  The generalized method of characteristics is used to obtain rank-2 solutions
of the classical equations of hydrodynamics in (3+1) dimensions describing the
motion of a fluid medium in the presence of gravitational and Coriolis forces.
We determine the necessary and sufficient conditions which guarantee the
existence of solutions expressed in terms of Riemann invariants for an
inhomogeneous quasilinear system of partial differential equations. The paper
contains a detailed exposition of the theory of simple wave solutions and a
presentation of the main tool used to study the Cauchy problem. A systematic
use is made of the generalized method of characteristics in order to generate
several classes of wave solutions written in terms of Riemann invariants.
