Past exam of the mathematics course of the University of Cambridge / 2013 / iii / Paper 65 / 1 / b / ii / Solution

For the positive-frequency internal gravity wave branch, $\omega =N{K}_h/K$. Differentiating with respect to the components of the wave vector gives the group velocity,The wavefront-normal phase velocity is ${\mathbf{c}}_p=\omega \mathbf{k}/K^2$. Directly,The negative-frequency branch changes both propagation signs but not their orthogonality. Another proof is that $\omega$ is homogeneous of degree zero in the wave vector, so Euler's homogeneous-function identity gives $\mathbf{k}\cdot {\nabla }_{\mathbf{k}}\omega =0$. Energy propagates along constant-phase surfaces, rather than normal to them. When $m=0$, $\omega =N$ and ${\mathbf{c}}_g=0$; the scalar-product result remains true, but there is no nonzero group-velocity direction to describe geometrically.