping process between condensate and noncondensate atoms. It
is shown that the Landau criterion of the superfluidity for the uniform system
is modified due to the presence of the periodic optical lattice potential.
