Absolute growth rate
= Absolute growth rate
{title2=$\sigma(0)=\operatorname{Im}\omega_0$}
The absolute growth rate is the exponential <growth rate> of a localized impulse at a fixed position in the chosen observation frame. Under the convention $e^{i(kx-\omega t)}$, it is the <imaginary part> of the physically selected <absolute frequency>. Positive values give <absolute hydrodynamic instability>; negative values can still coexist with <convective hydrodynamic instability>.