In the temporal approach, experimenters consider the flow in a periodic ( cyclic ) box " moving " at mean speed ( absolute instability ).
2.
While the 600 mb height is unknown, it is likely that the atmospheric lapse rate is close to or exceeds the threshold for absolute instability, strongly driving convection.
3.
By considering modes with zero group velocity and determining whether they grow or decay, we can determine whether an initial condition which is localised around x = 0 moves away from x = 0 as it grows, with y ( 0, t ) \ rightarrow 0 ( convective instability ); or whether y ( 0, t ) \ rightarrow \ infty ( absolute instability ).
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