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.

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 ]).

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.

The shear squared is also somewhat elevated in the regions of small N^2.