Spatiotemporal diagrams in the space (r, t) for inertial waves entering from r/R₀ = 20 at the forcing frequency ω_f = 0.3, m = 1, k_zR₀ = 5, ν = 0 for an unstable column with Ω₀/f = 3 (panel a) and a stable column with Ω₀/f = 0.5 (panel b). Panel c displays the time evolution of wave energy E(t) for the unstable case (a, black solid line) and stable case (b, black dashed line). The blue line denotes the energy growth C exp(2ω_it) with a constant C chosen for comparison with the unstable case.