The Background Richardson number is less than 0.25 for altitudes between
95.5 and 96.5 km. It is also very small (even negative) right at the
surface.
![Ri](Ri.png)
In both of these locations, the Richardson number is small due to small values
of N^2
(Ri=N^2/[ dU/dz^2 + dV/dz^2 ]).
![N2](N2.png)
Small values of N^2 are associated with rapid cooling with height. More
precisely, N^2 will be negative if dT/dz <= -g/c_p. The plot below shows
rapid cooling both near the surface and near 95 km.
![temperature](T.png)
The shear squared is also somewhat elevated in the regions of small N^2.